🔌 Wire Gauge Calculator

Enter the circuit's amperage, one-way wire run distance and system voltage to check the voltage drop for common AWG wire gauges and see which ones keep drop under 3%. This is a general estimate for copper wire, not a substitute for a licensed electrician or your local electrical code.

Current (amps)

One-way distance (ft)

System voltage

How the voltage drop is worked out

Current has to travel out along one conductor and back along the other, so the resistance that matters is over twice your one-way distance. Each gauge has a resistance per 1,000 ft - the figures here are for copper - and Ohm's law turns that into volts lost.

round trip  = one-way distance x 2
resistance  = ohms per 1000 ft / 1000 x round trip
drop (V)    = amps x resistance
drop (%)    = drop / system voltage x 100

20 A over 50 ft at 120 V on 12 AWG: 100 ft round trip at 1.588 ohms per 1,000 ft is 0.159 ohms, so 3.18 V lost, or 2.65% - inside the 3% target, and 12 AWG is rated for 20 A, so it passes on both counts. Stretch that same circuit to 100 ft one way and the drop doubles to 5.3%; 10 AWG still misses at 3.3%, and you are into 8 AWG.

Reading the table

A gauge is only marked as fitting when it clears two separate tests, and they fail in different ways:

Ampacity is a safety limit - how much current the conductor can carry without overheating. 14 AWG is listed at 15 A and 12 AWG at 20 A, so 14 AWG fails a 20 A circuit at any length at all.
Voltage drop is a performance limit. It never trips a breaker; it just dims lights, slows motors and wastes power as heat in the wall. Long runs are where it bites, which is why distance pushes you up a size or two on outbuildings and well pumps.

Two things shift the answer that this page cannot see. Ampacity is reduced for high ambient temperature, for several current-carrying conductors bundled in one conduit, and for loads that run continuously - each of those means upsizing. And these resistances are copper: aluminium conductors have roughly 60% more resistance for the same gauge, so an aluminium run needs a larger size for the same result. Raising the voltage helps for free - the same 3.18 V lost is 2.65% on a 120 V circuit but only 1.3% on 240 V.

The 3% figure is a widely used design target for a branch circuit, with about 5% total from the service to the load - it is advice, not the safety limit that ampacity is. Everything binding here comes from your local electrical code and inspection regime: permitted conductor types, breaker sizing, derating and who may legally do the work. Treat this page as a sanity check before you talk to a licensed electrician, not as a substitute for one.

Frequently asked questions

What gauge wire do I need for a 20 amp circuit?

12 AWG copper. 14 AWG is only rated to 15 A, so it is out regardless of the run length. On runs past about 55 ft at 120 V you may want 10 AWG anyway, to hold the voltage drop under 3%.

How far can I run 12 gauge wire on a 20 amp circuit?

About 57 ft one way at 120 V before the drop passes 3%. At 240 V the same wire and current reach roughly twice as far, because the volts lost are the same but the percentage is halved.

Why is the distance doubled in the calculation?

Because current makes a round trip. It flows out on the hot conductor and back on the neutral, so a 50 ft run means 100 ft of copper in the circuit, and both directions drop voltage.