Fill limits
| Internal area | 10.01 in² |
|---|---|
| One conductor | 53% — 5.3053 in² |
| Two conductors | 31% — 3.1031 in² |
| More than two | 40% — 4.0040 in² |
Reference tables · Conduit fill
RMC (rigid metal conduit)
3-1/2" RMC conduit fill capacity
3-1/2" RMC has an internal area of 10.01 in². At the 40% limit that applies to more than two conductors, it holds up to 301 12 AWG THHN conductors.
Capacity
Maximum conductors by size and insulation
| Conductor | THHN / THWN-2 | XHHW-2 | THW |
|---|---|---|---|
| 14 AWG | 400 | 288 | 191 |
| 12 AWG | 301 | 221 | 154 |
| 10 AWG | 189 | 164 | 120 |
| 8 AWG | 109 | 91 | 72 |
| 6 AWG | 78 | 67 | 55 |
| 4 AWG | 48 | 49 | 41 |
| 3 AWG | 41 | 41 | 35 |
| 2 AWG | 34 | 34 | 30 |
| 1 AWG | 25 | 26 | 21 |
| 1/0 AWG | 21 | 21 | 18 |
| 2/0 AWG | 18 | 18 | 15 |
| 3/0 AWG | 14 | 15 | 12 |
| 4/0 AWG | 12 | 12 | 10 |
| 250 kcmil | 10 | 10 | 8 |
| 300 kcmil | 8 | 8 | 7 |
| 350 kcmil | 7 | 7 | 6 |
| 400 kcmil | 6 | 6 | 6 |
| 500 kcmil | 5 | 5 | 5 |
| 600 kcmil | 4 | 4 | 4 |
| 700 kcmil | 4 | 4 | 3 |
| 750 kcmil | 3 | 3 | 3 |
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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