100 Amp Wire Size Calculator

The 100-ampere wire size calculator will assist you in choosing the proper wire size for your next equipment installation.

Gauge needed for a 3% dropAWG 5 or thickerlower AWG numbers are thicker wire — the scale runs backwards

Wire resistance37.7946 mΩround trip of 60 m of copper at 20 °C

Conductor diameter5.8273 mm229.42 mils, AWG 3

Cross-sectional area26.6705 mm²52,634.8 circular mils

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

Resistance per metre629.9103 µΩ/m

Voltage drop3.7795 V1.575% of the supply

Power lost in the wire377.9462 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 implied2.3622 Ω

AWG -39.4013 mΩ0.249× the resistance — six gauges down is almost exactly a quarter

AWG 018.8499 mΩ0.499× the resistance

AWG 675.7794 mΩ2.005× the resistance

AWG 9151.9401 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.