Find the minimum conduit size or check the percent fill of EMT, IMC, RMC and PVC conduit for any mix of THHN, XHHW, THW, bare and custom conductors — using the NEC Chapter 9 fill limits (53% / 31% / 40%, 60% for nipples).
Conduit fill confirms the conductors physically fit. It does not check ampacity: with more than three current-carrying conductors in one raceway, apply the adjustment factors of NEC 310.15(C)(1). Verify against the code edition and local amendments that apply to your project.
| Trade size | Internal area | Allowed | Fill | Status |
|---|
How many wires of one size fit in each trade size — the same method as NEC Annex C, including the Note 7 rounding rule.
| Number of conductors or cables | Maximum fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| 3 or more | 40% |
| Nipple, not longer than 24 in (600 mm) | 60% |
The calculation itself is simple: add up the cross-sectional area of every conductor from NEC Table 5 (insulated) or Table 8 (bare), then compare the total with the allowed percentage of the conduit's internal area from Table 4. The smallest trade size whose allowed area is at least the total is the minimum size.
It looks backwards that two wires are allowed less space than three. The reason is jamming. Two round conductors lying side by side span the full diameter of the conduit; when they twist during the pull — and they always twist at a bend — they can wedge across the bore and lock. Three or more conductors settle into a rough triangle and slide past each other, so the code can allow a higher percentage. A single conductor cannot jam against itself at all, which is why it gets 53%.
Circuit: 3 × 10 AWG THHN + 1 × 10 AWG THHN ground in EMT.
Minimum size: 1/2″ (16) EMT. Fill passes — but three current-carrying conductors is the limit before NEC 310.15(C)(1) adjustment applies, so add one more circuit and the conductors themselves may need upsizing.
The most-searched fill question, calculated with the same method as Annex C:
| Wire | 1/2″ EMT | 3/4″ EMT | 1″ EMT | 1-1/4″ EMT | 1-1/2″ EMT | 2″ EMT |
|---|---|---|---|---|---|---|
| 14 AWG THHN | 12 | 22 | 35 | 61 | 84 | 138 |
| 12 AWG THHN | 9 | 16 | 26 | 45 | 61 | 101 |
| 10 AWG THHN | 5 | 10 | 16 | 28 | 38 | 63 |
| 8 AWG THHN | 3 | 6 | 9 | 16 | 22 | 36 |
| 6 AWG THHN | 1 | 4 | 7 | 12 | 16 | 26 |
| Trade size (metric) | EMT | IMC | RMC | PVC 40 | PVC 80 |
|---|---|---|---|---|---|
| 1/2″ (16) | 0.304 in² | 0.342 in² | 0.314 in² | 0.285 in² | 0.217 in² |
| 3/4″ (21) | 0.533 in² | 0.586 in² | 0.549 in² | 0.508 in² | 0.409 in² |
| 1″ (27) | — | — | — | — | — |
| 1-1/4″ (35) | 1.496 in² | 1.647 in² | 1.526 in² | 1.453 in² | 1.237 in² |
| 1-1/2″ (41) | 2.036 in² | 2.225 in² | 2.071 in² | 1.986 in² | 1.711 in² |
| 2″ (53) | — | — | — | — | — |
| 2-1/2″ (63) | 5.858 in² | 5.135 in² | 4.866 in² | 4.695 in² | 4.119 in² |
| 3″ (78) | — | — | — | — | — |
| 3-1/2″ (91) | 11.545 in² | 10.584 in² | 10.010 in² | 9.737 in² | 8.688 in² |
| 4″ (103) | — | — | — | — | — |
| 5″ (129) | — | — | — | — | — |
| 6″ (155) | — | — | — | — | — |
100% internal area in square inches. Multiply by 645.16 for mm².
The NEC is the most detailed published method, which is why it is used worldwide even on non-US projects. Other codes approach it differently: BS 7671 guidance uses a cable-factor method for conduit and a 45% space factor for trunking, and IEC 61386 defines conduit systems without a fill table. For IEC cables, enter the manufacturer's outside diameter as Custom (OD) and the 40% NEC limit gives a sound, widely accepted result — but follow the local code or project specification where it sets its own rule.
53% for one conductor, 31% for two and 40% for three or more (Chapter 9, Table 1). A nipple no longer than 24 in may be filled to 60%.
Two conductors span the full bore and can wedge across it when they twist during the pull. Three or more nest into a triangle and slide more freely.
Nine. 0.304 in² × 40% = 0.1216 in², and each 12 AWG THHN is 0.0133 in². In 3/4″ EMT the answer is sixteen.
Yes. Chapter 9 Note 3 requires grounding and bonding conductors to be included at their actual size, insulated or bare. They are not current-carrying conductors for ampacity adjustment.
A multiconductor cable counts as one conductor (Note 9). Enter its outside diameter as Custom; the area is π × D² ÷ 4. For an elliptical cable use the major diameter.
No. Fill only confirms they fit. More than three current-carrying conductors in one raceway triggers the ampacity adjustment of NEC 310.15(C)(1), which may require larger conductors and a larger conduit. Check conductor size with the Cable Size Calculator and Voltage Drop Calculator.
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