Wire Ampacity and Current Rating Calculator

What Is Ampacity? Ampacity is the maximum current a conductor can carry continuously without exceeding its rated temperature. Exceeding the ampacity causes overheating, insulation degradation, and potentially fire. The National Electrical Code (NEC) publishes ampacity tables based on laboratory testing and engineering analysis. All wiring in residential, commercial, and industrial buildings in the US must comply with NEC ampacity limits.

NEC Table 310.15(B)(16) This table is the primary reference for ampacity of insulated conductors in conduit or raceways. Values are given for three insulation temperature ratings: 60°C, 75°C, and 90°C. Higher temperature-rated insulation allows more current for the same conductor size. However, the limiting factor is often the device terminal rating — most breakers and outlets are rated for 60°C or 75°C only. In practice: use 75°C values for most applications, but do not exceed terminal ratings.

Temperature Rating Selection 60°C (140°F) insulation: older wiring types, some residential applications. 75°C (167°F) insulation: THWN, XHHW — most common in modern commercial and industrial wiring. 90°C (194°F) insulation: THHN, XHHW-2 — allows highest base ampacity, but you must check terminal ratings. The 90°C column may be used for derating calculations even if terminations limit the final value to the 75°C column.

Installation Method: Conduit vs. Free Air Heat dissipates better in open air than inside conduit surrounded by other wires. Free air ampacity is approximately 130% of conduit values for the same conductor. This calculator applies that factor when “Free air” is selected.

Conductor Count Derating When multiple current-carrying conductors share the same raceway, they generate cumulative heat. NEC Article 310.15(C) requires derating based on the number of conductors: 3 or fewer conductors: no derating (base table values apply). 4–6 conductors: multiply by 0.80 (reduce ampacity by 20%). 7–9 conductors: multiply by 0.70 (reduce by 30%). 10–20 conductors: multiply by 0.50 (reduce by 50%). Neutral conductors in balanced three-phase systems do not count for derating purposes.

Ambient Temperature Correction NEC Table 310.15(B)(2)(a) provides correction factors for ambient temperatures other than 30°C (86°F). When the ambient temperature exceeds 30°C, the allowable current decreases — less temperature differential is available for heat dissipation. The correction factor is approximately: CF = √((T_rated − T_ambient) / (T_rated − 30)) Where T_rated is the conductor insulation temperature rating.

Applying All Correction Factors Final corrected ampacity = Base Ampacity × Ambient Correction Factor × Conductor Count Factor All three adjustments are multiplicative — apply all that are relevant. The corrected ampacity is the maximum safe current for the specific installation conditions. Always select a conductor size where the corrected ampacity exceeds the design load current.