Ampacity Calculator
Look up the base ampacity of a conductor from NEC Table 310.16, apply the ambient-temperature correction factor (310.15(B)(1)) and the more-than-three-conductors adjustment factor (310.15(B)(3)(a)), then check the result against the termination temperature rating per 110.14(C). The lowest of these limits is the usable ampacity.
Standard reference is 30°C (86°F).
Adjustment kicks in above 3 conductors (310.15(B)(3)(a)).
Base (Table 310.16)
25 A
Temp CF
1.00
Count CF
1.00
Usable Ampacity
25.0 A
Calculation
Adjusted = 25 × 1.00 × 1.00 = 25.0 A
How the calculation works
Adjusted ampacity = Base · Ambient CF · Count CF Usable ampacity = min( Adjusted, Termination-limit )
- Base — NEC Table 310.16 entry for the conductor at its insulation rating (60/75/90°C).
- Ambient CF — NEC Table 310.15(B)(1), based on conductor insulation rating.
- Count CF — NEC 310.15(B)(3)(a) — kicks in for more than 3 current-carrying conductors.
- Termination limit — NEC 110.14(C). Conductor cannot be loaded above the rating of the column matching the lug/breaker temperature (typically 60°C for ≤100 A, 75°C for >100 A).
Test your knowledge
Practice ampacity questions and conductor sizing:
Educational tool only. Verify against the current NEC. Not a substitute for a licensed engineer.