4" FMC Conduit Fill
How many wires fit in 4" FMC conduit? Maximum counts for every wire size from #14 to 500 kcmil, with worked examples, under NEC 2026.
Updated
For reference only. Always verify with the NEC edition adopted by your AHJ.
Most wires in 4" FMC
The maximum number of conductors of one type and size that fit in 4" FMC conduit. #12 THHN is the one most people look for: 378.
| Wire size | THHN / THWN-2 | XHHW-2 | THW |
|---|---|---|---|
| #14 | 518 | 361 | 361 |
| #12 | 378 | 277 | 277 |
| #10 | 238 | 207 | 207 |
| #8 | 137 | 115 | 115 |
| #6 | 99 | 85 | 69 |
| #4 | 61 | 61 | 51 |
| #3 | 51 | 52 | 44 |
| #2 | 43 | 44 | 37 |
| #1 | 32 | 32 | 26 |
| #1/0 | 27 | 27 | 22 |
| #2/0 | 22 | 23 | 19 |
| #3/0 | 18 | 19 | 16 |
| #4/0 | 15 | 15 | 13 |
| 250 kcmil | 12 | 13 | 11 |
| 300 kcmil | 11 | 11 | 9 |
| 350 kcmil | 9 | 9 | 8 |
| 400 kcmil | 8 | 8 | 7 |
| 500 kcmil | 7 | 7 | 6 |
— means a single conductor of that size does not fit.
Worked examples
Most #12 THHN wires
The largest number of #12 THHN/THWN-2 conductors that fits, and the next one up.
- PASS378 × #12 THHN / THWN-240.0% filled, limit 40%
- FAIL379 × #12 THHN / THWN-240.1% filled, limit 40%
Largest four-wire THHN feeder: 500 kcmil
Three hots and a neutral of one size: this is the largest wire size in the chart, and it fits.
- PASS4 × 500 kcmil THHN / THWN-222.5% filled, limit 40%
Most 500 kcmil THHN wires
500 kcmil is the biggest THHN size where this conduit still holds 6 or more, enough for a couple of multi-wire circuits. Shown at the maximum and one over.
- PASS7 × 500 kcmil THHN / THWN-239.4% filled, limit 40%
- FAIL8 × 500 kcmil THHN / THWN-245.0% filled, limit 40%
Equipment whip in flex
A short flex connection to a motor or fixture: four #12 THHN in all.
- PASS4 × #12 THHN / THWN-20.4% filled, limit 40%
How 4" FMC fill is worked out
Table 4 gives 4" FMC a total inside area of 12.566 square inches. Three or more wires may fill 40% of that, 5.026 square inches. One wire may use 53% (6.660) and two wires 31% (3.895).
Each wire takes up its Table 5 area, which depends on insulation and size. Divide the allowed area by the area of one wire and you get the counts above. When every wire is the same size and the result ends in .8 or more, Note 7 lets you round up to the next whole wire.
Fill examples show wire counts only; for equipment ground sizes, see NEC 2026 Table 250.122. Mixed wire sizes add up area by area rather than by count. The calculator does that for you.
4" FMC questions
- How many #12 THHN wires fit in 4" FMC conduit?
- Up to 378 #12 THHN/THWN-2 conductors fit in 4" FMC under NEC 2026 Chapter 9 (40% fill for three or more wires).
- How many #10 THHN wires fit in 4" FMC conduit?
- Up to 238 #10 THHN/THWN-2 conductors fit in 4" FMC under NEC 2026 Chapter 9 (40% fill for three or more wires).
- How many #14 THHN wires fit in 4" FMC conduit?
- Up to 518 #14 THHN/THWN-2 conductors fit in 4" FMC under NEC 2026 Chapter 9 (40% fill for three or more wires).
Keep going
You can read the NEC 2026 for free at NFPA free access. Need a different combination? Use the conduit fill calculator.