Heavy-wall threaded steel conduit — the thickest and strongest raceway in the NEC. Used for service masts, hazardous locations, and anywhere physical damage is likely. Trade size 1" is covered by NEC Article 344, and its internal dimensions come from NEC Chapter 9, Table 4.
| Trade size | 1" |
| Total internal area (100%) | 0.887 in² |
| 1 conductor (53%) | 0.470 in² |
| 2 conductors (31%) | 0.275 in² |
| Over 2 conductors (40%) | 0.355 in² |
| Wire size | Area (in²) | Max conductors |
|---|---|---|
| 14 | 0.0097 | 36 |
| 12 | 0.0133 | 26 |
| 10 | 0.0211 | 16 |
| 8 | 0.0366 | 9 |
| 6 | 0.0507 | 7 |
| 4 | 0.0824 | 4 |
| 3 | 0.0973 | 3 |
| 2 | 0.1158 | 3 |
| 1 | 0.1562 | 1 |
| 1/0 | 0.1855 | 1 |
| 2/0 | 0.2223 | 1 |
| 3/0 | 0.2679 | 1 |
| 4/0 | 0.3237 | 1 |
| 250 kcmil | 0.3970 | 1 |
| 300 kcmil | 0.4608 | 1 |
| 350 kcmil | 0.5242 | — |
| 400 kcmil | 0.5863 | — |
| 500 kcmil | 0.7073 | — |
| Wire size | Area (in²) | Max conductors |
|---|---|---|
| 14 | 0.0139 | 25 |
| 12 | 0.0181 | 19 |
| 10 | 0.0243 | 14 |
| 8 | 0.0437 | 8 |
| 6 | 0.0590 | 6 |
| 4 | 0.0814 | 4 |
| 3 | 0.0962 | 3 |
| 2 | 0.1146 | 3 |
| 1 | 0.1534 | 1 |
| 1/0 | 0.1825 | 1 |
| 2/0 | 0.2190 | 1 |
| 3/0 | 0.2642 | 1 |
| 4/0 | 0.3197 | 1 |
| 250 kcmil | 0.3904 | 1 |
| 350 kcmil | 0.5153 | — |
| 500 kcmil | 0.6984 | — |
Conduit fill is a pure area calculation. Take the internal area of the raceway from NEC Chapter 9, Table 4, multiply by the allowable percentage from Table 1, then divide by the area of one conductor from Table 5. Round down — you cannot install a fraction of a wire.
Max conductors = floor( conduit area × fill % ÷ conductor area )
For 1" Rigid (RMC) with 12 AWG THHN: floor( 0.887 × 0.40 ÷ 0.0133 ) = 26 conductors
The three percentages in NEC Chapter 9, Table 1 are not arbitrary. A single conductor gets 53% because it can be pulled straight down the middle. Exactly two conductors get only 31% because two circles inside a circle waste the most space. Three or more conductors nest efficiently again, so the limit rises to 40%.
Fill is only half the job. Once you have more than three current-carrying conductors in the same raceway, NEC 310.15(C)(1) also derates their ampacity — 80% for 4–6 conductors, 70% for 7–9, 50% for 10–20. A conduit can be legally filled and still leave your conductors undersized for the load. See NEC Table 310.15 for the full derating factors.
Nipples are the exception: a conduit or tubing length not exceeding 24 inches between boxes or enclosures may be filled to 60% per NEC Chapter 9, Table 1, Note 4 — and no ampacity derating applies to it. For 1" Rigid (RMC) that raises the usable area from 0.355 in² to 0.532 in².