The fill limits (NEC Chapter 9, Table 1)
| Number of conductors | Maximum fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| 3 or more | 40% |
| Nipple 24 in (600 mm) or shorter | 60% |
Two conductors get the lowest limit because two round wires can jam side by side across the conduit during a pull.
How to calculate conduit fill
- Look up each conductor's area in NEC Chapter 9 Table 5 (for example, THHN/THWN-2: 14 AWG 0.0097 in², 12 AWG 0.0133 in², 10 AWG 0.0211 in², 8 AWG 0.0366 in²).
- Add the areas of every conductor, including equipment grounding conductors.
- Look up the conduit's internal area in Table 4 (EMT: 1/2" 0.304 in², 3/4" 0.533 in², 1" 0.864 in²).
- Divide total conductor area by conduit area. The result must not exceed the Table 1 limit.
Example: mixed sizes in 3/4" EMT
Three 10 AWG THHN, three 12 AWG THHN and one 10 AWG THHN ground:
4 × 0.0211 + 3 × 0.0133 = 0.0844 + 0.0399 = 0.1243 in²
0.1243 ÷ 0.533 = 23.3% → under 40%, OK
The same wires in 1/2" EMT would be 0.1243 ÷ 0.304 = 40.9%, which is over the limit.
Example: how many of one size fit
Allowed area = conduit area × 0.40
Number of wires = allowed area ÷ one wire's area, rounded down
For 12 AWG THHN in 1/2" EMT: 0.304 × 0.40 = 0.1216 in², and 0.1216 ÷ 0.0133 = 9.1, so 9 conductors. Informative Annex C tables list these counts for every conduit type.
Notes that change the answer
- Rounding (Note 7): when all conductors are the same size, you may round up to the next whole number if the decimal is 0.8 or more.
- Grounds count (Note 3): equipment grounding and bonding conductors are included in the fill calculation.
- Cables (Note 9): a multiconductor cable is treated as one conductor using its overall cross-section. Oval cable such as NM uses the major diameter as a circle.
- Bare conductors (Note 8): use Table 8 dimensions for bare conductors.
- Conduit type matters: PVC Schedule 80 has thinner bores than EMT, so the same trade size holds fewer wires. Always use the Table 4 row for the actual raceway.
Fill is not the only limit
Putting more than three current-carrying conductors in one raceway reduces their ampacity under NEC 310.15(C)(1): 80% for 4 to 6, 70% for 7 to 9 and 50% for 10 to 20. Run the ampacity derating calculator after you size the conduit. Also check the 360° bend limit between pull points.
Quick reference only. Confirm against the code edition your jurisdiction has adopted and your inspector.