How many 12 gauge wires can I run in 1 inch conduit?

One inch EMT holds 26 number 12 THHN conductors at the NEC's 40 percent fill. Schedule 40 PVC at the same size takes a few less because its wall is thicker.

Fill is rarely the binding limit, though. Past three current-carrying conductors, ampacity derating kicks in: at seven to nine wires the 70 percent factor leaves a 12 AWG THHN at 21 amps, barely clearing its 20 amp breaker, and at ten or more the 50 percent factor drops it to 15 amps, which no longer qualifies.

So the honest planning number for 20 amp circuits is around 9 current-carrying wires per pipe, not 26.

Grounds and most neutrals do not count for derating, which is where careful counting earns back capacity.

The conduit size calculator reports both the fill limit and the derated ampacity together.