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.