Amp to Wire Size Calculator

The amp to wire size calculator will help you choose the proper wire size for your next equipment installation.

Gauge needed for a 3% dropAWG 12 or thickerlower AWG numbers are thicker wire — the scale runs backwards

Wire resistance127.7286 mΩround trip of 40 m of copper at 20 °C

Conductor diameter2.5882 mm101.9 mils, AWG 10

Cross-sectional area5.2612 mm²10,383 circular mils

Resistivity at 20 °C16.8 nΩ·m0% above the 20 °C figure

Resistance per metre3.1932 mΩ/m

Voltage drop3.8319 V1.597% of the supply

Power lost in the wire114.9557 Wall of it as heat, along the whole run

Against a 3% drop limitwithin limit3% on a branch circuit is the usual recommendation, 5% overall

Load resistance implied7.8723 Ω

AWG 431.7721 mΩ0.249× the resistance — six gauges down is almost exactly a quarter

AWG 763.704 mΩ0.499× the resistance

AWG 13256.0999 mΩ2.005× the resistance

AWG 16513.4883 mΩ4.02× the resistance

The formula

R = ρL ÷ A; AWG diameter = 0.127 mm × 92^((36−n)/39)

The gauge scale runs backwards, and geometrically

American Wire Gauge numbers get smaller as the wire gets thicker, and the series is geometric: 36 AWG is 0.005 inches, and each 39 gauges spans a factor of 92 in diameter. Two useful consequences fall out of that. Three gauges down doubles the cross-sectional area, and six gauges down quadruples it — so six gauges down quarters the resistance, almost exactly.

Because the formula is exact, this page computes diameters rather than looking them up. Published tables round, and rounded diameters get squared into areas, which compounds the error.

Voltage drop is usually what sizes a cable, not the current rating. A long run at modest current can easily exceed the customary 3% limit while remaining far below the wire's thermal capacity, and the fix is a thicker conductor. Remember the current goes out and comes back, so the resistance that matters is the round trip — using the one-way length halves the answer.

Temperature matters more than people expect. Copper's resistivity rises about 0.4% per degree, so a cable running at 75 °C has roughly 22% more resistance than the 20 °C table value. Sizing from cold figures understates the drop under load.