Where and How to Connect Your Irrigation System to Your Water Supply:
This page provides some specific rules, tips, and techniques for tapping into a house water supply pipe for a new irrigation system. Where and how you tap into the water supply can be critically important, not just for the proper operation of the irrigation system, but also for the preservation of your sanity!
Standard threaded fittings as used in most irrigation and plumbing are very slightly tapered. This creates a more positive seal as the male thread is twisted into the female thread, to prevent leaks. So the diameter of the male threads increases slightly at the back of the threaded portion, while the diameter of the female threads decreases further inside the fitting. In most cases this taper is not sufficient to create a reliable seal, so a sealer is used. For irrigation systems the recommended sealer is PTFE thread seal tape (often called Teflon® tape.) Do not use “pipe dope” (liquid or paste type sealers) on irrigation systems! If it comes in a tube or bottle, don’t use it. Look on the bottom of many sprinklers and you will see the warning “Don’t use pipe dope”. This is not an insult aimed at people who use pipes rather than tubes! When pipe dope or paste works it’s way inside the pipe (which it will) the water will carry it to the valves, drip emitters, and/or sprinklers and clog them up and ruin them! So the sprinkler manufacturer is warning you not to use them. Only use PTFE tape type sealers. If tape gets into a valve or sprinkler it can be removed and the damage is not permanent.
Using PTFE Thread Sealant Tape (Teflon® Tape)
When joining male and female threaded fittings, put a nice thick layer of PTFE thread seal tape on the male threads before you screw them into the fittings. Pull the tape tight onto the male thread so that the tape molds into the threads. Wrap it in the direction of the threads so it doesn’t unwind off when you screw the fitting on. (If you are looking at the end of the male fitting that would be clockwise.)
How much tape to use? The old standard was “3 wraps”. However now they are selling low-cost PTFE tape that is thinner and requires more wraps. When you have enough tape on the male threads the shape of the threads will be just barely visible through the tape. I personally prefer to err on the side of using too much tape, it is not fun to find you didn’t use enough after the water is turned on and you discover a leak.
Connecting the Fittings
Once the tape is on the male thread screw the male thread into the female threaded fitting. If the joint is between two metal pieces put a wrench on it and tighten it as tight as you can get it. If one or both of the fittings is plastic just tighten it by hand. If you are an average guy or gal you can add one more full turn using a wrench after it is hand tight. If you have ever been called a gorilla for your strength or grip, stop at hand tight. Over-tightening plastic fittings splits the female fitting, resulting in a leak. But not tightening them enough also gets you a leak.
Avoid joining a male metal threaded fitting to a plastic female threaded fitting. This will be a disaster if you are not very careful. The male metal end does not give at all, and the female plastic fitting is likely to split open unless it has heavy reinforcement. If you must join male metal to female plastic, use lots of PTFE tape and hand tighten only. No wrenches!!
High density polyethylene (HDPE, trademark name is Marlex®), street ells often don’t need PTFE tape to seal, but I still use a little on them. It will not hurt to use a couple of wraps of PTFE tape on HDPE fittings. HDPE is a shiny, black, plastic that is slightly softer than PVC and feels slightly “oily”. HDPE is good for places where the fitting needs to be able to rotate and not “seize up” over time. Metal, PVC and PBS plastic threads will seize up and not turn easily once assembled. The HDPE fittings are idea for sprinkler risers where you want the threaded joints to remain pliable and able to move and absorb impacts. Warning: The black barbed insert fittings used to connect to flexible tubing risers (often called “Funny Pipe®”) are not HDPE. HDPE is seldom, if ever, used for any fitting with barbs. This is because the slippery surface of HDPE makes the tube slide off the barbs! If it has barbs, be sure to use PTFE tape on any threads.
Rule of thumb: If you can’t scratch the plastic with your fingernail, use PTFE thread seal tape on the threads!
Grab a piece of string about 6″(152mm) long. Strip away any insulation so you can get at the pipe and wrap the string around it. Measure how many inches of string it takes to go around the pipe once. This is the circumference of the pipe (yikes, bad memories of high school geometry!). Using the circumference we can calculate the diameter of the pipe. But school’s out so let’s forget about doing geometry calculations! Based on the string length use the table below to find your pipe size.
For Copper or PEX Pipe
2.75″ (70mm) = 3/4″ pipe
3.53″ (90mm) = 1″ pipe
4.32″ (110mm) = 1 1/4″ pipe
5.10″ (130mm) = 1 1/2″ pipe
For Steel Pipe or PVC Plastic Pipe
3.25″ (83mm) = 3/4″ pipe
4.00″(102mm) = 1″ pipe
5.00″(127mm) = 1 1/4″ pipe
6.00″(152mm) = 1 1/2″ pipe
For Flexible Polyethylene Pipe
2.96-3.33″ (75-85mm) = 3/4″ pipe
3.74-4.24″ (95-108mm) = 1″ pipe
4.90-5.57″ (124-141mm) = 1 1/4″ pipe
5.70-6.28″ (145-160mm) = 1 1/2″ pipe
Your string length will probably not be exactly the same as the lengths in the chart. Measurements vary a little, depending on how much the string stretches, dirt on the pipe, manufacturing tolerance of the pipe, how accurate you are at measuring, etc.
The short section of tube that attaches a sprinkler to the underground lateral pipe is called a “riser”. But the riser does much more than attach the sprinkler. It must hold the sprinkler in the correct position, it must allow you to adjust the sprinkler location, and we can also use it to protect the sprinkler from damage. The riser type you use is an important choice and deserves some attention. A good riser choice can save you time and money over the years. This article will take you through the many choices and the pros and cons of each. (“Lateral pipe” is the name given to the pipes that go from the zone valve to the sprinkler heads.)
Basic Terminology and Definitions Used for Pipe Fittings:
Every industry has it’s own “code language”, sometime terms are a short-hand way to communicate faster and more accurately, some are just a way to identify “insiders” from “outsiders.” The following will help you to speak “Irrigation” (as well as “Plumbing” which uses most of the same terms.)
SSS, SST, SS, ST, etc…
These are terms used in describing the connections for PVC fittings. “S” stands for “SLIP” “socket”, or “spigot”, which means that the connection is a solvent weld (or glued) type. By the way, both sockets and spigots CAN be threaded also, which is why we use the term SLIP to specify that the connection is solvent welded. I’ve been told that slip really only means a socket, but common usage is for both sockets and spigots. In case you hadn’t guessed by now, there is a bit of confusion on this subject! (Webster’s Unabridged Dictionary doesn’t have the plumbing definition among it’s 69 definitions for the word slip.) At any rate, if you go to the hardware store to get a 3/4″ tee with solvent weld inlet and outlet and a 1/2″ threaded side outlet, you would ask for a “three quarter by three quarter by half tee, slip, slip, thread” and you would write it as “3/4 x 3/4 x 1/2 TEE SST”. (You can say “one-half” if you want to be technically correct, but most of us lazy people just say “half”.)
Lock, Loc, Push-Fit, etc…
There are an assortment of different terms used for these easy to assemble, glue-free fittings, most often the names in some way imply that the tube is “locked” into place. In irrigation most PEX connections are made using these push in and lock type fittings, where the tube is pushed into the fitting and a sharp metal clip locks it in place. At the time I am writing this article, the lock type fittings are starting to be made for all types of plastic tube and pipe, including poly tubing, PVC, and drip tubing. One note, when assembling using these locking fittings, be sure you press the tube or pipe very firmly into the fitting forcing it all the way in, it needs to go fully into the fitting in order to form a tight connection and not leak.
Spigots, Sockets, and Slip.
A spigot is the equivalent of a male end. A socket is a female end. In other words a spigot fits into a socket. Slip can mean either a spigot or socket, but normally means socket, and normally means the connection is solvent welded. For more on the subject see “SSS, SST, SS, ST, etc..” above.
Male and Female.
Oh come on now. Surely you can figure this one out! (Hint: many plumbing terms were originated by sex-crazed males.) Well all-right, male means spigot, female means socket. What do you mean that didn’t help?
You can say “glued” if you want, but that’s not totally correct (you do get partial credit). The cement (glue) used for connecting PVC parts is sticky like standard glue, but in addition it actually melts the plastic, creating a true weld. Thus the term “solvent weld”. By the way, the solvent for PVC is acetone (nail polish remover), so you can remove the PVC cement if you spill it with acetone. (I saved the front seat of my truck from disaster this way after knocking over a complete can of purple color pvc cement onto it. It was a lot of work and took a lot of acetone, but it all came out of the seat fabric eventually.) You can also use acetone to clean PVC pipe, but be careful as the acetone will melt the pipe if you use too much!
Abbreviation used for galvanized steel. Always avoid screwing a male metal fitting into a female plastic fitting. The plastic fitting will split when you tighten the joint. Always work male plastic into female metal when going from metal to plastic parts. If you absolutely must use a plastic female fitting place a worm-gear clamp around the very end of the female fitting and tighten it slightly before you insert the male end. Use lots of Teflon tape or pipe dope and don’t over tighten!
Cu, Type M, Type L, Type K.
All are abbreviations used for copper tube and fittings. Type M, L, and K describe the thickness of the copper pipe wall. M has the thinnest wall and K has the thickest. K is the strongest of the three. K is the only one that should have threads cut into it, the others are weakened too much by cutting threads. K is a good choice for use where physical strength is required, such as supporting the weight of a backflow preventer, although L will support a smaller 3/4″ or 1″ backflow preventer in most situations. L is the most common choice for standard uses, especially if the tube will be in or under buildings. Type M is more likely to develop pin holes in it over time. Widespread use of type M in homes during the 60’s and 70’s has lead to the creation of an entire industry devoted to replacing or rehabilitating failed copper pipe in homes.
A type of fitting used with polyethylene pipe. Insert fittings have barbs that are inserted into the open end of the tube. You must use a clamp placed over the tube and tightened to physically hold barbed fittings in place. Do not rely on the barbs to hold the tube on the insert fitting. Crimp and worm gear clamps are both used.
SCH 40, SCH 80.
Terms used with PVC fittings that indicate the specification standard the fitting was constructed to meet. SCH 80 is usually a gray color and is stronger than SCH 40 which is usually a white color.
Uniform Plumbing Code, National Sanitation Foundation. Indicates that the PVC fitting complies with standard code requirements. An indication of a top-quality PVC fitting.
Female Iron Pipe Thread, Male Iron Pipe Thread. Specifies the end is threaded and the thread pattern used is standard iron pipe style.
Poly Vinyl Chloride. The material most plastic fittings are constructed out of. See SCH 40, SCH 80 above.
The other plastic that some fittings are made of. Marlex is actually a brand name of a specific High Density Poly Ethylene. Normal PVC threaded joints “seize up” and will not turn freely. The HDPE has a oily surface which acts as a lubricant. HDPE fittings are used in situations where the threaded connection needs to remain flexible, such as swing joint risers. (A swing joint allows the sprinkler to move up and down for adjustment and to protect it from being damaged if run over by a vehicle. Swing joints are the standard type of sprinkler riser used in parks and golf courses.)
