First, let’s start with what “schedule” pipe is. “Schedule” PVC pipe is an older style of labeling pipe based on the standard dimensions of ductile iron and steel pipe ie. the thickness of the pipe walls. Schedule 40 pipe conforms to the old “standard” label for piping and is considered the same up to 10″ in diameter. All Schedule 40 piping, no matter the diameter of the pipe, has 3/8″ walls. Schedule 80 pipe has 1/2″ walls and conforms to the old “extra strong” label up to 8″ in diameter.
With schedule piping, obviously, the smaller the diameter of the pipe, the more pressure the pipe can take as the wall thickness of the pipe is the same. Conversely, the larger the pipe, the less pressure can be taken and the lower the pressure rating. For instance, a 10″ schedule 40 pipe can only take 145 psi. However, a 3/4″ Schedule 40 can take significantly more at 480 psi.
“Class” pipe is a different labeling system. Rather than categorization by wall thickness, the “class” designation rates the internal pressure that the pipe can take instead. So a “class 200″ pipe can take 200 psi, no matter what the diameter of pipe is. If it is 10″, it takes 200 psi. If it’s 3/4″, it takes 200 psi. Consequently, you might expect the wall thickness to vary depending on size of pipe.
The point being here that for irrigation system purposes, in which the water pressure you wish to maintain is roughly around 60 psi with a maximum around 90, smaller diameter class pipe which is cheaper is more than sufficient for the job. A lot of people think schedule 40 pipe is better. But it is total overkill at smaller diameters and a totally unnecessary additional expense, particularly since a shovel will break it just as easily as thinner class rated pipe.
So there you go. Now you know all about it.