Fill limits
| Internal area | 12.882 in² |
|---|---|
| One conductor | 53% — 6.8275 in² |
| Two conductors | 31% — 3.9934 in² |
| More than two | 40% — 5.1528 in² |
Reference tables · Conduit fill
RMC (rigid metal conduit)
4" RMC conduit fill capacity
4" RMC has an internal area of 12.882 in². At the 40% limit that applies to more than two conductors, it holds up to 387 12 AWG THHN conductors.
Capacity
Maximum conductors by size and insulation
| Conductor | THHN / THWN-2 | XHHW-2 | THW |
|---|---|---|---|
| 14 AWG | 400 | 370 | 246 |
| 12 AWG | 387 | 284 | 198 |
| 10 AWG | 244 | 212 | 154 |
| 8 AWG | 140 | 117 | 92 |
| 6 AWG | 101 | 87 | 70 |
| 4 AWG | 62 | 63 | 52 |
| 3 AWG | 52 | 53 | 45 |
| 2 AWG | 44 | 44 | 38 |
| 1 AWG | 32 | 33 | 27 |
| 1/0 AWG | 27 | 28 | 23 |
| 2/0 AWG | 23 | 23 | 19 |
| 3/0 AWG | 19 | 19 | 16 |
| 4/0 AWG | 15 | 16 | 13 |
| 250 kcmil | 12 | 13 | 11 |
| 300 kcmil | 11 | 11 | 9 |
| 350 kcmil | 9 | 9 | 8 |
| 400 kcmil | 8 | 8 | 7 |
| 500 kcmil | 7 | 7 | 6 |
| 600 kcmil | 5 | 5 | 5 |
| 700 kcmil | 5 | 5 | 4 |
| 750 kcmil | 4 | 4 | 4 |
Fitting is not the same as finishing
These counts are the mathematical fill limit, not a promise the pull will go well. A raceway at 39% fill with four 90° bends and a hundred feet between pull points is legal and miserable. Experienced installers routinely size up a trade size on long or bendy runs, and the material cost difference is usually smaller than the labour cost of a fight.
Two other things this table does not model. Nipples 24 inches and shorter between enclosures are permitted a much higher fill. And a mixed set of conductor sizes has to be summed by area rather than looked up by count — the conduit fill calculator handles that.
Remember also that conductor count drives ampacity derating separately. Filling this conduit with current-carrying conductors will reduce what each one can carry — see the bundling adjustment table.
Source: NFPA 70, National Electrical Code, Chapter 9, Table 1; NFPA 70, National Electrical Code, Chapter 9, Table 4; NFPA 70, National Electrical Code, Chapter 9, Table 5. Published by NFPA.NEC 2023
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