Calculate cable ampacity derating for ambient temperatures above 30°C per NEC Table 310.15(B)(1).
Rated ampacity at standard 30°C ambient per NEC Table 310.16. This is the maximum current the cable can safely carry under standard conditions.
A
Actual ambient temperature where the cable is installed. NEC base is 30°C. Attics, rooftops, and engine rooms often exceed this significantly.
°C
Maximum insulation temperature. 60°C: TW, UF. 75°C: THW, THWN, XHHW. 90°C: THHN, THWN-2, XHHW-2. Higher rating allows more current at elevated temperatures.
Per NEC 310.15(C)(1): 4-6 conductors=80%, 7-9=70%, 10-20=50%, 21-30=45%, 31-40=40%, 41+=35% derating when bundled in raceway.
Results
Temperature Derating Factor-
Conductor Fill Derating Factor-
Combined Derating Factor-
Derated Ampacity-A
Ampacity Reduction-A
References:
Temperature derating: sqrt((T_rated - T_ambient) / (T_rated - 30)) per NEC Table 310.15(B)(1). Conductor fill derating per NEC 310.15(C)(1). Both factors multiply together. When ambient exceeds insulation rating, cable cannot be used.