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Conduit Fill Calculator

Mix your THHN conductor sizes, pick EMT, PVC, or rigid, and get the fill percent, a pass/fail, and the smallest conduit that fits — per NEC Chapter 9.

Conductors (THHN / THWN-2) — enter how many of each
14 AWG
12 AWG
10 AWG
8 AWG
6 AWG
4 AWG
3 AWG
2 AWG
1 AWG
1/0
2/0
3/0
4/0
250 kcmil
350 kcmil
500 kcmil
Every wire counts toward fill, ground included. THHN and THWN-2 share the same Table 5 area.
Conduit fill
—
Add the conductors going in the pipe
NEC Ch.9: Table 1 fill (1 wire 53%, 2 wires 31%, 3+ 40%), Table 4 conduit areas, Table 5 conductor areas.
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How conduit fill works

The NEC caps how much of a raceway you can pack with copper so the wires can pull without damage and shed heat. Chapter 9, Table 1 sets the limit by conductor count: one wire can fill 53% of the pipe, two wires 31%, and three or more 40%. You add up the cross-sectional area of every conductor going in — grounds included — from Table 5, then compare it to the allowed slice of the conduit's interior area from Table 4. Overfill it and you fight the pull, nick insulation, and trap heat. This tool sums your exact mix and hands you the smallest conduit that stays legal.

Questions

What is the NEC conduit fill limit?
Chapter 9, Table 1: one conductor 53%, two conductors 31%, three or more 40% — of the conduit's total interior area. Each conductor's area comes from Table 5.
How many THHN wires fit in a conduit?
Depends on wire and conduit size. As a benchmark, 1/2-inch EMT (0.122 in² allowed at 40%) holds about nine 12 AWG THHN or twelve 14 AWG. Enter your exact mix above for the real answer.
Does the ground count?
Yes — every conductor in the raceway counts toward fill by its Table 5 area, including the equipment ground. Add it to the count.
Is this free and accurate?
Free, in your browser, NEC 2023 Chapter 9 Table 1, Table 4 conduit areas, and Table 5 THHN/THWN-2 areas. A reference tool — verify against your adopted code.

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For reference and estimating only. Based on NEC (NFPA 70) 2023 Chapter 9 — Table 1 fill percentages, Table 4 conduit dimensions, and Table 5 THHN/THWN-2 conductor areas. Not a substitute for the National Electrical Code, a licensed electrician's judgment, or your authority having jurisdiction. Verify against your locally adopted code before installing.