Teflon Tape, Pipe Dope
Most threaded joints need a sealer placed on the threads before the connection is made. The sealer serves two purposes. First it seals the joint (like you couldn’t figure that out). Second, it lubricates the joint, which makes it much easier to thread the pieces together. Teflon tape is my preferred product , it is easy to use and clean. Wrap 3 layers around the male threads, wrapping in the same direction as the threads (so it doesn’t unwrap when you start threading the fittings together). Don’t cover the end thread, that will help avoid “cross-threading” the joint. Be careful not to allow “strings” of Teflon tape to get into the pipes where they can clog sprinklers or emitters. Never use pipe dope with sprinklers, it can gum up the nozzles, and in gear-driven sprinklers it jams the turbines. Most sprinklers say “no pipe dope” on them, the manufacturer is not calling you a derogate name and saying not to use pipe!
The Basic Fittings (drawings are of PVC fittings but names also apply to polyethylene and PEX.)
Bell Reducer. A bell reducer has female threads on both ends. Bell reducers are generally not available in PVC (so, despite what the caption says above, this drawing is not of a PVC fitting).
Cap. A cap may have a solvent weld socket end or a female threaded end. The other end is closed off. If a solvent weld cap is used to provide for a future connection point, be sure to leave several inches of pipe before the cap! When the cap is cut off for the future connection there will need to be enough pipe present to glue a new fitting onto! I can’t begin to tell you how many times I’ve seen a solvent weld cap butted right up against another fitting, making it impossible to ever use the capped connection again!
Coupling. A coupling connects two sections of pipe together. Couplings may have solvent weld socket ends or female threaded ends.
Cross. A cross connects four pipe sections together. Crosses may have solvent weld socket ends or female threaded ends (no female threads available for PVC). Crosses are special order parts at many suppliers. Crosses create a great deal of stress on the pipe because they have four connection points. In theory this is the same principle that makes a 3 leg stool (a “tee”) more steady than a 4 leg stool (a “cross”). I recommend that you avoid using crosses in most situations. Use two tees.
Female Adapter. Female adapters are used to add a female threaded pipe connection on a solvent welded pipe. Never use female adapters when converting to a metallic pipe. The metal male pipe threads tend to split the PVC fittings. Place a metal coupling on the metallic pipe then use a PVC male adapter. Metal male threads should never be inserted into any female threaded PVC fitting!
Male Adapter. Male adapters are used to add a male threaded pipe connection to a solvent weld pipe section.
Plug. Used to plug a unused fitting outlet. May have female threads or a solvent weld spigot. In most cases a threaded plug is used to provide a connection point for future use. If solvent welded in place the plug is never going to be removed!
Side Outlet Ell. Side outlet ells are an ell with a side outlet. (Well duh…) They most commonly have two 3/4″ or 1″ solvent weld sockets, with a 1/2″ side outlet having female threads. Side outlet ells are common in residential sprinkler systems, but are seldom used in commercial installations. The side outlet is listed last when stating the side outlet ell size. Example: 1x1x1/2 SO ELL SST has a 1/2″ threaded side outlet.
Tee.The most common fitting! Available with all female thread sockets, all solvent weld sockets, or with opposed solvent weld sockets and a side outlet with female threads. Many configurations of “reducer tees” are available, meaning that one or more of the sockets is smaller than the others. Tees are always labeled as “NxNxN TEE with the side outlet as the last size. The largest of the other two sockets is always listed first. Thus a 1×3/4×1/2 TEE SST has a 1/2″ threaded side outlet (T for threaded) with the remaining sockets being 1″ and 3/4” solvent weld sockets (SS for slip, slip). On a “bullhead tee” the side outlet is the largest socket on the tee (thus it looks somewhat like a bull’s head I guess). The side outlet is referred to as the “bullhead”.
Definition of irrigation mainline: The mainline is all the pipes between the water source (POC) and the irrigation zone control valves. Another definition is that mainline is any pipe that is always pressurized with water.
Worksheet for Choosing Your Mainline Pipe or Tube
Excessive Water Pressure: In all cases if your static water pressure exceeds 100 PSI it is advisable to install a pressure regulating valve at the irrigation connection point to maintain a pressure lower than 100 PSI. All of these pipes or tubes may burst at higher pressures.
Temperate Areas (ground doesn’t freeze in winter)
Rocky Soil: Is your ground very rocky, so that it would be impossible to keep rocks larger than 2″ diameter from contacting the pipe? If “yes” consider using PEX tubing for you mainline.
Normal soil: If ground is not rocky consider using SCH 40 PVC pipe for your mainline. If pipe larger than 2″ is needed use Cl 315 PVC pipe.
Frost Areas (ground freezes at least a couple inches deep in winter.)
If your static water pressure is less than 60 PSI the use of 125 PSI rated poly tube may be sufficient if cost is a major issue. However it would be better to use 160 PSI tube if you can afford it.
For static water pressure between 60 and 80 PSI, use 160 PSI rated poly tube.
For static water pressure between 80 and 100 PSI, use 200 PSI rated poly tube.
Make sure you provide a method to blow out or drain the water from the mainline completely during winter.
Pipe or Tube Size: There is no easy way to say what exact size you should use. If you really don’t want to do calculations the following is a very rough educated guess that will work for most (but not all) situations. Use a pipe or tube size that is the next size larger than the water supply pipe the irrigation system (POC) is tapped into. Do not use smaller than 3/4″. Ie; if the irrigation system mainline is going to tap into a 1″ house water supply pipe (POC), then use a 1 1/4″ size mainline for the irrigation.
Pipe Depth: Bury the mainline pipe at least 18″ deep from the top of the pipe to the ground surface. It is critical that this pipe be protected from accidental damage and light frosts.
No water running through the house! To avoid nasty surprises, avoid using a water supply (POC) for your irrigation system that passes through a house inside the walls, under floors, or through the attic.
Keep reading for in-depth details and answers to “why?”
The path ahead viewed dimly through the fog??? What we’re about to embark on here is known as “doing it the right way”. We are going to start by figuring out what the maximum water supply would be if you had perfect conditions, such as a very short pipe from the water meter to your house, lots of water pressure, a small yard, a happy family, a low interest rate mortgage, and good neighbors! Then we are going to modify that number later in the tutorial to reflect your actual conditions (long pipe, lousy water pressure, bad neighbors, whatever.)
The end result is that we will determine what the exact, optimum water supply is for designing your sprinkler system. What that means for you is that your sprinkler system will use less water, last longer, and there won’t be dry spots! Now it’s going to be a little more work than “guesstimating” would be, but it will be worth it. Don’t get discouraged, be patient, and it will all come together. The worst thing you could do right now is to try deciding what sprinkler you want to use. That would be “putting the cart before the horse”. Trust me, I know what I’m doing. Now let’s get on with it…
A. Find your water supply pipe.
Hopefully you already know where the water service pipe comes onto your property, or at least where it enters your house.
Mild Winter Climates: In milder climate areas there is typically a shut-off valve and or a water meter at the location where the pipe enters the property. From there the pipe generally goes to the house, then surfaces above ground where a house shut-off valve is located, then the pipe turns and runs into the side of the house. Often this location where the pipe enters the house is where the tap for the irrigation system will be made.
Note the “W” etched in the curb in front of the concrete water meter box in the photo above. Often there will be some type of mark on a curb at the location that the water supply pipe to the house runs under it.
The photo above is of a typical mild-climate water supply line where it enters the house. This one has a rather unusual model of pressure regulator (the gizmo with the white adjustment knob on top) to reduce the water pressure. Many houses do not have a pressure regulator. A ball valve (with a blue handle, the handle is in the “off” position) is on the incoming water supply pipe. The pipe going into the wall is the house supply. The pipe exiting the photo at the lower left goes to a hose bib.
Cold Winter Climates: In colder climates the water line often enters directly into the basement or crawl space under the house from underground. This water pipe to the house is often buried very deep to keep it below the frost line. The shut-off valve, and possibly a water meter, are often located in the basement or crawl space to help protect them from freezing.
The photo above shows a typical water supply line in a cold-winter climate. A copper water pipe enters through floor, goes up into a ball valve (yellow handle), then through a pressure regulator, then a remote-reading water meter. You would tap in for the sprinklers after the water meter. The mainline supply size would be measured on the copper pipe coming out of the floor. The water pressure in this case could be measured at any water faucet after the regulator (probably any faucet in the whole house would work). Photo credit and thanks to Ed Pletsch.
What type of pipe is it?
Once you find your supply pipe you need to know what type of pipe or tubing it is. Keep in mind that there may be more than one type of pipe or tubing used at different locations! Often copper is used under concrete slabs and then it converts to PEX for other locations.
Steel Pipe. Steel pipe comes in two types, black steel (used mostly for gas lines) and galvanized steel (galv. steel) which is used for water pipes. Galvanized steel pipe will be a silver gray color, and a magnet will stick to it. It will have threaded joints. Steel pipe is made in conformance with IPS (iron pipe size) standards. Galvanized pipe is often found on homes in inland areas, especially on less expensive tract homes.
Brass Pipe. Brass pipe is sometimes used for homes. Like copper it can take on a greenish tint with age. A magnet will not stick to it. It will have threaded joints. Brass pipe is made in made in conformance with IPS (iron pipe size) standards. Brass is not very common except for short sections of pipe, due to cost.
PVC Pipe. PVC plastic pipe is almost always white or gray, and is more rigid than the other commonly used types of plastic water pipe. Standard PVC pipe is made in conformance with IPS (iron pipe size) standards. It should have the letters “PVC” printed on the pipe. PVC is fairly rigid, and it is not easily scratched with your fingernail. PVC tends to be more commonly used in mild climate areas. Another type of PVC called CPVC is sometimes used inside homes and often is found in older mobile homes. It is similar to regular PVC, but will be labeled “CPVC”. Most often it is a yellowish, gold, or tan color. CPVC in homes is usually made to copper tube sizes (look for “SDR-11” printed on the pipe), but is also sometimes iron pipe size (labeled IPS). PVC is often used for house supply pipes in mild winter areas.
Copper Tube. Copper tube is very common in homes. It takes on a dirty green color as it ages. A magnet will not stick to it. Most joints will be soldered, look for silver color solder at the joints to identify it. Copper tube has a different diameter than iron pipes, and is made in sizes known as CTS (copper tube size). Copper has been the standard “high quality” tube used on better homes for decades. Often used in areas near the coast where salt air causes rapid corrosion of steel.
