Conduit Fill

Fill is not about whether the wires physically fit. It is about heat and about friction: a crowded raceway cannot shed the heat its conductors make, and a crowded pull strips insulation on the way in. So the codes cap the fraction of the raceway's internal cross-section that conductors may occupy — conventionally 53 % for a single conductor, 31 % for exactly two, and 40 % for three or more. Those three numbers are the well-known figures of North American practice, and they are the ones this page assumes you are aiming at, but the permitted value for your raceway, your conductor type and your edition of the code is something you must read in the code book. This calculator gives you the ratio; it does not give you permission.

Two mistakes account for most bad fill calculations. The first is using the conductor's nominal size rather than its actual area over the insulation: a 12 AWG conductor's copper is about 3.31 mm², but the thing you are pulling — copper plus insulation — is two or three times that, and it is the outside dimension the raceway sees. Every code book tabulates the overall area for each conductor type; look it up and enter it here. The second is forgetting that trade size is a name, not a dimension. Half-inch conduit does not have a half-inch bore, and the bore differs between EMT, rigid and PVC of the same trade size — so when this page tells you the internal area a raceway needs, take that number to the dimension table rather than to the label on the stick.

One more practicality: a mixed pull. This page multiplies one conductor area by a count, which is exactly right when everything in the raceway is the same. When it is not, add the actual areas of all the conductors yourself, enter that sum as the conductor area, and set the count to 1. And note that when the answer is a conductor count you round down — the arithmetic does not care that the next conductor puts you over the line, but the inspector will.