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

At the NEC 40% fill limit for three or more conductors, 3-1/2 inch PVC (internal area 9.737 sq in) holds 293 #12 THHN, 401 #14, 184 #10, or 106 #8 conductors. The math is the conduit area times 0.40 (9.737 × 0.40 = 3.895 sq in allowable), divided by the conductor's area from NEC Chapter 9 Table 5.

293 #12 THHN conductors fit in 3-1/2 inch PVC at the 40% fill limit. Each #12 THHN is 0.0133 sq in, so 3.895 sq in allowable ÷ 0.0133 rounds to 293 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.

184 #10 THHN conductors fit in 3-1/2 inch PVC at the NEC 40% fill limit. Each #10 THHN is 0.0211 sq in, so 3.895 sq in allowable (9.737 × 0.40) ÷ 0.0211 rounds to 184 conductors (NEC Chapter 9 Note 7). THWN-2 has the identical area and gives the same count. Count the equipment ground as a conductor.

401 #14 THHN conductors fit in 3-1/2 inch PVC at 40% fill. Each #14 THHN is 0.0097 sq in, so 3.895 sq in ÷ 0.0097 rounds to 401 (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 PVC (internal area 9.737 sq in, Schedule 40 PVC) that is 5.161 sq in for one wire, 3.018 sq in for two, and 3.895 sq in for three or more. The 40% case is the common one.