PEX and PE Tube. Both are both polyethylene (poly) products. Both tend to be used in areas with severe winters and/or rocky soil. There is a lot of confusion over these two poly-based products. Be careful, both are sometimes called “poly”, especially by the sales people in the big home improvement stores. True PEX is a stronger form of cross-linked polyethylene that has become popular in recent years. Both PEX and PE are flexible, and both have a glossy appearance and slick surface. So how do you tell which one you have? Older PE is almost always black, and in most cases PEX is not black. PE is almost never used inside a house if the house was built to code. The surest thing to look for are the letters “PEX” printed on the tubing. Making things even worse, white PEX looks a lot like PVC, especially if it is old or dirty! PEX is easily scratched with a fingernail, PVC scratches, but not easily. PEX was not invented until the ’70s, and it is seldom found in homes built before 1975. (It wasn’t officially sold in the USA until 1985. Of course, if your house has been remodeled, you could still have it in a older house.) PEX is almost always made to conform with CTS sizes. The heavy duty PE tube used for plumbing is most often made to a uniform size standard (labeled “SDR-7”), but many different PE products used for irrigation do not conform with this size standard. Be careful when working with PE tube, if possible take a sample with you when you go shopping for parts so you can test fit them at the store. PEX is quickly becoming the default tube for piping new homes due to low cost and ease of installation.
Warning: PEX pipe has a very thick wall, thus it has a smaller inside area for the water to flow through. This means it has much higher pressure losses when the water passes through it. For this reason you need to be careful when replacing a copper or PE tube with a PEX tube. Often when replacing a copper or PE tube with a PEX tube it is necessary to use PEX that is one size larger than the tube it is replacing. So if you are replacing a 3/4″ copper tube with PEX, you should consider using 1″ size PEX tube for the replacement. Otherwise you may notice a drop in water pressure after the replacement is made.
One good hint to the type of pipe is the way the pipes are connected to each other. PEX and PE are never glued at the joints. Sometimes PEX & PE are heat welded together, but most of the time they are connected together with fittings using clamps, teeth, or compression-nuts that hold the tube onto or into the fitting. (“Fittings” is the term we use for the various connectors that are used to join two or more pipes together.) If the pipe has glued joints it is almost always going to be PVC or ABS. (ABS plastic is typically black rigid pipe, almost always 3″ or larger in diameter, and is mostly used for sewer and drainage pipes. ABS can be other colors so don’t assume a pipe is PVC just because it is white or gray!) Another hint is that poly pipe tends to be used in colder climates, and PVC tends to be used in warmer climates. If you have to regularly shovel snow from the driveway, chances are the pipe is PE or PEX. Copper pipe is often soldered to the fittings. Look for the silver color solder at joints. Steel and brass pipe have threaded connections, a few threads almost always are visible at the joints. Confused yet? Your best bet is to find lettering on the pipe that says what type it is.
B. Find your Water Meter:
Now we need to know if you have a water meter. Most, but not all, water companies use a water meter to measure the amount of water you use. If you don’t have a meter, there will almost always be a shut-off valve at the point your house water line connects to the water provider’s pipes. Often the valves are buried, sometimes several feet down, and a sleeve comes up to the surface with a small lid or box over it. The water company uses a special tool that can reach down and open or close the valve. Often grass has completely grown over the lid and you can’t find it. Try probing the ground with a pitchfork, metal rake, or screwdriver to find the hard cover of the box.
The water meters are normally installed in an underground box as close as possible to the property line. This is usually at the street or alley. Most of the time the box will have “water meter” or the water company name stamped on the lid. In areas with severely cold winters the water meter is often installed in the house basement or a utility room of the house. If you still can’t find it, call your water company and request their assistance.
Try to find a size stamped on the meter. If you can’t find a size, ask your water company or just assume the meter is the next size SMALLER than the pipe running to the house. It is common for the meter to be one size smaller than the pipe. Standard water meter sizes are: 5/8″, 3/4″, 1″, 1 1/2″.
Spiders and snakes: If the meter is in a box, watch out for spiders and ants in the meter box! Most of the “pro” irrigation repair guys I know carry a can of spider spray with them! Sometimes we find snakes, rats, gophers, and other beasts in the boxes too! I found a turtle shell in a box once. No tunnels or holes into the box that I could find. I have no idea how it got in there.
Enter the meter size on your Design Data Form. If you don’t have a meter, enter 0 (zero).
C. Measure Your Water Pressure
Water pressure is the energy that powers your sprinkler system, so it is very important. If you work with it, it will make your sprinklers do the “rain dance”. If you ignore it, it can bite you hard in the wallet! For this tutorial I use the pressure units “PSI” which means “pounds per square inch”. When pros talk about pressure readings we almost never say the words “pounds per square inch”, we just say the letter names “P. S. I”. Outside of the United States pressure is most often measured in “bars”.
First off, grab the phone and call up your water supplier. Ask them for the “static water pressure” for your neighborhood. Don’t be shy, people call them all the time to ask! They may give you a pressure range, like 40-60 PSI. If so, write down the LOW number of this range. You can also measure your own water pressure using a pressure gauge that attaches to a hose bib on your house (you can purchase a 0-120 PSI gauge with a hose adapter on it at pretty much any hardware store).
Pressure regulators (also called pressure reducing valves)
Pressure regulators are devices used to reduce the water pressure and are commonly found on home water supplies in towns with hills. It takes lots of water pressure to lift water uphill. So in order to get the water to the houses on top of the hill the water pressure in the water system has to be very high. But this causes the pressure at the homes at the bottom of the hill to be too high. So pressure regulators are installed on the water supply pipes to homes in the lower areas of town, where the pressures are very high. The pressure regulators are generally set to someplace between 50 and 65 PSI.
If the water company tells you your neighborhood pressure is over 65 PSI, you probably have a pressure regulator installed someplace on the water supply line to the house. The pressure regulator reduces the water pressure in your house, so that it doesn’t damage your plumbing fixtures. Look around and see if you can find it (see the pressure regulators in the pictures above). The regulator may be installed near the water meter or at the point where the water supply pipe enters the house. This is important, because if you have a regulator and you tap into the water supply for your sprinklers after the regulator, the pressure will be a lot lower.
If you have a pressure regulator on your house you must use a gauge to test the water pressure yourself. Most pressure regulators are adjustable, so the water company has no idea what pressure the regulator is set at. When in doubt, test the water pressure with a gauge.
At this point you should make at least a preliminary decision as to where you want to tap into the house water supply pipe for the irrigation system water. Typically, the closer you can tap to the point the water enters your property, the better. Of course, you must tap into the pipe after the water meter. In areas where it gets very cold some people like to tap into a pipe in the basement or someplace else inside a heated building. That way they don’t have to worry about the shut-off valve for the irrigation freezing. (Be sure to install a drain valve after the shut-off valve to drain the water out of the irrigation pipe during freezing weather!) If you have a pressure regulator, consider if it would be better to tap before or after it.
A static water pressure higher than 70 PSI can damage the fixtures and appliances in a household. If you measure a static water pressure higher than 70 PSI when you do your water pressure check as described below, then you should consider installing a pressure regulator on your house water supply if there is not one already. It will help your faucets, pipes, washing machine, dish washer, etc. to all last a lot longer. Make sure it is a good quality brass-body pressure regulator.
For a pressure regulator to work accurately the pressure setting on it must be at least 15 PSI lower than the inlet pressure. So if your static pressure is 70 PSI, the highest pressure you should set on the pressure regulator would be 55 PSI. 55 PSI is a good pressure for both the needs of a house and a sprinkler system.
Hose Bibs as a Water Supply Source = BAD!
Using a hose bib or even a “sprinkler system stub-out pipe” provided on the side of the house for sprinklers is not a good idea. There are often unknown restrictions in the house piping that cause the water supply from these hose bibs to be severely limited. The water running through the house pipes can also be very noisy at night and disturbs some people’s sleep. Do this only as a last resort, when there is no other reasonable way to get water for your sprinkler system. I would suggest you assume the pipe is 1/2″ size, even if it appears larger. If you have concrete that prevents running a new pipe around the house, call a boring contractor and find out how much it would cost to bore a 1″ pipe under the concrete. It may be worth the price. Directional boring technology now allows them to bore and install curved pipes around obstacles.
How to Measure the Water Pressure with a Gauge
Important: If you want to test the pressure yourself, everything that uses water in your home: faucets, ice makers, toilets, etc., MUST be turned off when you take the measurement (that’s why its called “static” water pressure, the water isn’t moving.) Everything! This is critical or you will get a false low reading! You can test the pressure at any faucet that is at about the same height as the proposed irrigation tap. If all the water is turned off, the pressure will be exactly the same regardless of where you test it. (Try it and see!) The easiest place to test the pressure is usually a hose bib or garden valve on the outside wall of the house.
To test the water pressure using a gauge, attach the gauge to a water outlet, like a hose bib or washing machine connection. The place where you attach the gauge can be anywhere in the house, as long as it is about the same height (elevation) as the place where you will tap in the sprinkler system supply. Ie; don’t check it on the 3rd floor if you plan to attach the sprinklers at the first floor! (It is one of those weird, hard to understand hydraulic laws that as long as the water is not flowing the pressure is the same at any point on a pipe that has the same elevation above sea level.) Double check that all the water so water is turned off and not flowing in the house pipes. Then turn on the valve the gauge is connected to and allow the water to enter the gauge. Read the pressure on the gauge. That’s all there is to it, it’s very easy to do! Turn off the water and disconnect the gauge, you’re done!
OK, I realize I may have confused you, because earlier I told you not to use a hose bib to tap the sprinklers into, and now I just told you that you can use a hose bib to measure the static pressure. This is because you can get an accurate pressure measurement from a hose bib– if the water is not flowing, as described. The small pipe can’t restrict the flow if the water isn’t flowing! Confused? Hydraulics is hard to understand. I may sound crazy but I know what I’m doing! Often users of the tutorial have an “ah ha!” moment when they get about 95% done with their first design and suddenly it all makes sense.
The static water pressure that you were given (or you measured with a gauge) is your Design Pressure. Write down the “Design Pressure” on your Design Data Form!
D. Measure the Maximum Available Flow (GPM)
Flow is the traveling companion of water pressure. Pressure is the “energy” that moves the water through the pipes. Flow is the measure of how much water is moved in a given amount of time. Flow is measured in this tutorial using Gallons per Minute (GPM). Other common units used to measure flow include cubic feet per second (commonly used to measure river flows here in the USA), liters per minute, cubic meters per hour, and many others. Now that you know your Design Pressure you need to determine how much water you can use at a time, or your available flow.
Measure Your Supply Pipe Size
You need to find the water supply pipe and measure it’s size. Grab a piece of string about 6″(152mm) long, then find the location where your water supply pipe enters the house. Strip away any insulation, so you can get at the pipe and wrap the string around it. Measure how many inches of string it takes to go around the pipe once.
The string length is the circumference of the pipe (yikes, bad memories of high school geometry!). Using the circumference we can calculate the diameter of the pipe, which allows us to look up the pipe size, from which we can calculate the flow of water using the formula… zzzzzzzzzz….. Let’s forget all those calculations! Based on the string length use the table below to find your pipe size.
