Breaker Size Calculator

The breaker size calculator will help you obtain the correct circuit breaker size for your home panel based on the electrical load.

Voltage drop3.0464 V2.539% of the supply

Wire resistance152.3223 mΩround trip of 30 m of copper at 20 °C

Conductor diameter2.0525 mm80.81 mils, AWG 12

Cross-sectional area3.3088 mm²6,529.9 circular mils

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

Resistance per metre5.0774 mΩ/m

Power lost in the wire60.9289 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

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

Load resistance implied5.8477 Ω

AWG 637.8897 mΩ0.249× the resistance — six gauges down is almost exactly a quarter

AWG 975.97 mΩ0.499× the resistance

AWG 15305.4111 mΩ2.005× the resistance

AWG 18612.3588 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.