Wire Ampacity Calculator - NEC Table 310.16

Ampacity is the maximum current a conductor can carry continuously under specific conditions without exceeding its temperature rating. The National Electrical Code establishes base ampacity values in Table 310.16 (now 310.15(B)(16)), but actual installation conditions often require derating these values. Understanding ampacity calculations and derating factors is essential for safe electrical installations that prevent conductor overheating and maintain system integrity.

Temperature correction factors apply when ambient temperature exceeds the standard 30°C (86°F) assumption. Higher temperatures reduce a conductor's heat dissipation capacity, requiring larger wire sizes or reduced current loading. NEC Table 310.15(B)(2)(a) provides temperature correction multipliers: for example, at 40°C ambient with 75°C rated conductors, apply a 0.88 correction factor.

Conductor fill adjustment factors account for heat buildup when multiple current-carrying conductors share a raceway or cable. More than three conductors require derating per NEC Table 310.15(B)(3)(a): four to six conductors use 80%, seven to nine use 70%, and ten to twenty use 50% of base ampacity. Neutral conductors carrying only unbalanced current typically don't count, but in certain harmonic-rich environments, neutrals must be counted as current-carrying conductors.