For Copper Pipe & PEX Tube
2.75″ (70mm) = 3/4″ pipe
3.53″ (90mm) = 1″ pipe
4.32″ (110mm) = 1¼” pipe
5.10″ (130mm) = 1½” pipe
For Steel, Brass or PVC Plastic Pipe
3.25″ (83mm) = 3/4″ pipe
4.00″(102mm) = 1″ pipe
5.00″(127mm) = 1¼” pipe
6.00″(152mm) = 1½” pipe
For most PE Tube
2.96-3.33″ (75-85mm) = 3/4″ pipe
3.74-4.24″ (95-108mm) = 1″ pipe
4.90-5.57″ (124-141mm) = 1¼” pipe
5.70-6.28″ (145-160mm) = 1½” pipe
Your string length will vary a little, depending on such unavoidable variables as string stretch, dirt on pipe, manufacturing tolerance, what kind of mood you’re in, etc.
Enter the supply pipe size on your Design Data Form! Also make a note of the type– copper, brass, steel, PVC, PEX, or PE.
Find Your Maximum Available GPM:
Your maximum available GPM is the maximum flow of water you have available for your sprinkler system. Actually, it would be more accurate to call this the Maximum Safe GPM. In most cases it is possible to push a higher flow (GPM) through the pipe. However, at high flows the water actually damages the inside of the pipe.
Use the smallest pipe to determine the Maximum Available Flow. Often the water supply coming into your property will not be a single type and size of pipe. You may have a plastic pipe running underground from the water company to your house. When the pipe enters the house it might switch from plastic to copper pipe, or possibly it might be galvanized steel. Then as the water supply pipe runs through the house it likely branches off in several directions with the pipe becoming smaller and smaller in size as it goes. When determining your Maximum Available GPM you will need to check the Maximum Available Flow for each of the types of pipe that the water will pass through, then use the lowest value as the Maximum Available GPM for your sprinkler design. You only need to be concerned about the pipes the water will pass through before it reaches the point where you are going to tap into it for the irrigation system.
There is an exception to the statement above. Often a short section of a smaller pipe size will be present on the water supply for one reason or another. Maybe the plumber didn’t want to drill a larger hole in the wall for the pipe. As long as this smaller pipe section is less than 5 feet long, you can ignore it and use the larger pipe size to determine maximum flow. The higher flow will be able to squeeze through the smaller pipe. The smaller pipe may wear out faster over time, but typically these short pipes are in places where they are easy to replace. Plus, the smaller pipe is often brass or steel, which has a higher resistance to wear than copper or plastic. You have to make a judgement call on this. In most cases I choose to ignore the small section of pipe.
Small Valves. It is not uncommon to find that a shut-off valve installed on the water supply pipe is a smaller size than the pipe. Don’t worry about it. It will not impact the available flow and valves are constructed to handle higher flows than the pipe.
Example 1: You find the water supply pipe entering the house, examine and measure it, and find that it is 1″ copper pipe. But you’re an ambitious type, so you also have done some digging around in the yard and discovered that the pipe going to the house through the yard is 1 1/4″ PE plastic. It just changes to copper about 6 feet away from the house (this is actually a fairly common situation.) After the copper pipe enters the house it quickly branches off in multiple directions and becomes smaller, but this doesn’t matter to you, because you have already decided that you are going to tap your irrigation system into the 1″ copper pipe right where it enters the house. So the irrigation water will not pass through any of those smaller pipes inside the house and you can ignore them.
Looking at the table you find that 1 1/4″ PE gives a flow of 23 GPM. But looking at 1″ copper pipe in the table shows a flow of only 18 GPM. Since the copper pipe is over 5 feet long you can’t just ignore it. This means you must use the lower 18 GPM value. But wait a minute! What if instead of tapping into the copper pipe, you decide to tap into the PE pipe out in the yard before it switches to copper? Now you can use the higher 23 GPM value because the water will no longer go through the 1″ copper pipe!
Example 2: You found you have a 3/4″ copper pipe that comes into the basement but you have no idea where or what type of pipe is used in the yard. It’s 0 degrees outside, and you couldn’t get a shovel into the frozen ground even if you wanted to, which you don’t! In this case it’s reasonably safe to assume the pipe in the yard is 3/4″ copper also. So you would use 11 GPM from the table.
Example 3: You have no idea where the water pipe enters the house, you have no idea where it is in the yard, and you have no desire to try to find out. In this case you must face reality, it’s time to hire a sprinkler contractor!
Maximum Available GPM Table (Maximum Safe GPM)
Maximum Available GPM (Maximum Safe GPM)
PE (poly) Tube
PEX (CTS) Tube
6 GPM(7 ft/sec)
6 GPM(7 ft/sec)
6 GPM(7 ft/sec)
6 GPM(7 ft/sec)
3 GPM(7 ft/sec*)
11 GPM(7 ft/sec)
11 GPM(7 ft/sec)
11 GPM(7 ft/sec)
11 GPM(7 ft/sec)
7 GPM(7 ft/sec*)
18 GPM(7 ft/sec)
18 GPM(7 ft/sec)
18 GPM(7 ft/sec)
18 GPM(7 ft/sec)
12 GPM(7 ft/sec*)
23 GPM(5 ft/sec)
23 GPM(5 ft/sec)
23 GPM(5 ft/sec)
23 GPM(5 ft/sec)
32 GPM(5 ft/sec)
32 GPM(5 ft/sec)
32 GPM(5 ft/sec)
32 GPM(5 ft/sec)
52 GPM(5 ft/sec)
52 GPM(5 ft/sec)
52 GPM(5 ft/sec)
52 GPM(5 ft/sec)
CTS = Copper tubing size.
Caution: The values in the table above are the maximum safe flows for the given size and type of pipe.
These values are NOT the amount of flow you actually will use for your sprinkler system! Step #2 will show you how to modify these values to reflect your actual flow.
Velocities (ft/sec) are shown for reference only.
* PEX tube has an extremely small inside diameter when compared with the other pipe/tube types, this limits flow. Some manufacturers suggest that PEX will not be damaged by higher flows, up to 10 ft/sec. I don’t feel there is sufficient evidence yet to warrant damaging your plumbing by using flows that are too high, so I am sticking with the old industry standard for plastic pipe of 7 ft/sec maximum velocity. If you wish to take the chance, values at 10 ft/sec are
1/2″=6 GPM, 3/4″=11 GPM and 1″=18 GPM. Use these higher values at your own risk. They could cause serious damage to your both your house plumbing & irrigation piping. Read More on Water Hammer.
A flow test is optional, but suggested if you are not positive about the size or type of water supply pipe. The flow test should be run at a faucet as close as possible to the point you will tap into the water pipe for your irrigation system.
Get a 5-gallon bucket. Old paint buckets work great. Since most 5-gallon buckets actually hold more than 5 gallons of water, you will need to calibrate the bucket as follows: Find an accurate measuring container, and measure out 5 gallons of water into your bucket. Then mark the water level on the side of the bucket with a marking pen so you can easily see it. ?The test is simple. Put the bucket under your water outlet pipe and time how long it takes to fill the bucket to 5 gallons. The formula for calculating the flow in GPM is: 300 divided by the seconds it takes to fill a 5 gallon bucket = GPM.
If the result of the bucket test is lower than the Maximum Available GPM from the table above, something is restricting the flow. It may be the faucet you are using for the test, or there may be a restriction someplace in the house water supply pipe. You can try to find the restriction and get rid of it, or you can simply use the lower flow test GPM for your Initial Design Flow below.
If the result of the bucket test is higher than the Maximum Available GPM you determined in the table above, use the lower value from the table. The Maximum Available GPM Table gives you the maximum SAFE flow. The bucket test is only used to determine if there is an unseen restriction in the water supply pipe that reduces the flow below the level given in the table. Yes, many sprinkler tutorials and sprinkler salespersons may tell you a bucket test should be used for the design flow, they are wrong! In most cases a bucket test like this one gives you an unsafe flow. See the answers to common questions at the bottom of this page for details on why this happens.
Enter your Maximum Available GPM on your Design Data Form.
E. Initial Design Flow
Your Design Flow is the maximum amount of water you will design your sprinkler system to use. For now, use the same number as the Maximum Available GPM, or use the actual Flow Test GPM, whichever is lower.
You will probably need to reduce your Design Flow later, so additional lines are provided for Adjusted Design Flows on the Design Data Form. The initial flows here are very optimistic, 20 to 30% too high for most situations. You will make the adjustment, if needed, later in step #2. Don’t worry about it now. This is just an advanced warning so you won’t be surprised when you need to change the flow later.
Enter your Design Flow on your Design Data Form. Use a pencil so you can change it later!!!!
F. Do you have enough water available?
You are going to need about 20 GPM of water to irrigate 1 acre of grass with sprinklers. One acre is equal to 43,560 square feet (or 4047 square meters). So if you have a 2 acre grass yard you will need to have 40 GPM of water available in order to water it. If you have shrubs, they typically only use 1/2 as much water as grass, so 20 GPM would water 2 acres of shrubs.
There are only so many hours in the day to water. The amount of water needed varies with the climate, these values are typical for hot summer areas where most sprinkler systems are installed (daily high temperatures over 90 degrees F., 32 degrees C.) These values assume you would be willing to water as many as 10 hours per day. If you are willing to water more hours per day you can increase the area irrigated by a similar percentage.
If you don’t have enough water I can suggest a few ideas for you to look into.
Another option is to use drip irrigation for shrub areas. With drip irrigation you only water the area the plant foliage actually covers. Therefore, if the plants only cover half the actual ground area, you only need half the water.
Consider reducing the amount of lawn and replacing it with shrubs. Shrubs use about half the amount of water as lawn.
Another option for getting a higher flow is to install a larger water supply pipe. A description of how to do that is at the bottom of this page.
Why is the flow you measured with a bucket often too high? The GPM rates in the Maximum Available GPM Table above are based on a SAFE water velocity. When you do the bucket test, there are few restrictions on the flow, so the water velocity may easily exceed that safe limit. If you design your sprinkler system to exceed these flows some really bad things can happen. The first of these is called “water hammer”. Water hammer is a pressure surge which declares its presence by destroying the weakest point in your plumbing. The weakest point is usually that little water tube that runs between the shut off valve and the toilet in your bathroom, or possibly the ones that go to the sink faucets. The result is a flooded house, and that’s something you don’t need. Water hammer is exponentially related to water pressure. The higher the water pressure, the greater the water hammer danger. If your water pressure is over 80 PSI, I suggest that you reduce your maximum flow found in the table above by 20% and read carefully the High Pressure Alert below! The other bad thing that happens at high flows is called “scrubbing”. Scrubbing is what happens when the high water velocity actually scrubs molecules loose from the inside of the pipe. Eventually it wears away enough that the pipe develops a leak. The higher the velocity, the more scrubbing you get. A little scrubbing may take 20-30 years to create a leak. With a higher velocity the problem becomes much worse. I have seen 7 year old homes need a total replacement of all the copper pipes due to scrubbing damage. This is extremely expensive to repair! In my 30-year-old neighborhood, most of the homes have now had to replace the water supply pipes to the house due to scrubbing damage caused by sprinkler systems installed back in the bad old days before any of us realized the dangers of high flows. There are still a lot of old tutorials and literature being published that were written before the dangers of high flows were discovered, so be careful when comparing advice on this topic. A lot of industry professionals still haven’t gotten the word on this either!
