How Many Wires Fit in 3-1/2" RMC?

At the NEC 40% fill limit for three or more conductors, 3-1/2 inch RMC (internal area 10.01 sq in) holds 301 #12 THHN, 412 #14, 189 #10, or 109 #8 conductors. The math is the conduit area times 0.40 (10.01 × 0.40 = 4.004 sq in allowable), divided by the conductor's area from NEC Chapter 9 Table 5.

301 #12 THHN conductors fit in 3-1/2 inch RMC at the 40% fill limit. Each #12 THHN is 0.0133 sq in, so 4.004 sq in allowable ÷ 0.0133 rounds to 301 conductors (NEC Chapter 9 Note 7 rounds up when the decimal is 0.8 or more). The same count applies to THWN-2, which has the identical area.

189 #10 THHN conductors fit in 3-1/2 inch RMC at the NEC 40% fill limit. Each #10 THHN is 0.0211 sq in, so 4.004 sq in allowable (10.01 × 0.40) ÷ 0.0211 rounds to 189 conductors (NEC Chapter 9 Note 7). THWN-2 has the identical area and gives the same count. Count the equipment ground as a conductor.

412 #14 THHN conductors fit in 3-1/2 inch RMC at 40% fill. Each #14 THHN is 0.0097 sq in, so 4.004 sq in ÷ 0.0097 rounds to 412 (NEC Chapter 9 Note 7). Remember the ground counts as a conductor, and more than three current-carrying conductors trigger ampacity derating per NEC 310.15(C)(1).

The maximum fill depends on how many conductors are in the raceway (NEC Chapter 9, Table 1): 53% for one conductor, 31% for two, and 40% for three or more. For 3-1/2 inch RMC (internal area 10.01 sq in, RMC (rigid metal conduit)) that is 5.305 sq in for one wire, 3.103 sq in for two, and 4.004 sq in for three or more. The 40% case is the common one.