Wire Ampacity Calculator
Find the safe current-carrying capacity of a copper or aluminum wire based on AWG size, ambient temperature, and number of bundled conductors. Essential for NEC-compliant circuit design.
Last updated: September 2026
Formula below · 2 sources (ieee.org, Wikipedia) · Updated Sep 2026
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About this calculator
Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. This calculator follows the NEC method for 90 °C-insulated copper building wire (THHN/THWN-2, the usual choice): start from the 90 °C column of NEC Table 310.16 (14 AWG 25 A, 12 AWG 30 A, 10 AWG 40 A, 8 AWG 55 A, 6 AWG 75 A, 4 AWG 95 A), then apply two adjustments. Ambient temperature above 30 °C: factor = √((90 − T) / 60), the NEC 310.15(B) equation (0.91 at 40 °C, 0.82 at 50 °C). More than three current-carrying conductors together: 80% for 4–6 and 70% for 7–9 (NEC Table 310.15(C)(1)). The adjusted value is then capped at the 60 °C column (14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A, 8 AWG 40 A, 6 AWG 55 A, 4 AWG 70 A), because terminations on circuits of 100 A or less are limited to 60 °C unless they are marked for 75 °C (NEC 110.14(C)(1)(a)), and because NEC 240.4(D) limits 14, 12 and 10 AWG to 15, 20 and 30 A overcurrent protection. Formula: Ampacity = min(base90 × temperature factor × bundling factor, 60 °C value). Aluminum conductors have lower ampacities; use the aluminum columns of Table 310.16.
How to use
Example: 10 AWG copper THHN, ambient temperature 40 °C, 5 current-carrying conductors in conduit. Step 1 — 90 °C base ampacity for 10 AWG: 40 A. Step 2 — temperature factor: √((90 − 40) / 60) = 0.913. Step 3 — bundling factor for 4–6 conductors: 0.8. Step 4 — adjusted: 40 × 0.913 × 0.8 = 29.2 A. Step 5 — cap at the 60 °C / 240.4(D) value of 30 A: 29.2 A is lower, so the usable ampacity is 29.2 A, and the largest breaker you may use is 25 A (NEC 240.4(B) does not allow rounding up past 30 A for 10 AWG). At 30 °C with three or fewer conductors, 12 AWG gives min(30, 20) = 20 A.
Frequently asked questions
How does ambient temperature affect wire ampacity and why does it matter?
Wire insulation has a maximum rated temperature (commonly 60 °C, 75 °C, or 90 °C). The conductor itself generates heat proportional to I²R, so the allowable current depends on how hot the surrounding environment already is. At higher ambient temperatures, less heat can dissipate before the insulation limit is reached, so ampacity must be reduced. Ignoring temperature derating can cause insulation degradation, fire risk, or nuisance tripping of protective devices.
What is the difference between copper and aluminum wire ampacity for the same AWG size?
Aluminum has higher resistivity than copper, so it carries less current for the same cross-sectional area. An aluminum conductor of the same AWG size has roughly 84% of the ampacity of its copper equivalent. In practice, aluminum wiring requires one or two AWG sizes larger to match copper's current capacity. Aluminum is common in large-feeder and service-entrance applications where its lighter weight and lower cost outweigh the need for larger conductors.
Why does bundling multiple conductors in a conduit reduce their ampacity?
When current-carrying conductors are bundled together, heat from adjacent wires accumulates and reduces each conductor’s ability to dissipate its own heat. NEC Table 310.15(C)(1) requires adjustment: 4–6 current-carrying conductors in a raceway or cable to 80%, 7–9 to 70% (and lower for more). Neutrals carrying only unbalanced current and grounding conductors are not counted. The adjustment applies to the 90 °C value, and the final ampacity still may not exceed the termination and 240.4(D) limits.