But, but, but… you didn’t hear any water hammer when you did the bucket test, and nothing broke, so what’s the deal? After all, that higher flow could save some serious money on sprinkler parts! The deal is that you are only human. You can’t close the valve fast enough by hand to create water hammer, but don’t worry, an automatic sprinkler valve can! It can snap that valve closed almost instantly. The higher the water pressure, the faster the valve closes. When that valve snaps closed, it sends a shock wave through the pipe (water hammer). It may take weeks or even years for it to wear down the weak point in your plumbing and break, but it will! Then the cost savings on sprinkler parts will seem trivial. Do it right the first time! Water hammer and scrubbing are insidious and relentless. They just keep working away, little by little, day after day. Clunk, clunk, clunk, chew, chew… until the day you come home to a flooded house.
Clunk, clunk, clunk? Pipe noise!!! I hear those loud noises every time the washing machine or dishwasher runs! Is that water hammer??? You bet it is, and you better do something about it! First, if the water pressure in your house is over 65 PSI, install a pressure regulator to lower the pressure. If that doesn’t get rid of it, go down to your local hardware store and buy a water hammer arrestor. You can get one that screws onto the washing machine or dishwasher fill pipe. They cost about $10-15 and they work pretty well for water hammer caused by appliances. They don’t work nearly as well for water hammer caused by sprinkler systems. This is because many sprinkler systems exceed the maximum water velocities by so much that the arrestor is over-whelmed by it. I’ve written a whole tutorial on this topic: Water Hammer and Air in Pipes.
PVC pipe that is exposed to sunlight should be painted to prevent sunlight from destroying the plastic. But what if you need to repair the PVC pipe later? You can’t glue pipe that has paint on it. In fact, you need to remove all of that paint from the area to be glued, even a small amount may cause leaks in the glued joint.
Using a small brush “paint” acetone onto the area of pipe where you want to remove the paint. (PVC primer will also work as a replacement for acetone, but the purple color makes it hard to make sure all the paint is off.) Allow the acetone to soak in for a few seconds then scrape off the paint with a knife blade or other metal edged tool like a putty knife. If some of the paint doesn’t easily come off re-coat the paint with more acetone. Once most of the paint is off, apply another coat of acetone. This time use a paper towel to wipe off the remaining paint. You may need to wipe down the PVC pipe 2-3 times with fresh paper towels to get a nice clean white surface. Now you can glue your joint using the normal method for gluing PVC pipe as described on the cement/glue can. The acetone will leave the pipe surface feeling sticky, that’s OK, you don’t need to wait for it to dry before you glue the joint. The primary ingredient in the purple PVC primer that is used to clean and prepare PVC pipe for gluing is acetone.
Use caution when working with acetone. Read and follow the warnings on the can. Repeated skin exposure to acetone may be harmful to your health.
Q. How do I calculate sprinkler risers losses in a sprinkler zone where the risers are extra long, 3 ft or more above ground? I have 10 risers in a zone for my proposed sprinkler irrigation system.
A. If you are using my Sprinkler System Design Tutorial and a standard riser of the recommended size, then you don’t need to worry about pressure lose in the riser, the tutorial has friction loss for the risers built-in to the formulas it uses. So you can ignore the riser pressure loss. Some standard risers are shown on the page on Sprinkler Risers in the Irrigation Installation Tutorial. The recommended size for a riser? In most cases it should be the same size as the threaded inlet on the sprinkler. But please actually read that page on risers, as there are some exceptions to that rule for certain types of standard risers!
OK, I realize that didn’t answer your question, you are asking about a non-standard riser that uses a long pipe to hold the sprinkler high above the ground. In that case you must calculate what the friction loss will be in the longer-than-normal riser pipe. (In this case that would be the 3 ft long pipe you described in your question above.) To do that you simply use the same friction loss spreadsheets that you use to calculate the friction loss in any other pipe. Just use this link to get the proper spreadsheet from my website for the type of pipe you are using. Then open the spreadsheet and on the first line enter the pipe size, GPM of the sprinkler you will install on the riser, and the length of the riser. Enter an error factor of 1.4 rather than the default 1.1. This is because even your “longer” riser is shorter than the typical pipe length that the default error factor is based on. Now read the friction loss. That’s it, you have the friction loss for your non-standard riser! Don’t worry about the fittings like ells and couplings that are part of the riser, that is part of what the error factor is compensating for.
When adding the riser friction loss into the total friction loss calculations for your whole sprinkler system, just add in the loss for a single riser. Use the friction loss value for the riser that has the highest friction loss. (This is most likely the one with the highest GPM sprinkler, or it may be the longest riser if you have different riser lengths. You may have to calculate the friction loss for several different risers to figure out which of them has the highest loss.) Why do you add in the friction loss for only one sprinkler, rather than the combined loss for all of them? Because as a single drop of water goes through the sprinkler system it only goes through one sprinkler, not all of the sprinklers. You have to think about the water as a collection of millions of drops, not as one solid body. So the pressure loss is what a single drop would experience as it travels through the system. As a drop of water enters the sprinkler system it travels through a water meter, lots of pipe, a valve or two, then it finally blows out through a single sprinkler onto the landscape. The pressure loss calculation for the whole sprinkler system is determined by what the worst case pressure loss values would be for a single drop of water traveling through the sprinkler system.
OK, so you calculated the friction loss, but what if it is a really high value, or maybe the calculator complained about the velocity being to high. In this case you need to use a larger size pipe for your riser. For the velocity in a riser you can go all the way up to the 7 ft/sec maximum without too much risk. Velocities in the marginal “use caution” zone are generally OK for risers. High velocity in a riser will seldom cause a water hammer problem, unless you are using a special type of sprinkler that has a solenoid valve built in to it. Those sprinklers are called “valve-in-head sprinklers”, they are very expensive, and are mostly used for golf course greens.
PVC pipe types labeled “schedule” (abbreviated “SCH“) are made based on the traditional dimensions used for steel pipe. Unfortunately steel has very different strength characteristics from plastic, so it is a system that isn’t very logical for use with PVC pipe. But when plastic first came along it was made to the same size standards that were already in use for steel. The common PVC pipe schedules you will see in stores are SCH 40 and SCH 80. As the pipe sizes rated SCH increase, the strength and pressure rating of SCH pipe decreases. So 1/2″ SCH 40 PVC pipe is very strong, while 2″ SCH 40 PVC has comparatively a low pressure rating, and is more easily damaged. In sizes 1/2″ to 1 1/2” SCH 40 is a thick wall pipe with a reasonably high pressure rating and good resistance to physical damage. It is often used for mainlines and other situations where a tough high pressure pipe is needed. Sch 80 is generally used for making threaded plastic nipples because the plastic walls are thick enough to have threads cut into them (although most now have molded threads rather than threads “cut” with a die.)
As you can see, the pressure ratings drop as the pipe size increases. Note that the industry standard rule is that your normal operating pressure should not exceed 1/2 of the rated pipe pressure. In other words, you shouldn’t use 1 1/2″ pipe for pressures higher than 165 PSI (330 x 0.5 = 165 PSI). This is because pressure surges created by closing valves can easily double the water pressure in the pipe. This rule applies to all PVC pipe, including that labeled SCH and CL.
Class rated pipe
PVC pipe types labeled “Class” (abbreviated “CL“) are based on the pipe’s pressure rating. So Cl 200 PVC pipe is rated for 200 PSI of water pressure. Cl 315 PVC pipe is rated for 315 PSI of water pressure. The strength of CL labeled pipe is directly related to the pressure rating. The standard “Cl” pipes are Cl 125, Cl 160, Cl 200 and Cl 315. Of these Cl 200 and Cl 315 are most common. Cl 125 is sold as a low cost pipe for use in sprinkler laterals for those for whom low price is everything. It has a very thin wall and breaks easily if not handled carefully or nicked with a digging tool.
1/2″ size pipe is generally only available in SCH 40. This is because of the thin wall of 1/2″ pipe makes it very easy to break. I don’t recommend using 1/2″ PVC pipe at all, however if you must, you should use SCH 40. Sometimes you will find 1/2″ Cl 125 PVC pipe at discount stores due to the very low price.
The Class system is obviously a more logical system for labeling pipe as you know immediately how strong the pipe is based on the label. Unfortunately the more confusing “SCH” system became entrenched in the industry and remains.
What Pipe Type to Use
All PVC pipe labeled for a given size in the USA has the same outside diameter. So any pipe labeled as 3/4″ will be the same diameter, whether it is SCH 40 or Cl 200 or any other type. That allows the same fittings to be used to join the various pipe types together. Most fittings are made to SCH 40 standards, although SCH 80 fittings are available, typically only at specialty plumbing and irrigation stores. Technically most codes require SCH 80 fittings for pipe sizes 2″ or over. In practice I’ve noticed that SCH 40 fittings are often used up to 3″ size. When dealing with sizes 4″ and above the use of non-glued “rubber ring-joint” fittings is recommended and usually required by code as well. Glueing joints on 3″ and larger PVC pipe is very, very difficult.
“Mainlines” are all of the pipes that are under constant pressure, that is, the pipes that are before the sprinkler zone valves. In most of the industry SCH 40 PVC pipe is used for irrigation mainlines up to 1 1/2″ size. For 2″ size and larger Cl 315 PVC is used. Most building codes prohibit the use of 2″ and larger SCH 40 PVC pipe for pressurized water lines. Depending on the jurisdiction, this rule may or may not be applied to irrigation systems. Those same codes generally require that all pressurized PVC pipes (mainlines) be buried at least 18″ deep to protect them from accidental damage, regardless of the type or size of pipe used.
“Lateral” pipes are the pipes after the sprinkler zone valve. These pipes are only pressurized when the sprinklers are operating. For lateral pipes the standard is to use Cl 200 PVC pipe. Where budget is a concern and you can find it, sometimes Cl 160 is used. As previously mentioned I recommend you avoid Cl 125 PVC pipe. Laterals can be buried any depth, but I generally recommend at least 10″ deep to avoid a lot of maintenance problems with broken pipes.
Well, if you have been reading my tutorials, I guess it’s pretty obvious that I’m not recommending you use 1/2″ pipe. I have a number of reasons for this.
1/2″ PVC pipe is generally not available in most areas except in SCH 40 type.
The water capacity of 1/2″ pipe or tube is very low.
1/2″ PVC pipe is hard to glue together without using too much glue. The glue piles up on the inside of the pipe when you insert it into the fitting and blocks the water flow. Too much glue also weakens the wall of the pipe and a leak develops after a few years.
Using 1/2″ pipe means you have to have another size of spare pipe and fittings on hand for repairs.
But the biggest reason is that 1/2″ pipe leaves no flexibility for future changes or additions to your sprinkler system. If you ever need to add another sprinkler to the pipe you’re screwed.
My conclusion: The small amount of money saved by using 1/2″ pipe just isn’t worth the hassle and risk.
