A pipe pressure rating is pure hoop-stress arithmetic. ASTM D1785 rates PVC pipe at P = 2·S·t/(D − t), where S is the hydrostatic design stress of the compound (2,000 psi for PVC 1120 in water at 73 °F), t the minimum wall, and D the outside diameter. The formula explains the pattern in every rating table: small pipe carries startlingly high ratings because its wall is thick relative to its diameter, and a heavier-wall schedule always outrates a lighter one in the same size and material — Schedule 40 and 80 share one outside diameter per nominal size, so the extra Schedule 80 wall grows inward and buys pressure capacity (and thread depth) at the cost of bore.
A published rating is a starting point that two derates and one caveat whittle down. Temperature: the 73 °F rating falls fast as the fluid warms — at 100 °F a PVC pipe keeps only 62% of it, and at 140 °F, PVC’s maximum service temperature, just 22%. Threading: cut threads halve the wall at the root, so threaded Schedule 80 PVC is rated at 50% of its plain-end value, and threading Schedule 40 is not an accepted practice at all. And the rating is for water without surge — pressure spikes from valve slam ride on top of the working pressure and must be added before comparing against it, which is exactly the check a water-hammer calculation feeds.