Question: why is cpvc smaller than pvc?
Answer: CPVC is the same o.d. as copper, but since the wall is thicker, 1/2" CPVC is MUCH smaller than 1/2" copper. PVC is sch. 40 so its o.d. is the same as steel pipe but its i.d. is also smaller than steel pipe. In other words, plastic pipe is ALWAYS smaller, internally, than the corresponding size metal pipe.