As water moves through a pipe it loses pressure due to a phenomenon commonly called “friction loss”. Much of this loss is caused by turbulence, but we call it friction loss for simplicity. The amount of friction loss is determined by the type of pipe, the diameter of the pipe, the amount of water flowing through the pipe, and the length of the pipe. A complex formula (called the Williams/Hazen Formula) predicts the amount of pressure that will be lost due to friction loss. The water also loses pressure each time it passes through a valve, a backflow preventer, or anything else it encounters on it’s way to the sprinkler head. Even a bend in the pipe causes pressure loss! Don’t panic over the formula, we’ll use a pipe sizing chart or a friction loss calculator!
You need lots of pressure at those sprinkler heads!
The sprinkler head needs a minimum amount of water pressure to work properly. The manufacturer’s performance charts tell you how much pressure is required to achieve a specific radius for the water. As the pressure increases so does the flow (GPM) and the radius of the throw. So in order to assure that there is enough pressure to make the sprinklers operate as they should, we need to calculate the pressure losses between the water source and the sprinkler head. If the pressure loss is found to be too great, then we must reduce. The easiest way to do that is to use a larger size pipe.
DETERMINING THE SPRINKLER PIPE SIZE
There are several methods used to determine pipe sizes of sprinkler system lateral pipes. I’m going to explain two methods. One method is faster but less accurate, the other is very accurate but takes more time.
Pros: The fastest and easiest method. Requires a single, simple calculation and uses a chart to determine the sizes.
Cons: The learning curve to use it is a bit more difficult to understand. It uses an averaging system to arrive at pipe sizes.
See step-by-step tutorial for the Chart Method
Trial & Error:
Pros: Very accurate, calculates the pressure loss in each pipe section using a spreadsheet. Easier to understand.
Cons: Time consuming, need to enter data into the spreadsheet, uses trial and error to establish pipe sizes.
See step-by-step tutorial for the Trial & Error method
Which method should you use? For a beginner with a small irrigation system probably the Trial & Error system will be easier. Below are overviews of each method for experienced designers to use. Unless you are experienced you should probably read the full tutorial for the method you select.
Overview: CHART METHOD FOR LATERAL IRRIGATION PIPE SIZING
( ____ PSI x 100) / ____ Feet Total Length = PSI/100
____ PSI. Insert the maximum PSI loss for the valve circuit laterals into the formula where it says “____PSI.”
____ Feet. Insert the distance from the valve to the farthest sprinkler on the valve circuit in the space labeled “____ Feet Total Length” in the formula.
Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure.
The Pipe Size Table or Chart:
Sprinkler Pipe Sizing Chart for Laterals
PSI/100 = Desired PSI Loss in Lateral x 100 / Total length of Lateral
Flows shown red are over 5 feet/second. Use caution!
Find your PSI/100 value in the top blue row.
Read down the column to the value equal to, or higher than, the GPM in the pipe section.
Read across to the pipe size for that section in the right column.
Repeat steps 2 & 3 for next pipe section.
This table uses an averaging formula based on the assumption that all flows for any given size of pipe will not be at the maximum GPM for that size of pipe. In rare cases the PSI loss for the entire lateral may exceed the desired loss by up to 10%. This table assumes the use of Cl 200 PVC pipe, adjustments to the pipe sizes are required for other pipe types, such as poly or SCH 40 PVC.
Pipe Sizing Chart, Copyright 1979, Jess Stryker, All rights reserved.
Notes about the Pipe Sizing Chart:
Warning: The sprinkler pipe sizing chart is based on using Cl 200 PVC pipe. It also works for Class 125 (not recommended) and Class 160 (hard to find).
Schedule 40 PVC: If you plan to use Schedule 40 PVC pipe (“SCH 40”) for the laterals you need to make an adjustment before using the chart. Reduce the PSI/100 value you just calculated for the valve circuits to 1/2 the original values.
Polyethylene, Polybutylene: After you obtain your pipe size from the chart you need to increase it by one size to get the proper size for poly pipe. In other words, if the chart says ¾” PVC pipe, then you should use 1″ poly pipe. 1″ would become 1¼”, 1¼” becomes 1½”, 1½” becomes 2″, etc.
Go to the next line down and repeat steps 4-7 for the next pipe section.
The spreadsheet calculator will tell you the velocity and PSI Loss for each pipe section.
At the bottom of the calculator it will tell you the pressure loss total of all sections combined.
Change the pipe size if the velocity or total pressure loss is too high.
You must calculate the pressure loss for each of the possible water paths in the valve circuit.
Here is an example of the possible water paths for a valve circuit, shown in red, blue, and magenta.
Start with the water route that is the longest. In this case that would be the red route. There are 9 pipe sections in this route labeled 1-9. Enter the data from this route into the calculator. Use a larger pipe size if the velocity is not safe. Check that the friction loss “Total of All Sections” does not exceed your maximum allowable amount.
Write the pipe size for each section on your plan.
Now repeat the process for the blue water route and then the magenta color route.
It will not harm anything to use a larger pipe size. Period. If you are uncertain whether to use a 3/4″ or 1″ pipe, then you should use the 1″. Using a larger size pipe is ALWAYS the safest choice.
No, I don’t own stock in an irrigation pipe manufacturer and I’m not getting kickbacks for pushing bigger pipe! Unlike clothing, pipe can never be “too large”. Contrary to what might appear to be true, forcing water into a smaller pipe REDUCES the water pressure, and hurts sprinkler performance. This is because the smaller pipe creates more pressure loss due to friction and turbulence as the water flows through it. It’s another of those hard to grasp hydraulic principles! Just remember that when it comes to pipe, bigger is better! I’m always amazed at how many irrigation equipment sales people don’t know this most basic of irrigation rules. I’ve had clients tell me they were told to use a smaller pipe to keep the pressure up by tech support people at some of the major sprinkler manufacturer’s. That’s an industry disgrace!
So one more time to drill it into your head– You don’t decrease the pipe size to keep the pressure up- or down for that matter. That is totally, completely, wrong. The reason we use smaller pipe is to save money. Which of course, is a good reason! For those who want more specifics on this, there is a very boring scientific explanation at the bottom of this page.
Is it Pipe or Tube? For the most part I use the term “pipe” rather than “tube” on this page and elsewhere. Bad habit of mine (note that by reading carefully, you have found one of my faults!) The difference is the material they are made from. Steel and PVC plastic are generally called pipe. Polyethylene, PEX, and copper are usually referred to as tube. I often screw up and call tube pipe! 🙂
TRIAL & ERROR METHOD TO DETERMINE LATERAL PIPE SIZE USING A SPREADSHEET
This method involves trying various pipe sizes until a good combination is found.
Definitions you need to know:
Lateral pipe: all the pipes between the control valve and the sprinkler heads.
Mainline: The pipe that goes from the water source to the control valves.
Control Valve: The valve that turns on and off a group of sprinklers. Most often it is an electric valve operated by a timer.
Valve circuit: a single valve, and all the pipe, fittings and sprinkler heads downstream from it. In other words, all the sprinkler heads that start working when you turn on the valve are part of the same valve circuit.
GPM: Gallons per minute, a measure of water flow rate. Use primarily in the United States.
PSI: Pounds per square inch, a measure of water pressure. Use primarily in the United States.
You will need a spreadsheet Friction Loss Calculator.
Here’s a page with calculators for almost every type of pipe: Friction Loss Calculator Spreadsheets
Grab the appropriate spreadsheet for the type pipe you plan to use.
Now you just enter the appropriate data for each section of pipe into the calculator and then read the total pressure loss at the bottom of the spreadsheet. If the pressure loss is too high, then try making one of the lengths of pipe larger. The calculator will also give you the water velocity in each section of pipe, and warn you if the velocity is too high.
The best way to teach this is probably to walk you through a couple of examples.
If I may make a suggestion, download the spreadsheet for Cl200 PVC now, and open it up in a separate window. Then think about each step, enter the values I show into the spreadsheet, and actually try to duplicate what I do in the examples below. Something about actually doing this helps engage people’s brains. People tell me they read it twice and still don’t get it when they just read it, but as soon as they actually TRY it, then it suddenly makes sense. It’s called learning by doing, and it is considered the best teaching method. This process is simple, HOWEVER, it is not obvious and sounds illogical to those not trained in hydraulics.
A Simple Example:
The sketch above is an example of a very simple valve circuit with 5 sprinkler heads. In this example the sprinklers are 15′ apart and each sprinkler uses 3.7 GPM of water. The red numbers on the sketch are the total water flow for each pipe section in GPM.
Let’s assume we want to use Cl200 PVC pipe and we want a maximum total of 4 PSI of pressure loss in our lateral pipes.
If you are working through the Sprinkler Design Tutorial the maximum total pressure loss is entered on your Design Data Form in the Pressure Loss Table section. There you will see a figure you entered called “_____ PSI – Laterals”. That is the maximum PSI loss for the laterals, use that number here. If in doubt, 3 PSI is a reasonably safe value for most sprinkler systems.
If you don’t understand how to calculate the water flow in each section (the red numbers) you should take a look at the Sprinkler Pipe Layout page.
Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure. Example: 20 PSI sprinkler operating pressure. 20 x 0.20 = 4 PSI maximum pressure loss in circuit laterals.
If you don’t understand pressure losses in irrigation, see Pressure Loss & Selecting Your Sprinkler Equipment.
For advice on types of pipe (Cl200, poly, etc.) see Irrigation System Lateral Pipes.
To use the spreadsheet friction loss calculator to determine the pressure loss:
Download and open the Friction Loss Calculator.
There is a line on the spreadsheet for each section of pipe. So for this example you will enter data for 5 pipe sections.
Start with the pipe section closest to the valve as section #1, and work out to the farthest sprinkler head.
Start by selecting 3/4″ pipe for the pipe or tube size for all the sections. (See “why not 1/2″?”)
Enter the GPM for the section of pipe.
Enter the length of the section of pipe.
Use an error factor of 1.1
Go to the next line down and repeat steps 4-7 for the next pipe section.
The spreadsheet calculator will tell you the velocity and PSI Loss for each pipe section.
At the bottom of the calculator it will tell you the pressure loss total of all sections combined.
Here’s what the spreadsheet calculator looks like after we enter the data requested for each of the pipe sections using the example in the sketch above.
Note that the “Total of all Sections” shown at the bottom exceeds the 4 PSI maximum limit we set for pressure loss. Also notice that the velocity in two of the sections (highlighted in red) exceeds the safe level. The marginally high velocity highlighted in yellow is considered acceptable by most experts, since these are lateral pipes. (The marginal velocity level would not be as acceptable in mainlines.) Start by fixing the velocity problems. To decrease the velocity in those sections we will need to increase the pipe size. So let’s increase the pipe size for the two sections highlighted with red to 1″. Here’s what it looks like after the change:
Now the velocities are all within acceptable levels. Also note that increasing the pipe sizes reduced the pressure loss “Total of All Sections” shown at the bottom to 2.9 PSI, which is well below our maximum level of 4 PSI. That’s good, no more changes are needed. It is not possible for the pressure loss to be “too low.” As long as it is under the maximum it is fantastic. So what would happen if the pressure loss was still too high? If there was still too much pressure loss we would need to try increasing the size of some of the pipes to lower the friction loss.
