3-1/2" RMC Conduit Fill Chart

The NEC numbers behind 3-1/2" RMC

NEC 2023 Chapter 9, Table 4 lists the interior cross-sectional area of every standard trade size for each conduit family. For 3-1/2" RMC (Article 344), the total 100% area is 10.0100 in². The allowable conductor fill is then derived from Table 1: 53% for a single conductor (5.3053 in²), 31% for two (3.1031 in²), and 40% for three or more (4.0040 in²) — covering nearly all real-world raceway pulls.

Worked example — four #12 THHN conductors

Each #12 THHN conductor occupies 0.0133 in² (NEC Ch.9 Table 5). Four conductors fill 4 × 0.0133 = 0.0532 in², which is 0.0532 ÷ 10.0100 = 0.5% of total area. Because that is well below the 40% NEC limit for 4.0040 in², the pull is compliant — and you can keep adding conductors up to 301× #12 THHN before exceeding the cap.

If you exceed the fill cap

Step up one trade size. Increase a 3-1/2" RMC to 4" when the calculated fill crosses 40%. Reducing one trade size is rarely worth the labor: pull tension climbs fast, jam probability spikes (NEC Ch.9 Informational Note re: 2.8–3.2 conduit-ID to wire-OD ratio), and you lose room for de-rating or future moves.

Source citations

NFPA 70 (2023) — National Electrical Code. Chapter 9, Table 1: percent of cross section of conduit and tubing for conductors. Table 4: dimensions and percent area of conduit and tubing (Article 344). Table 5: dimensions of insulated conductors.