Ampacity calculator
Start from the insulation column of NEC Table 310.16, multiply by the ambient correction of Table 310.15(B)(1) and the adjustment of Table 310.15(C)(1), then hold the result to the terminal rating (110.14(C)) and to NEC 240.4(D).
Allowable ampacity
20 A
30 A (90 C column) x 1 (ambient) x 1 (conductors) = 30 A. Terminal limit 25 A (75 C column). NEC 240.4(D) limit: 20 A.
- The table assumes not more than three current-carrying conductors in a raceway, cable or directly buried, at an ambient temperature of 30 C. More conductors or more heat call for the adjustments in 310.15; the ampacity calculator applies them.
- NEC 240.4(D) caps overcurrent protection at 15 A for 14 AWG copper, 20 A for 12 AWG copper and 30 A for 10 AWG copper, and at 15 A and 25 A for 12 and 10 AWG aluminum.
- NM-B and UF-B cable are held to the 60 C column (334.80, 340.80). Dwelling services and main feeders may use the 83 percent rule of 310.12.