So we now have pipe sizes that will work for each section of pipe in our lateral. I’m often asked at this point if it would be OK to make some of the pipes 1/2″ since the pressure loss is so low? The answer is yes, but you might not want to do it. See my explanation of the problems associated with the use of 1/2″ pipe.
A More Complex Example:
Now lets look at a more complex valve circuit. (Please note that this circuit is much larger than that found on a typical residential irrigation system. It would require much more water than most residences have available and is just used to show you an example of a much more complex layout.) As with the previous example we will assume that our maximum pressure loss value for the valve circuit is 4 PSI.
This valve circuit involves numerous paths the water may take. This makes the calculation a bit more complex, as a separate calculation is needed for each possible route that the water might take through the laterals on it’s way to the last sprinkler at the end of a pipe. If you look at the example above you will notice there are 3 sprinklers that are at the end of pipes, each sprinkler at the end of a pipe represents a different route the water can take. So this circuit has 3 and will therefore require 3 separate pressure loss calculations. The next drawing shows the possible water routes in magenta, blue, and red colors.
It may help to think of each path as the shortest route that a single drop of water could take to go from the valve to the last sprinkler on a pipe branch. For some people it helps to think of it as a road map and your looking for the shortest route to each of the dead ends at the end of the roads.
Start your calculations with the water route that is the longest. In this case that would be the route highlighted in red. There are 9 pipe sections in this route, I have labeled them 1-9 for clarity. Just as before, enter the data from this route into the calculator, and make all the pipe 3/4″ size. Here’s what the spreadsheet looks like:
As you can see there are a number of pipe sections highlighted red due to unsafe velocity. Change those pipe sections to larger pipe sizes until all the velocities are within safe levels.
Here’s the resulting spreadsheet calculator with the smallest possible pipe sizes. However, notice the Total of All Sections is 4.4 PSI, which is more than our 4 PSI maximum:
So we need to make some of the pipe sections larger in order to reduce the pressure loss (or friction loss.) Start by increasing the size of one of the the smaller pipe sections. Changing a 3/4″ pipe to a 1″ size is a lot less expensive than changing a 1″ pipe to 1 1/4″. So for the example lets change section 7 from 3/4″ to 1″. Doing that drops the Total of all Sections value to 3.77 in our example, below the 4 PSI maximum we set earlier. So now everything is good, these sizes will work for the “red” highlighted water route.
Now we add the pipe sizes from the spreadsheet to our circuit drawing (note that the pipe sizes for the red highlighted sections have been added on the next drawing below.)
Now we relabel our sections to follow the blue highlighted route.
Using the blue highlighted water route, repeat the same process used for the red one. Enter the GPM and pipe lengths for each section in the spreadsheet. This time we already know the sizes for sections 1-6, they were entered into the spreadsheet when we did the red section. So we just enter those for the new blue sections 7 and 8, again using 3/4″ size pipe. And it looks like this on the spreadsheet calculator:
Using 3/4″ pipe size for our two new sections works good. The velocity is safe and the Total of All Sections is 3.29 PSI, so the pressure loss for this route is also within the 4 PSI maximum we set. Write the size of the two new sections on the drawing and the blue water route is done.
Now all that remains is to do are the calculations for the magenta highlighted water route. That is done the same way, entering the data into the spreadsheet calculator for each pipe section. Start with 3/4″ then change to larger sizes until the velocity is safe. Then check that the total of all Sections is less than 4 PSI as before. Here’s the data entered into the spreadsheet calculator:
Now all that remains is to insert our lateral pipe sizes from the spreadsheet calculators into the drawing of the valve circuit.
All done! So the pressure loss for the entire circuit is the same as that for the highest water route. In this case the red route was highest at 3.77 PSI. So the pressure loss for the lateral circuit shown here is 3.77 PSI.
Often I get asked at this point why the “Total of all Sections” pressure losses for all 3 routes wasn’t added together? The pressure loss for the red route was 3.77 PSI, for the blue section it was 3.29 PSI, and for the magenta route it was 3.02 PSI. So the confusion is that it seems like there should be a total loss of 10.08 PSI! Nope, the pressure loss for the entire lateral is 3.77 PSI, the loss of the highest route. To understand this think of a single drop of water again. It can only travel on one route from the valve to the farthest sprinkler. It is not going to go backwards and try another route! So the pressure loss for the entire valve circuit is equal to the pressure loss from the valve to the farthest sprinkler.
This method is based on the assumption that you are using Cl 200 PVC pipe for the lateral pipes. With minor adjustments this method will also work reasonably well for SCH 40 PVC pipe or polyethylene irrigation tube. For other types of pipe or tube you will need to use the Trial & Error method to determine the pipe sizes.
While the Pipe Sizing Chart method described here seems rather complex when you read it the first time, it is actually extremely fast and easy once you figure it out. You will start with a simple calculation to obtain a “PSI/100” value. Then you will use that value in the Pipe Sizing Chart to figure out the maximum flow for various sizes of pipe. You will only do this once for each sprinkler system. Once you have that schedule you will fly through inserting pipe sizes into your plan. Most designers who “design in their heads” are using this method or a close variation of it. It is the method I use when designing my systems.
Lateral pipe: The pipes between the control valve and the sprinkler heads are called “laterals”.
Mainline: The pipes that go from the water source to the control valves are called “mainlines”.
Control Valve: The control valve is the valve used to turn on and off a group of sprinklers. Often it is an electric solenoid valve operated by a timer.
Valve circuit: A valve circuit consists of a single control valve, and all the fittings, pipes, and sprinkler heads that it turns on.
GPM: Gallons per minute, a measure of water flow rate. Use primarily in the United States.
PSI: Pounds per square inch, a measure of water pressure. Use primarily in the United States.
BASIC RULES TO KEEP IN MIND
When in doubt, always use a larger diameter pipe!
You may always use a larger size pipe. No, I don’t own stock in a irrigation pipe manufacturer. But using a larger size of pipe will not cause any harm to how well your sprinkler system works. Using a larger pipe will NOT noticeably reduce the water pressure. (Yes, I did condition that statement with a “noticeably”.) The only damage done by using a larger size of pipe is to your pocketbook. Larger pipe generally costs more. But from a irrigation system performance perspective you will NEVER hurt anything by using a larger size pipe. Now I realize that somewhere out there, somewhere will tell you this is not true. They are going to tell you that you need a smaller pipe to squeeze the water and create more pressure. They are totally wrong of course, but as you read this you are probably uncertain who is right, since they will claim I am wrong! Ask them to provide you with a scientific, documented explanation of why they are right. I will also provide both a basic and a very scientific explanation with references for you. Here’s mine: Using A Smaller Pipe to Increase Water Pressure. OK, sorry, I’ll climb down off my soapbox now.
Is it Pipe or Tube? I tend to call everything pipe. Habit, since here in La La Land (Los Angeles, California) we use mostly PVC pipe for irrigation. However some types of “pipe” are technically defined as “tube”. The difference is the material they are constructed of. Steel and PVC plastic are generally called pipe. Polyethylene, PEX, and copper are usually called tube or tubing. If I say pipe where I should say tube, please accept my apologies.
CALCULATING THE PSI/100 VALUE:
The PSI/100 value is a value used in the Pipe Sizing Chart (we’ll get to the chart in a moment.) The PSI/100 value determines which column of the chart you will use when finding the pipe sizes. A simple calculation will give you the PSI/100 value.
The PSI/100 formula:
( ____ PSI x 100) / ____ Feet Total Length = PSI/100
For those who prefer variables, this is the same formula written using variables: (LPSI * 100) / FTL = PSI/100
Here are the values to insert in the blank spots (“____” ), or variables, in the formula:
____ PSI. (LPSI) Insert the maximum PSI loss for all laterals on the valve circuit into the formula where it says “____PSI.” .
If you are working through the Sprinkler Design Tutorial look on your Design Data Form for the Pressure Loss Table. There you will see a figure you entered called “_____ PSI – Laterals”. That is the maximum PSI loss for the laterals, use that number here. If in doubt, 3 PSI is a reasonably safe value for most sprinkler systems. If you don’t understand pressure losses in irrigation, see the Pressure Loss & Selecting Your Sprinkler Equipment and Lateral Pressure Loss pages. Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure. Example: 20 PSI sprinkler operating pressure. 20 x 0.20 = 4 PSI maximum pressure loss in circuit laterals.
____ Feet Total Length. (FTL) Insert the distance from the control valve to the farthest sprinkler in the space labeled “____ Feet Total Length” in the formula.
For this value you need to figure out the total length of pipe (in feet) that the water needs to travel through in order to get from the valve to the farthest sprinkler. Measure only the pipe sections that the water would pass through on the way from the control valve to that farthest sprinkler. Don’t add in the length of any side spurs going off to other heads that aren’t on the longest route. In the example below, the route from the control valve to the farthest sprinkler that you would measure the distance of is shown in red. Totaling each of the pipe sections along that route results in 118′. So 118 feet would be the ___ feet value you would use in the PSI/100 formula.
Example of A Typical Valve Circuit
Now use the PSI/100 formula above to calculate the PSI/100 value. ( ____ PSI x 100) / ____ Feet Total Length = PSI/100
Write down the PSI/100 value. ____________
Example: Let’s say the value “____ PSI – Laterals” is 4 PSI. Let’s also assume that the total length of the lateral as measured above is 118 feet. Those values inserted in the formula would look like this: (4 PSI x 100) / 118 feet Now do the math. 4 times 100 = 400. Then 400 divided by 118 = 3.389 Round that number to 3.4. Therefore when using this example your PSI/100 value to use in the Pipe Sizing Chart would be 3.4 PSI/100 .
You can repeat this procedure for each valve circuit. But the usual method is–
It is possible to use the same PSI/100 value for all the valve circuits. That’s how most professionals (myself included) do it. The only catch is that you must use the “worst case” PSI/100 value. In other words you need to figure out which of the valve circuits on your entire sprinkler system has the longest “Feet Total Length” between the valve and last sprinkler. Then use that valve circuit to calculate your worst case PSI/100 for the entire sprinkler system. The advantage of using the same PSI/100 value for everything is uniformity of design and, obviously, doing only one PSI/100 calculation for the entire sprinkler system saves time. For example, a pipe with five half circle spray heads downstream would always be the same size pipe. This is much less confusing for the installer, which is the main reason we do it this way.
Pipe Sections and GPM:
Each section of lateral pipe may be a different size. For example, the first section of pipe leading away from the valve might be 1 1/4″. The next two sections might be 1″, and the rest of the sections might be 3/4″. The pipe size to each section is based on the actual GPM flow passing through that section of pipe, so you will need to know what the GPM flow is for each section. If you have been working through the Sprinkler Design Tutorial you have already figured this out and written these GPM values down on your plan in an earlier step. If not, you will need to take a few minutes to do this now. See the page on Sprinkler Pipe Layout for instructions on figuring out the GPM for each pipe section.
