How conduit fill works
The NEC limits how much of a conduit's cross-section wires may occupy, so they can be pulled without damage and can shed heat. Add up the area of every conductor, including grounds, and compare it with the allowed percentage of the conduit's internal area.
Example: three #6 THHN + one #8 ground in EMT
3 × 0.0507 + 0.0366 = 0.1887 in². 1/2 in EMT allows 0.304 × 40% = 0.122 in² (too small). 3/4 in EMT allows 0.533 × 40% = 0.213 in², so 3/4 in EMT works at 35.4% fill.
THHN wire areas (NEC Ch. 9, Table 5)
| Size | Area (in²) | Size | Area (in²) |
|---|---|---|---|
| #14 | 0.0097 | #2 | 0.1158 |
| #12 | 0.0133 | #1/0 | 0.1855 |
| #10 | 0.0211 | #2/0 | 0.2223 |
| #8 | 0.0366 | #3/0 | 0.2679 |
| #6 | 0.0507 | #4/0 | 0.3237 |
| #4 | 0.0824 | 250 kcmil | 0.3970 |
EMT internal area (Table 4)
1/2 in: 0.304 · 3/4 in: 0.533 · 1 in: 0.864 · 1-1/4 in: 1.496 · 1-1/2 in: 2.036 · 2 in: 3.356 in².
Frequently asked questions
How many #12 wires fit in 1/2 inch EMT?
Nine #12 THHN conductors (0.304 × 40% ÷ 0.0133 = 9.1).
Do grounding wires count toward conduit fill?
Yes. Equipment grounding and bonding conductors count toward fill.
Does conduit fill change ampacity?
Yes. More than three current-carrying conductors in a raceway require ampacity adjustment under NEC 310.15(C)(1), separate from the fill limit.