The derating formula
Adjusted ampacity = table ampacity × temperature correction factor × conductor adjustment factor
Start from the column that matches the conductor insulation rating. The adjusted value must still meet the terminal limit of NEC 110.14(C) and the small-conductor limits of 240.4(D).
Ambient temperature correction (NEC Table 310.15(B)(1))
| Ambient | 60 °C wire | 75 °C wire | 90 °C wire |
|---|---|---|---|
| 10 °C or less | 1.29 | 1.20 | 1.15 |
| 11–15 °C | 1.22 | 1.15 | 1.12 |
| 16–20 °C | 1.15 | 1.11 | 1.08 |
| 21–25 °C | 1.08 | 1.05 | 1.04 |
| 26–30 °C | 1.00 | 1.00 | 1.00 |
| 31–35 °C | 0.91 | 0.94 | 0.96 |
| 36–40 °C | 0.82 | 0.88 | 0.91 |
| 41–45 °C | 0.71 | 0.82 | 0.87 |
| 46–50 °C | 0.58 | 0.75 | 0.82 |
| 51–55 °C | 0.41 | 0.67 | 0.76 |
| 56–60 °C | n/a | 0.58 | 0.71 |
| 61–65 °C | n/a | 0.47 | 0.65 |
| 66–70 °C | n/a | 0.33 | 0.58 |
| 71–75 °C | n/a | n/a | 0.50 |
| 76–80 °C | n/a | n/a | 0.41 |
| 81–85 °C | n/a | n/a | 0.29 |
Rooftop runs: under NEC 310.15(B)(2), raceways or cables exposed to sunlight less than 7/8 in (23 mm) above a roof add 33 °C (60 °F) to the outdoor temperature before you pick the factor. XHHW-2 conductors are exempt.
More than three current-carrying conductors (NEC Table 310.15(C)(1))
| Current-carrying conductors | Percent of ampacity |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41 and above | 35% |
Count phase conductors, plus neutrals that carry current (see the FAQ below). Do not count equipment grounding conductors or spare conductors.
Example 1: six 12 AWG THHN in one conduit
12 AWG copper THHN is 30 A in the 90 °C column. Six current-carrying conductors give an 80% factor: 30 × 0.80 = 24 A. That is above 20 A, so each circuit can still be protected at 20 A under 240.4(D).
Example 2: nine 12 AWG THHN in a 40 °C attic
30 A × 0.91 (40 °C, 90 °C wire) × 0.70 (nine conductors) = 19.1 A. That is below 20 A, so the circuits need a 15 A breaker or 10 AWG conductors. 10 AWG gives 40 × 0.91 × 0.70 = 25.5 A, enough for 20 A circuits.
Example 3: 3 AWG copper feeder on a hot roof
On a 35 °C day, a conduit 1/2 in above the roof sees 35 + 33 = 68 °C. The 90 °C factor is 0.58: 115 A × 0.58 = 66.7 A. Raising the conduit 7/8 in or more above the roof, or using XHHW-2, removes the 33 °C adder.
In Canada
The CEC uses the same idea with its own tables: Table 5A for ambient temperature correction and Table 5C for more than three conductors in a raceway or cable, applied through Rule 4-004. Check the factors in the edition your province has adopted.
Try the numbers in the derating calculator, or let the wire size calculator pick the conductor.
This guide is a quick reference. Always confirm against the code edition your jurisdiction has adopted, the equipment markings and your inspector.