Ambient temperature correction factors
Conductor ampacity is published at a 30 °C (86 °F) ambient. A rooftop in summer is far hotter, so NEC 310.15(B)(1) applies a correction factor that reduces usable ampacity — the single most-missed derate on a PV plan check.
Temperature correction factors (based on 30 °C ambient)
| Ambient temperature | 60 °C insulation | 75 °C | 90 °C |
|---|---|---|---|
| ≤ 10 °C (50 °F) | 1.29 | 1.2 | 1.15 |
| ≤ 15 °C (59 °F) | 1.22 | 1.15 | 1.12 |
| ≤ 20 °C (68 °F) | 1.15 | 1.11 | 1.08 |
| ≤ 25 °C (77 °F) | 1.08 | 1.05 | 1.04 |
| ≤ 30 °C (86 °F) | 1 | 1 | 1 |
| ≤ 35 °C (95 °F) | 0.91 | 0.94 | 0.96 |
| ≤ 40 °C (104 °F) | 0.82 | 0.88 | 0.91 |
| ≤ 45 °C (113 °F) | 0.71 | 0.82 | 0.87 |
| ≤ 50 °C (122 °F) | 0.58 | 0.75 | 0.82 |
| ≤ 55 °C (131 °F) | 0.41 | 0.67 | 0.76 |
| ≤ 60 °C (140 °F) | — | 0.58 | 0.71 |
| ≤ 65 °C (149 °F) | — | 0.47 | 0.65 |
| ≤ 70 °C (158 °F) | — | 0.33 | 0.58 |
| ≤ 75 °C (167 °F) | — | — | 0.5 |
| ≤ 80 °C (176 °F) | — | — | 0.41 |
How it stacks
Corrected ampacity is: table ampacity × temperature correction × conduit-fill adjustment. Both derates apply from the conductor's own insulation rating (usually 90 °C for THWN-2/PV wire) — but the final result is still capped by the 110.14 termination rating, normally 75 °C.
Rooftop ambient in practice
Many jurisdictions expect a rooftop adder above the local 2% design high — conduit in direct sun above a dark roof runs materially hotter than the shaded air temperature. Confirm the ambient your AHJ expects; our permit guides by jurisdiction list the design criteria we hold for each.
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