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.
About this calculator
Ampacity is the maximum continuous current a conductor can carry without exceeding its temperature rating. The base ampacity depends on wire gauge (AWG) and material: for example, 12 AWG copper has a base of 30 A. Aluminum carries about 84% of the current of the same AWG copper wire. Two correction factors are then applied. The temperature correction factor reduces capacity as ambient temperature rises above the 30 °C baseline: factor ≈ 1 − (T − 30) × 0.005. The bundling (fill) correction applies when multiple conductors share a conduit: ≤3 conductors → no derating; 4–6 conductors → multiply by 0.8; 7 or more → multiply by 0.7. The combined formula is: Ampacity = base_ampacity × material_factor × temperature_factor × bundling_factor. These rules align with NEC Table 310.15.
How to use
Example: 10 AWG copper wire, ambient temperature 40 °C, 5 conductors in conduit. Step 1 — base ampacity for 10 AWG copper: 40 A. Step 2 — material factor for copper: 1.0. Step 3 — temperature correction: 1 − (40 − 30) × 0.005 = 1 − 0.05 = 0.95. Step 4 — bundling factor for 5 conductors (4–6 range): 0.8. Step 5 — multiply: 40 × 1.0 × 0.95 × 0.8 = 30.4 A. The wire safely carries 30.4 A under these conditions, down from its 40 A base rating.
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 conductors are bundled together, heat from adjacent wires accumulates and reduces each conductor's ability to dissipate its own heat. The NEC requires derating to compensate: 4–6 current-carrying conductors in a raceway derate to 80%, and 7 or more derate to 70% of the base ampacity. Proper conduit fill calculations and ampacity derating prevent overheating, which is one of the leading causes of electrical fires in building wiring.