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 3-1/2" is covered by NEC Article 344, and its internal dimensions come from NEC Chapter 9, Table 4.
| Trade size | 3-1/2" |
| Total internal area (100%) | 10.010 in² |
| 1 conductor (53%) | 5.305 in² |
| 2 conductors (31%) | 3.103 in² |
| Over 2 conductors (40%) | 4.004 in² |
| Wire size | Area (in²) | Max conductors |
|---|---|---|
| 14 | 0.0097 | 412 |
| 12 | 0.0133 | 301 |
| 10 | 0.0211 | 189 |
| 8 | 0.0366 | 109 |
| 6 | 0.0507 | 79 |
| 4 | 0.0824 | 48 |
| 3 | 0.0973 | 41 |
| 2 | 0.1158 | 34 |
| 1 | 0.1562 | 25 |
| 1/0 | 0.1855 | 21 |
| 2/0 | 0.2223 | 18 |
| 3/0 | 0.2679 | 15 |
| 4/0 | 0.3237 | 12 |
| 250 kcmil | 0.3970 | 10 |
| 300 kcmil | 0.4608 | 8 |
| 350 kcmil | 0.5242 | 7 |
| 400 kcmil | 0.5863 | 7 |
| 500 kcmil | 0.7073 | 5 |
| Wire size | Area (in²) | Max conductors |
|---|---|---|
| 14 | 0.0139 | 288 |
| 12 | 0.0181 | 221 |
| 10 | 0.0243 | 164 |
| 8 | 0.0437 | 91 |
| 6 | 0.0590 | 68 |
| 4 | 0.0814 | 49 |
| 3 | 0.0962 | 41 |
| 2 | 0.1146 | 35 |
| 1 | 0.1534 | 26 |
| 1/0 | 0.1825 | 22 |
| 2/0 | 0.2190 | 18 |
| 3/0 | 0.2642 | 15 |
| 4/0 | 0.3197 | 12 |
| 250 kcmil | 0.3904 | 10 |
| 350 kcmil | 0.5153 | 7 |
| 500 kcmil | 0.6984 | 5 |
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, except that NEC Chapter 9, Note 7 lets you round up to the next whole number when the result has a decimal of 0.8 or more and all conductors are the same size.
Max conductors = conduit area × fill % ÷ conductor area (Note 7 rounding)
For 3-1/2" Rigid (RMC) with 12 AWG THHN: 10.010 × 0.40 ÷ 0.0133 = 301.05 → 301 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 3-1/2" Rigid (RMC) that raises the usable area from 4.004 in² to 6.006 in².
A 3-1/2-inch Rigid (RMC) conduit has 10.010 square inches of internal area. With more than two conductors the NEC limits fill to 40%, or 4.004 in². That works out to 301 #12 THHN, 412 #14 THHN, 189 #10 THHN, or 109 #8 THHN conductors.
301 conductors. Each 12 AWG THHN wire is 0.0133 in² per NEC Chapter 9, Table 5, and the 40% fill area of 3-1/2-inch Rigid (RMC) is 4.004 in².
NEC Chapter 9, Table 1 sets three limits: 53% for a single conductor, 31% for exactly two conductors, and 40% for three or more. The lower two-conductor figure exists because two round conductors pack poorly inside a round raceway.
Yes. Every conductor in the raceway counts toward the cross-sectional fill calculation, including equipment grounding conductors and neutrals. Grounding conductors do not count as current-carrying conductors for the separate NEC 310.15(C)(1) derating calculation, but they still occupy space.