What Size Conduit for 16 × 12 AWG THWN-2?
Short answer: 3/4" EMT, or 1" in Schedule 40 PVC. 16 × 12 AWG THWN-2 occupies 0.2128 in², and 3/4" EMT allows 0.2132 in² at the 40% fill limit — a working fill of 39.9%.
Every trade size, EMT and PVC
Note that EMT and PVC do not agree here: 3/4" EMT works but PVC needs 1". Schedule 40 PVC has less internal area than EMT at the same trade size, and this set falls right on that boundary.
| Trade size | EMT area | EMT fill | EMT | PVC area | PVC fill | PVC |
|---|---|---|---|---|---|---|
| 1/2" | 0.304 in² | 70% | no | 0.285 in² | 74.7% | no |
| 3/4" | 0.533 in² | 39.9% | fits | 0.508 in² | 41.9% | no |
| 1" | 0.864 in² | 24.6% | fits | 0.832 in² | 25.6% | fits |
| 1-1/4" | 1.496 in² | 14.2% | fits | 1.453 in² | 14.6% | fits |
| 1-1/2" | 2.036 in² | 10.5% | fits | 1.986 in² | 10.7% | fits |
| 2" | 3.356 in² | 6.3% | fits | 3.291 in² | 6.5% | fits |
| 2-1/2" | 5.858 in² | 3.6% | fits | 4.695 in² | 4.5% | fits |
| 3" | 8.846 in² | 2.4% | fits | 7.268 in² | 2.9% | fits |
| 4" | 14.753 in² | 1.4% | fits | 12.554 in² | 1.7% | fits |
How the number is built
- Conductor area — 12 AWG THWN-2 is 0.0133 in² each, from Chapter 9 Table 5.
- Total — 0.0133 × 16 = 0.2128 in².
- Fill limit — Chapter 9 Table 1 allows 40% for more than two conductors.
- Raceway — 3/4" EMT has 0.533 in² internal area, so 0.533 × 0.4 = 0.2132 in² usable.
Do not size this from THWN habit alone
THWN-2 at 12 AWG is 0.0133 in². If any part of this run uses USE-2 / PV wire instead, recompute — PV wire of the same gauge is 36% larger in area and can push you to the next trade size.
What 16 conductors costs you in ampacity
Fill is only half the problem, and it is the half people check. Every current-carrying conductor in this raceway also counts toward the 310.15(C)(1) bundling adjustment, which at 16 conductors is a factor of 0.5. Combined with the rooftop temperature correction, that is what a 12 AWG conductor actually delivers here:
| Ambient | Temp correction | Fill adj. (16 CCC) | 12 AWG usable ampacity |
|---|---|---|---|
| 86 °F | 1 | 0.5 | 15 A |
| 104 °F | 0.91 | 0.5 | 13.7 A |
| 122 °F | 0.82 | 0.5 | 12.3 A |
| 140 °F | 0.71 | 0.5 | 10.6 A |
A conduit that physically fits 16 conductors can still force a larger gauge on ampacity grounds — 50% of the conductor's rating is gone to bundling before the heat correction is even applied. Size the conductor first, then the raceway around it.
Related
Computed with OneLine Studio's NEC engine — the same deterministic code tables that size a real permit set, not an estimate. Conductor ampacity from NEC 310.16, correction factors from 310.15(B)(1), fill adjustment from 310.15(C)(1). Always confirm the enforced NEC edition and any local amendments with your AHJ.
FAQ
What size conduit for 16 12 AWG THWN-2 conductors?
3/4" EMT, or 1" in Schedule 40 PVC. The set occupies 0.2128 in² and 3/4" EMT allows 0.2132 in² at the 40% fill limit, giving 39.9% actual fill.
What is the maximum conduit fill percentage?
Chapter 9 Table 1: 53% for a single conductor, 31% for exactly two, and 40% for three or more. This set of 16 uses the 40% limit.
Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.
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