Conduit Fill Calculator

The NEC limits how full a conduit can be: 53% for a single conductor, 31% for two, and 40% for three or more (Chapter 9, Table 1). Add up the cross-sectional areas of your conductors (Table 5) and compare with that percentage of the conduit’s internal area (Table 4). For example, a ¾″ EMT holds sixteen 12 AWG THHN conductors at the 40% limit. The calculator does the table lookups and the arithmetic.

The 40% limit for three or more conductors leaves room to pull the wires without damaging the insulation, and lets heat escape. A packed conduit is hard to pull and traps heat, which derates the conductors. The limits tighten for one or two conductors (53% and 31%) because a single or pair of wires jams and heats differently — those are the Chapter 9, Table 1 figures the calculator uses.

Yes. Every conductor in the raceway counts toward fill, including the equipment grounding conductor (EGC) and the neutral — even though the ground carries no current in normal operation, it still takes up space. It does not, however, count as a current-carrying conductor for ampacity derating; fill and derating are separate checks.

A lot. THHN/THWN-2 is thin, so more fit; older THW and some XHHW are fatter for the same gauge and fewer fit. Pick the insulation printed on your wire — the calculator switches to that insulation’s Table 5 areas, so the fill and conduit size come out right.

No — conduit fill and ampacity derating are two different rules. Once you have more than three current-carrying conductors in a raceway, NEC 310.15 derates their ampacity (for example, 4–6 conductors to 80%), which can force a larger wire — separate from whether they physically fit. Size the wire (and check derating) with the cable size calculator, then use this tool for the conduit.

A conduit nipple 24 inches or shorter is allowed 60% fill (NEC Chapter 9, Note 4) and the derating factors don’t apply to it. This calculator uses the standard 40/31/53% limits for a normal run; for a true nipple you can fit more, but keep the 60% in mind rather than overstuffing it.