Ambient Temperature Correction Factors

Multipliers applied to ampacities based on a 30°C (86°F) ambient. A dash means that insulation rating is not listed at that ambient — the conductor cannot be used there.
Ambient temperature	60°C conductor	75°C conductor	90°C conductor
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	—	0.58	0.71
61–65°C	—	0.47	0.65
66–70°C	—	0.33	0.58
71–75°C	—	—	0.50
76–80°C	—	—	0.41
81–85°C	—	—	0.29

Source: NFPA 70, National Electrical Code, Table 310.15(B)(1)(1). Published by NFPA.NEC 2023

Why ambient temperature changes ampacity

Current limits are heat limits. A conductor is rated for the current at which its insulation reaches its temperature rating, and that depends on how much cooler the surroundings are. At 30°C ambient a 75°C conductor has 45°C of headroom to dissipate heat into. At 50°C ambient it has 25°C — barely half — so it carries substantially less.

Below the 30°C basis the factors exceed 1.00, which means a conductor in a cold location may legitimately carry more than its table value. That direction is rarely used in practice, because ambient can change and few designers want a circuit that depends on the weather.

Where this bites

Attics, rooftops (where solar gain can add substantially to the measured air temperature), boiler and mechanical rooms, and equipment enclosures. Rooftop conductor runs in particular have their own provisions worth knowing about.

Apply this factor and the bundling adjustment together — they multiply. The ampacity lookup does both and shows each step.

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Ambient Temperature Correction Factors

Why ambient temperature changes ampacity

Where this bites

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