3-1/2" FMC Conduit Fill Chart
How many wires fit in 3-1/2" FMC (Flexible Metal Conduit). Maximum conductor counts per AWG at the NEC 2023 40% fill cap for three or more conductors.
MAXIMUM CONDUCTORS BY WIRE GAUGE
40% fill (3+ conductors per NEC 2023 Ch.9 Table 1). 3-1/2" FMC interior area = 9.6210 in².
| AWG | THHN / THWN-2 | XHHW-2 | RHH / RHW-2 |
|---|---|---|---|
| #14 | 396 | 396 | 184 |
| #12 | 289 | 289 | 148 |
| #10 | 182 | 182 | 115 |
| #8 | 105 | 105 | 69 |
| #6 | 75 | 75 | 53 |
| #4 | 46 | 46 | 39 |
| #3 | 39 | 39 | 33 |
| #2 | 33 | 33 | 28 |
| #1 | 24 | 24 | 20 |
| #1/0 | 20 | 20 | 17 |
| #2/0 | 17 | 17 | 14 |
| #4/0 | 11 | 11 | 10 |
WORKED EXAMPLE & NEC REFERENCES
The NEC numbers behind 3-1/2" FMC
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" FMC (Article 348), the total 100% area is 9.6210 in². The allowable conductor fill is then derived from Table 1: 53% for a single conductor (5.0991 in²), 31% for two (2.9825 in²), and 40% for three or more (3.8484 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 ÷ 9.6210 = 0.6% of total area. Because that is well below the 40% NEC limit for 3.8484 in², the pull is compliant — and you can keep adding conductors up to 289× #12 THHN before exceeding the cap.
If you exceed the fill cap
Step up one trade size. Increase a 3-1/2" FMC 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 348). Table 5: dimensions of insulated conductors.