4" FMC Conduit Fill Chart
How many wires fit in 4" 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). 4" FMC interior area = 12.5660 in².
| AWG | THHN / THWN-2 | XHHW-2 | RHH / RHW-2 |
|---|---|---|---|
| #14 | 518 | 518 | 240 |
| #12 | 377 | 377 | 193 |
| #10 | 238 | 238 | 150 |
| #8 | 137 | 137 | 90 |
| #6 | 99 | 99 | 69 |
| #4 | 61 | 61 | 51 |
| #3 | 51 | 51 | 44 |
| #2 | 43 | 43 | 37 |
| #1 | 32 | 32 | 26 |
| #1/0 | 27 | 27 | 22 |
| #2/0 | 22 | 22 | 19 |
| #4/0 | 15 | 15 | 13 |
WORKED EXAMPLE & NEC REFERENCES
The NEC numbers behind 4" FMC
NEC 2023 Chapter 9, Table 4 lists the interior cross-sectional area of every standard trade size for each conduit family. For 4" FMC (Article 348), the total 100% area is 12.5660 in². The allowable conductor fill is then derived from Table 1: 53% for a single conductor (6.6600 in²), 31% for two (3.8955 in²), and 40% for three or more (5.0264 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 ÷ 12.5660 = 0.4% of total area. Because that is well below the 40% NEC limit for 5.0264 in², the pull is compliant — and you can keep adding conductors up to 377× #12 THHN before exceeding the cap.
If you exceed the fill cap
Step up one trade size. Increase a 4" FMC to the next available trade size 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.