Conduit Fill Calculator — NEC Chapter 9, 40% Check
Who this is for: For electricians pulling mixed wire lists through a run — data + power in one sleeve — who need the 40% answer fast and want to see how many more conductors fit before stepping up a trade size.
Pick conduit type and trade size, list your wires — get the fill percentage against the NEC 40% limit and the max count of that wire.
Quick answer: Conduit fill = total wire area ÷ conduit internal area, capped at 40% for three or more conductors (53% for one, 31% for two). Nine 12 AWG THHN in 1/2" EMT = 39.4% — just inside the limit. The tenth wire pushes past 40%, so you step up to 3/4".
| Quick reference — max 12 THHN | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" |
|---|---|---|---|---|---|---|
| EMT | 9 | 16 | 25 | 44 | 61 | 100 |
| Fill limits (Ch.9 T1) | 1 wire 53% · 2 wires 31% · ≥3 wires 40% |
|---|---|
| Nipple ≤24 in | 60% allowed |
| Raceways | EMT · PVC-40 · IMC · RMC, 1/2"–4" |
| Wire areas | NEC Table 5 (THHN/THWN) |
| Example | 9 × 12 AWG in 1/2" EMT = 39.4% ✓ (10th wire fails) |
| Status | Figures follow NEC 2017+ reference tables (310.16 ampacity, Chapter 9 conduit fill, 314.16 box fill, 240.4(D) small-conductor caps) and standard equal-friction duct sizing, compiled September 2026. Estimates only — final designs belong to a licensed electrician / HVAC designer per local code. |
How does conduit fill math work?
NEC Chapter 9, Table 1 sets the maximum fill: one conductor 53%, two 31%, three or more 40% of the raceway's internal area (a nipple 24 inches or shorter gets 60%). This calculator takes each wire's approximate area from Table 5 (THHN/THWN values), sums your list, and divides by the Table 4 total area for the conduit type and trade size you chose. The result shows the percentage, the pass/fail against your list's limit, and how many conductors of the first wire in your list would fit at the 40% line — the fastest way to answer 'can I add one more?'. For example, nine 12 AWG THHN in 1/2" EMT sits at 39.4% and passes; the tenth pushes to 43.8% and fails.
Common uses
- Deciding between 3/4" and 1" EMT before ordering material
- Adding a data cable to an existing power conduit without a re-pull
- Checking a PVC run quoted by a supplier for a permit submittal
- Estimating pull difficulty — fill over ~35% runs noticeably stiffer
Why two conductors get a tighter limit than three
Two round conductors in a round hole stack one-on-one and jam diagonally, so the code caps them at 31% — less than the 40% allowed for three or more, which settle into a more pullable bundle. It's a pulling-difficulty rule as much as a heat rule. If your two-conductor run needs one more wire later, re-check at three: the allowed area jumps from 31% to 40%, so the third wire is often 'free'.