Conquer the Washington State WDM 1 Exam 2026 – Dive Into Success and Lead with Confidence!

Fittings are sized to the pipe’s outside diameter, not just the nominal size. For certain nominal sizes, ductile-iron pipe and PVC pipe share the same outside diameter, so bends, tees, and other fittings designed for ductile-iron will fit PVC as well. This matching of outer dimensions allows using common fittings across these materials, sometimes with a transition or adaptation as required by the joint type. Copper or cast-iron pipes do not generally have the same outside diameter as PVC at the same nominal size, so those fittings aren’t interchangeable without adapters. The key idea is that compatibility comes from identical outer dimensions, which is why ductile-iron pipe fittings can be used with PVC.