Choose a conduit, add your conductors, and check the fill against NEC Chapter 9. Applies the correct 53% / 31% / 40% limit for the conductor count, handles mixed wire sizes, and tells you the smallest conduit that would work.
Count every conductor, including neutrals and equipment grounding conductors. A 12/2 circuit pulled as loose wire is 3 conductors.
NEC Chapter 9, Table 1 gives the maximum percentage of a raceway's internal cross-section that conductors may occupy. The limit changes with the number of conductors, not their size:
| Number of conductors | Max fill | Why |
|---|---|---|
| 1 conductor | 53% | A single wire runs straight down the centre of the raceway. |
| 2 conductors | 31% | Two circles inside a circle waste the most space — the lowest limit in the table. |
| 3 or more conductors | 40% | Multiple conductors nest efficiently, so the limit rises again. |
| Nipple, 24 in. or shorter | 60% | Table 1, Note 4. Short nipples between enclosures are also exempt from ampacity derating. |
Six 12 AWG THHN and three 10 AWG THHN in 3/4-inch EMT:
6 × 0.0133 in² = 0.0798 in²
3 × 0.0211 in² = 0.0633 in²
Total conductor area = 0.1431 in²
3/4-inch EMT internal area = 0.533 in², and 9 conductors means the 40% limit applies: 0.213 in²
0.1431 ÷ 0.533 = 26.8% fill — passes, with room to spare
Note that nine current-carrying conductors would trigger a 70% ampacity adjustment under NEC 310.15(C)(1), dropping 12 AWG THHN from 30 A to 21 A before the 240.4(D) 20 A cap is even applied. Passing fill is not the same as passing ampacity.
Disclaimer: This calculator is for reference only, based on NEC Chapter 9, Tables 1, 4 and 5. It does not check bend allowances, box fill, conductor derating, or local amendments. Always consult a licensed electrician and your local authority having jurisdiction for actual electrical work.
Enter the conduit type and trade size, then add each conductor size and how many of them. The calculator sums the conductor areas from NEC Chapter 9, Table 5, compares that total against the allowable fill area from Tables 1 and 4, and reports the fill percentage with a pass or fail verdict.
It depends on the total conductor count. One conductor may fill 53% of the raceway, exactly two conductors 31%, and three or more conductors 40%. The calculator selects the correct limit automatically from the number of conductors you enter.
Yes, and the calculation still works — you add the individual conductor areas together and compare the sum to the allowable fill area. The NEC Annex C tables only cover conduits filled with conductors that are all the same size and insulation, so mixed pulls have to be worked out by area, which is what this calculator does.
Yes, for fill. Every conductor physically in the raceway occupies area, including equipment grounding conductors and neutrals. The separate current-carrying conductor count used for ampacity derating under NEC 310.15(C)(1) excludes grounds and most neutrals.