When does a water heater need a drain pan, and what size is the drain?

A pan is required where a leak would cause damage; it must drain through a pipe at least 3/4 inch in diameter to an approved location (IRC P2801.6).

The IRC requires a drain pan under a storage-tank water heater installed where leaking water would cause damage — for example in an attic or above finished living space. The pan must be at least 1-1/2 inches (38 mm) deep.

The pan must drain through a pipe no smaller than 3/4 inch (19 mm) in diameter, running full-size to an indirect waste receptor or to the building exterior, where it ends 6 to 24 inches above grade.

The pan catches slow leaks and condensate. It is separate from, and not a substitute for, the relief-valve discharge line.