The derating formula
Adjusted ampacity = Table 310.16 ampacity × temperature correction factor × adjustment factor
The result is then capped by the termination temperature rating (usually the 75 °C column) and, for 14–10 AWG, by the 240.4(D) overcurrent limits. Full table values are in the wire ampacity chart.
Ambient temperature correction (NEC Table 310.15(B)(1))
| Ambient (°C) | 60 °C | 75 °C | 90 °C |
|---|---|---|---|
| ≤ 10 | 1.29 | 1.2 | 1.15 |
| 11–15 | 1.22 | 1.15 | 1.12 |
| 16–20 | 1.15 | 1.11 | 1.08 |
| 21–25 | 1.08 | 1.05 | 1.04 |
| 26–30 | 1 | 1 | 1 |
| 31–35 | 0.91 | 0.94 | 0.96 |
| 36–40 | 0.82 | 0.88 | 0.91 |
| 41–45 | 0.71 | 0.82 | 0.87 |
| 46–50 | 0.58 | 0.75 | 0.82 |
| 51–55 | 0.41 | 0.67 | 0.76 |
| 56–60 | — | 0.58 | 0.71 |
| 61–65 | — | 0.47 | 0.65 |
| 66–70 | — | 0.33 | 0.58 |
| 71–75 | — | — | 0.5 |
| 76–80 | — | — | 0.41 |
| 81–85 | — | — | 0.29 |
More than three current-carrying conductors (NEC Table 310.15(C)(1))
| Conductors | Adjustment |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41 and above | 35% |
Worked examples
1. Six 10 AWG THHN in a warm mechanical room
10 AWG copper THHN is 40 A in the 90 °C column. Ambient is 40 °C (factor 0.91) and there are six current-carrying conductors (80%):
40 × 0.91 × 0.80 = 29.1 A
That is below the 75 °C limit of 35 A, so 29.1 A is the ampacity. 240.4(D) would cap protection at 30 A anyway. NEC 240.4(B) allows rounding up to a 30 A breaker for a non-standard ampacity, but not on a multioutlet branch circuit supplying cord-and-plug receptacles, where you would use 25 A or upsize to 8 AWG.
2. 12 AWG through a hot attic
Three current-carrying 12 AWG THHN conductors pass through an attic at 50 °C. 30 A × 0.82 = 24.6 A, which still supports a 20 A breaker. With six conductors bundled in the same space: 30 × 0.82 × 0.80 = 19.7 A, which is under 20 A, so you would need 10 AWG or a separate run.
3. A feeder in 75 °C wire
Four 4/0 aluminum XHHW conductors (a 3-phase feeder with a neutral carrying non-linear load, so it counts): 4/0 Al 90 °C is 205 A, × 0.80 = 164 A. That fits a 150 A breaker, or 175 A under 240.4(B) where the ampacity is not a standard size and the next standard rating is used. Check whether your design meets 240.4(B)'s conditions before rounding up.
Tips to avoid derating
- Split large conduit runs so no raceway carries more than three current-carrying conductors (or six, taking an 80% hit on 90 °C wire, which is often harmless for 12 AWG).
- Avoid running cables and conduit through hot attics and across sunny roofs where possible.
- Use 90 °C insulation so you can derate from the higher column.
- Remember bundled NM cable: NEC 334.80 applies these factors when more than two cables pass through the same insulated opening without spacing.
After derating, confirm the raceway with the conduit fill calculator and the run with the voltage drop calculator.