SCH Rated Pipe
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.)
Pressure ratings of SCH 40 PVC pipe:
3/4″ = 480 PSI
1″ = 450 PSI
1 1/4″ = 370 PSI
1 1/2″ = 330 PSI
2″ = 280 PSI
2 1/2″ = 300 (not a typo, 2.5″ pressure is an oddity)
3″ = 260 PSI
4″ = 220 PSI
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.