Derating for more than three wires
Fill tells you how many wires fit. Derating tells you what each may carry once they're in there. With more than three current-carrying conductors in one raceway, their ampacity is adjusted by Table 310.15(C)(1).
| Current-carrying conductors | Percent of Table 310.16 |
|---|---|
| 4 to 6 | 80 % |
| 7 to 9 | 70 % |
| 10 to 20 | 50 % |
| 21 to 30 | 45 % |
| 31 to 40 | 40 % |
| 41 and above | 35 % |
What counts as current-carrying
- Every ungrounded (hot) conductor.
- A neutral that carries only the unbalanced current of its circuit doesn't count (310.15(E)). On a 4-wire wye with mostly nonlinear loads, it does.
- Equipment grounds never count.
THHN copper, adjusted
THHN's 90 °C ampacity is the starting point for derating; the result still can't exceed what the terminals allow (usually the 75 °C column) or the small-conductor limits of 240.4(D).
| Size | 90 °C | 4-6 | 7-9 | 10-20 | 21-30 | 31-40 | 41+ |
|---|---|---|---|---|---|---|---|
| #14 | 25 | 20 | 17.5 | 12.5 | 11.3 | 10 | 8.8 |
| #12 | 30 | 24 | 21 | 15 | 13.5 | 12 | 10.5 |
| #10 | 40 | 32 | 28 | 20 | 18 | 16 | 14 |
| #8 | 55 | 44 | 38.5 | 27.5 | 24.8 | 22 | 19.3 |
| #6 | 75 | 60 | 52.5 | 37.5 | 33.8 | 30 | 26.3 |
| #4 | 95 | 76 | 66.5 | 47.5 | 42.8 | 38 | 33.3 |
| #3 | 115 | 92 | 80.5 | 57.5 | 51.8 | 46 | 40.3 |
| #2 | 130 | 104 | 91 | 65 | 58.5 | 52 | 45.5 |
| #1 | 145 | 116 | 101.5 | 72.5 | 65.3 | 58 | 50.8 |
| #1/0 | 170 | 136 | 119 | 85 | 76.5 | 68 | 59.5 |
| #2/0 | 195 | 156 | 136.5 | 97.5 | 87.8 | 78 | 68.3 |
| #3/0 | 225 | 180 | 157.5 | 112.5 | 101.3 | 90 | 78.8 |
| #4/0 | 260 | 208 | 182 | 130 | 117 | 104 | 91 |
Amps, 30 °C ambient. Other ambient temperatures take the correction factors of 310.15(B) as well.