THE PIPE SIZING TABLE or CHART:
before you use the chart…
Warning: The sprinkler pipe sizing table /chart is based on using Cl 200 PVC pipe. For other pipe types you will need to make an adjustment if you want to use the chart.
Schedule 40 PVC: If you plan to use Schedule 40 PVC pipe (“SCH 40”) for the laterals you need to make an adjustment before using the chart below, because SCH 40 PVC pipe has a much less water capacity than other PVC pipes. Reduce the PSI/100 value you just calculated for the valve circuits to 1/2 the original values.
Example for SCH 40 PVC pipe: In the example above you calculated a value of 3.4 PSI/100. But you have decided to use SCH 40 PVC pipe for the laterals, rather than Cl 200 PVC pipe. So you will need to reduce the PSI/100 value by half. 3.4 x 0.5 = 1.7 PSI/100. So your new value is 1.7 PSI/100. As you will see, this will result in much larger lateral pipes! This is why most people do not use SCH 40 PVC for laterals, and why I recommend you use Class 200 PVC. It makes a big difference in cost!
Class 125, Class 160, or Class 200 PVC pipe: The chart below is based on the use of Class 200 PVC pipe. It also works for Class 125 (not recommended) and Class 160 (hard to find).
Class 100 and 315 PVC pipe: As a general rule, these types of PVC pipe are not used for laterals.
Polyethylene, Polybutylene: Use the chart below. Then, after you obtain your pipe size from the chart you need to increase it by one size to get the proper size for poly pipe. In other words, if the chart says ¾” PVC pipe, then you should use 1″ poly pipe. 1″ would become 1¼”, 1¼” becomes 1½”, 1½” becomes 2″, etc. Note: PEX pipe is not the same thing as polyethylene irrigation pipe.
To use the chart you will use the PSI/100 value you calculated along with the GPM flow in the pipe section.
Sprinkler Pipe Sizing Chart for Laterals
PSI/100 (round down)
Flows shown in red are over 5 feet/second.
Sprinkler Pipe Sizing Chart, Copyright 1979, Jess Stryker, All rights reserved.
Permission is granted for reuse for any purpose and in any media, provided the copyright notice is maintained.
Sprinkler Pipe Sizing Table /Chart Instructions:
Start with the pipe section farthest from the valve (connecting to the last sprinkler head.)
Find the PSI/100 value in the top row (blue text, directly under the heading PSI/100.)
Read down that column and find a value equal to, or higher than, the GPM in the pipe section.
Now read across to the right column to find the pipe size to use for the pipe section.
Repeat steps 3-5 for the other pipe sections in the lateral valve circuit.
Flows over 5 ft/second are considered marginal (shown in red on chart.) Most experts believe that flows up to 7 ft/sec are acceptable for laterals. However flows over 7 ft/sec velocity are not considered safe, so they are not shown on the chart.
This table uses an averaging formula based on the assumption that all flows for any given size of pipe will not be at the maximum GPM for that size of pipe. In rare cases the PSI loss for the entire lateral may exceed the desired loss by up to 10%.
This table assumes the use of Cl 200 PVC pipe, adjustments to the pipe sizes are required for other pipe types, such as poly or SCH 40 PVC.
In the example above the flows for each pipe section are noted in gray text with an arrow pointing at the pipe section. The red pipe circuit has the longest distance between the control valve and the farthest sprinkler head. So for our example let’s use the red pipe circuit.
First we need to calculate the PSI/100 value.
We start with the maximum pressure loss we want in our lateral pipes. For this example we will use 4 PSI.
Now we measure the total pipe distance from the valve to the farthest head. I showed this route using a bold red line. It is 96 feet from the control valve to the farthest head when following this bold red route.
Now the PSI/100 formula with the values from this example inserted: ( _4_ PSI x 100) / _96_ Feet Total Length = _4.2_ PSI/100
Now we start using the chart to find the pipe sizes.
Our PSI/100 value is 4.2, so we look on the chart. Rounding down we see that 4.0 is the closest PSI/100 value on the chart, so we use the 4.0 column.
Now read down the 4.0 column. The numbers will tell us the maximum flow for each pipe size.
So the first number we see is 10. That would mean 10 GPM. Reading across to the right we see that 10 GPM is the maximum flow for 3/4″ size pipe.
Continuing in the 4.0 column, the next number is 20. Again we read across and see that 20 GPM will be the maximum flow for 1″ pipe.
Reading down one more line we see that 36 GPM is the maximum flow for 1 1/4″ pipe. And we can continue this on down the chart.
So now we can create a simple pipe size schedule to use for our plan, based on the values we took from the pipe sizing chart:
Up to 10 GPM = 3/4″ size pipe
Up to 20 GPM = 1″ size pipe
Up to 36 GPM = 1 1/4″ size pipe
Up to 48 GPM = 1 1/2″ size pipe
Up to 76 GPM = 2″ size pipe
Now go back and look at the flow for each section of pipe on your plan. Then based on the GPM flow, insert the pipe size from the schedule you made.
So the section with a flow of 2.5 GPM will be 3/4″ pipe.
The section with 1.3 GPM will also be 3/4″ pipe.
The section with 3.8 GPM will be 3/4″ pipe.
The section with 6.4 GPM will be 3/4″ pipe.
The section with 1.3 GPM will be 3/4″ pipe.
The section with 2.6 GPM will be 3/4″ pipe.
The section with 9.0 GPM will be 3/4″ pipe.
The section with 11.5 GPM will be 1″ pipe.
I’ve inserted these pipe sizes on the example sketch above.
See how fast and easy that is? Once you have the initial PSI/100 calculations done you can use the pipe sizing chart to create a custom pipe schedule for your plan. Then it is really fast to simply look at the flow in a pipe section, look it up on the schedule, and write in the pipe size! You can see why pros use this method, it allows them to fly through a large design with hundreds of sprinklers.
COMMON PROBLEMS AND QUESTIONS REGARDING USING THE PIPE SIZING CHART
Is your PSI/100 value off the chart? If your PSI/100 value is 6.0 or higher you should use the 6.0 column. At 6.0 you have reached the maximum safe capacity of the pipe sizes used on the chart.
Is the pipe size larger than the valve size? It is fairly normal for the first pipe after the valve to be one size larger than the valve. So you may have a 1″ mainline going into a 3/4″ valve and then have a 1″ lateral pipe coming out of the 3/4″ valve. This is very common, and is not a problem at all. So don’t worry if the pipe size you get from the chart is larger – or smaller – than the valve size.
Write the pipe size down next to the pipe on your plan. Repeat for each pipe section. Repeat for each valve circuit.
Here are some spreadsheets I have created to help you calculate the capacity and water pressure loss through pipes and tubes of various types and sizes. These should be useful for both figuring pressure loss in mainlines and laterals. Each spreadsheet allows for multiple sections of pipe of various sizes and flows. All you do is select the proper spreadsheet for the type of pipe you are going to use, select the pipe size from a drop down list, enter the flow through the pipe in GPM, then enter the length of the pipe in feet. The spreadsheet calculator will then do the math to give you the water velocity in the pipe along with the pressure loss in PSI for that section of pipe. If there are multiple sections of pipe the spreadsheet will also total all of them for the total pressure loss.
Full instructions for using the spreadsheets are included on the spreadsheets.
Please read this paragraph before you try to use the spreadsheets.
These spreadsheets use Apache Open Office. That means you need to have the Open Office program installed on your computer for them to work. I use Open Office because it is a free, safe program that is available for just about every desktop and laptop computer make and model. That means just about everyone can use it, and nobody has to make a major software investment just to use the spreadsheets. If you don’t already have Open Office you will need to install it on your computer before you can use the spreadsheets. You can uninstall it when you are done using it if you want. Download it free from http://www.openoffice.org . It does take a while to download Open Office. It is a full office productivity suite. (Check it out, there are some cool apps in it!)
Your browser may try to “open” the spreadsheets if you left click on the links, even if you don’t have Open Office installed. This is because most computers have some type of minimal spreadsheet reader installed on them, so the reader will try to open these spreadsheets. If they were just simple spreadsheets they would probably work with the readers. But they aren’t. In most cases you will get a corrupted version of the spreadsheet that does not work. This is because these friction loss calculators use very complex formulas that the “stripped down” spreadsheet readers can’t handle. You need to install Open Office. I may have already mentioned this. 🙂
Remember, you can un-install Open Office when you are done if you don’t like it.
If you try to open the spreadsheets directly using your Internet browser they will probably open as Read Only and the spreadsheet won’t work. This is because the browser will not open an executable file directly. (It is trying to protect you from possible viruses.)
Solution: Right click on the link and “Save” the file to your hard drive. The actual wording varies, so depending on your browser you may select “Save link as..”, “Save Target as…”, etc. This should save the spreadsheet file to your hard drive. Then open it directly from your computer without using the browser plug-in. Tablet and phone users: you may need to get to a real computer to use the spreadsheets.
If the spreadsheets don’t work for you..
Do you have Open Office installed? If not, install it.
Are you using Open Office to read the spreadsheet? Sometimes another spreadsheet program will try to open it instead of Open Office.
Have you tried saving the spreadsheet file to your desktop, starting up Open Office, then opening the spreadsheet with Open Office?
If the spreadsheet says it is “Read Only” you probably are using a non-compatible plug in. Install & use Open Office.
Just dragging the spreadsheet link to your desktop may not actually save the file. You need to right click on the link and select Save as…
If you have an older version of Open Office you may need to upgrade it.
Try rebooting. I’ve experienced a problem where if I try to open one of these spreadsheets in a browser the computer gets messed up and won’t start Open Office. Rebooting fixed the issue.
SPREADSHEET CALCULATORS FOR PVC PIPE
Do not try to open these spreadsheets by left clicking on the links. Save the spreadsheets to your hard-drive first. See the explanation above.
PVC fittings only come in 90 degree and 45 degree angles. Sometimes you need a smaller bend. A website reader asked if it is safe to bend PVC pipe and if so, how much can PVC pipe be bend without damaging the pipe?
The answer is that, yes, it is OK to bend PVC pipe,but don’t bend it too sharp or too much. Each pipe manufacturer has rules on what degree curve you can bend the pipe to based on the type and size of pipe. You could look that up but it would take a lot of time and even then figuring out how much a 15% bend is out in the yard is not very practical for the average homeowner. So here is a simpler “rule of thumb” that I basically just made up. But it seems to work reasonably well, it’s easy to do, and it gives you a nice, visual answer!
To determine how much is the maximum bend you should allow grab one end of a length of the pipe you plan to bend and hold it so the other end is off the ground. The amount the pipe bends on it’s own is about the maximum amount of bend you should allow.
You can also make any angle you want simply by using two 45 degree ells. This is easier to demonstrate than to explain. Get two 45 degree PVC ells. Lightly push them together onto either end of a very short piece of pipe. (Don’t glue them for now, this is just a learning experience. If you do ever use them on a irrigation system then you can glue them!) Now start twisting them in different directions. You will see that you can make any angle curve from 0 degree up to 90 degree! Add another 45 degree ell and you can make even more angles. Have fun. It’s cool!
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