The short answer
Conductors may fill 53% of a conduit's internal area when there is one, 31% when there are two, and 40% when there are three or more (NEC Chapter 9, Table 1). Add the conductors' areas from Chapter 9, Table 5 and compare the total with that percentage of the conduit's area from Chapter 9, Table 4.
Step by step
- Look up each conductor's area in square inches for its size and insulation type (Chapter 9, Table 5).
- Add the areas of all the conductors in the conduit.
- Look up the conduit's total internal area for its type and trade size (Chapter 9, Table 4).
- Take 53% of that area for one conductor, 31% for two, or 40% for three or more.
- The fill complies if the conductors' total area is no more than the allowed area.
Worked example
Nine 12 AWG THHN conductors take 0.1197 in² (0.0133 each). A 1/2" EMT has 0.304 in² inside, and 40% of it is 0.1216 in², so the nine conductors fit at 39.4% fill.
| EMT size | Area (in²) | 14 | 12 | 10 | 8 | 6 |
|---|---|---|---|---|---|---|
| 1/2" | 0.304 | 12 | 9 | 5 | 3 | 2 |
| 3/4" | 0.533 | 22 | 16 | 10 | 6 | 4 |
| 1" | 0.864 | 35 | 26 | 16 | 9 | 7 |
| 1-1/4" | 1.496 | 61 | 45 | 28 | 16 | 12 |
| 1-1/2" | 2.036 | 84 | 61 | 38 | 22 | 16 |
| 2" | 3.356 | 138 | 101 | 63 | 36 | 26 |
THHN/THWN-2 conductor areas (Chapter 9, Table 5)
| Conductor | Area (in²) |
|---|---|
| 14 AWG | 0.0097 |
| 12 AWG | 0.0133 |
| 10 AWG | 0.0211 |
| 8 AWG | 0.0366 |
| 6 AWG | 0.0507 |
| 4 AWG | 0.0824 |
| 2 AWG | 0.1158 |
| 1/0 AWG | 0.1855 |
Common mistakes
- Using 40% for two conductors: two conductors may fill only 31%, less than three.
- Mixing up insulation types: the areas here are THHN/THWN/THWN-2; XHHW and other types have different areas in Table 5.
- Forgetting the equipment grounding conductor: it counts toward the fill like any other conductor.
Code numbers and editions
Table numbers here are NEC 2023's. The counts above apply Chapter 9's Note 7, which lets a same-size result with a decimal of 0.8 or more round up, as Annex C's tables and the app's calculator do. Note 7 does not reach mixed sizes, which keep the area test.
Code adoption cycles change. Always confirm the edition and amendments in force with your Authority Having Jurisdiction (AHJ) before permitting work.