TradesHow this works
Conduit fill is about two things: getting the wire in without wrecking the insulation, and letting the bundle shed heat. The NEC sets a limit on how full the pipe can be, and it changes with how many conductors you are pulling. This tool adds up the area of your conductors, picks the smallest conduit that stays under the limit, and tells you how many more wires would fit.
Filling to the exact limit is legal but rarely smart. It is a hard pull and leaves no room for later. That is why the tool also offers the next size up, and why the future-capacity option recommends it.
Common questions
What is the 40% fill rule?
The NEC limits how full a conduit can be so the wires can be pulled without damage and can shed heat. For three or more conductors the limit is 40% of the conduit area. For two it is 31%, and for a single conductor it is 53%.
Why leave room instead of filling to the limit?
A conduit packed to the legal limit is hard to pull and leaves nothing for the day you need to add a circuit. Going one trade size up is cheap insurance, which is why the tool shows how much room is left and offers the next size.
Does this handle mixed wire sizes?
Not yet. It models one THHN size at a time, which covers most branch and feeder runs. Mixed sizes change the total area and are figured from the NEC tables directly.
Sources: NEC Chapter 9 Tables 1, 4, 5 (fill limits, conduit and conductor areas) (2023). Engine version 1.0.0. Risk tier 2.