Conduit fill for solar circuits
NEC Chapter 9 limits how much of a raceway's interior a conductor bundle may occupy, so conductors can be pulled without damage and can shed heat. The limits come from Chapter 9 Table 1; the conductor and raceway areas from Tables 4 and 5.
The fill limits (Chapter 9, Table 1)
| Conductors in the raceway | Maximum fill |
|---|---|
| 1 conductor | 53% |
| 2 conductors | 31% |
| Over 2 conductors | 40% |
Smallest EMT that fits a PV conductor bundle
| Conductor | 3 wires | 4 wires | 6 wires | 9 wires | 12 wires |
|---|---|---|---|---|---|
| 12 AWG PV wire | 1/2" (17.9%) | 1/2" (23.8%) | 1/2" (35.7%) | 3/4" (30.6%) | 1" (25.1%) |
| 10 AWG PV wire | 1/2" (24%) | 1/2" (32%) | 3/4" (27.4%) | 1" (25.3%) | 1" (33.8%) |
| 8 AWG PV wire | 3/4" (24.6%) | 3/4" (32.8%) | 1" (30.3%) | 1-1/4" (26.3%) | 1-1/4" (35.1%) |
| 6 AWG PV wire | 3/4" (33.2%) | 1" (27.3%) | 1-1/4" (23.7%) | 1-1/4" (35.5%) | 1-1/2" (34.8%) |
Computed with PV-wire outside diameters (thicker insulation than THWN-2), so a THWN-2 bundle of the same count often fits a smaller trade size.
EMT interior area and the 40% working limit
| Trade size | Total area | 40% fill (3+ conductors) |
|---|---|---|
| 1/2" | 0.304 in² | 0.122 in² |
| 3/4" | 0.533 in² | 0.213 in² |
| 1" | 0.864 in² | 0.346 in² |
| 1-1/4" | 1.496 in² | 0.598 in² |
| 1-1/2" | 2.036 in² | 0.814 in² |
| 2" | 3.356 in² | 1.342 in² |
| 2-1/2" | 5.858 in² | 2.343 in² |
| 3" | 8.846 in² | 3.538 in² |
| 4" | 14.753 in² | 5.901 in² |
Worked example: six 10 AWG PV conductors occupy 0.1458 in², so the smallest compliant EMT is 3/4" at 27.4% fill (limit 40%).
Two things people miss
- Fill and ampacity are separate checks. Passing 40% fill doesn't excuse the 310.15(C)(1) adjustment for more than three current-carrying conductors — see 310.16 ampacity and derating.
- The EGC counts for fill but is not a current-carrying conductor for the ampacity adjustment.
Conduit on a hot roof also raises ambient temperature — combine fill with the temperature correction before settling on a conductor size.
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