How far can I run 12 gauge wire on a 20 amp circuit?
About 47 feet at 120 V, or 93 feet at 240 V, before 12 AWG copper exceeds the 3% voltage-drop recommendation at 20 amps.
12 AWG copper is 1.93 ohms per 1,000 ft. At 120 V the 3% allowance is 3.6 V, and 3.6 × 1,000 ÷ (2 × 20 × 1.93) = 46.6 ft. Doubling the voltage doubles the allowable drop in volts, so it doubles the distance.
Going up a size buys real distance: 10 AWG reaches about 74 ft at 120 V and 8 AWG about 116 ft. The 3% figure is a recommendation from the informational notes to NEC 210.19(A), not enforceable code — an inspector will not fail you on it.
| Load | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
|---|---|---|---|---|---|
| 10 A | 58 ft | 93 ft | 148 ft | 231 ft | 366 ft |
| 15 A | 39 ft | 62 ft | 99 ft | 154 ft | 244 ft |
| 20 A | n/a | 46 ft | 74 ft | 115 ft | 183 ft |
| 30 A | n/a | n/a | 49 ft | 77 ft | 122 ft |
| 40 A | n/a | n/a | n/a | 57 ft | 91 ft |
| Load | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 4 AWG |
|---|---|---|---|---|---|
| 20 A | 93 ft | 148 ft | 231 ft | 366 ft | 584 ft |
| 30 A | n/a | 99 ft | 154 ft | 244 ft | 389 ft |
| 40 A | n/a | n/a | 115 ft | 183 ft | 292 ft |
| 50 A | n/a | n/a | 92 ft | 146 ft | 233 ft |
| 60 A | n/a | n/a | n/a | 122 ft | 194 ft |
Why doubling the voltage doubles the distance
The 3% allowance is a percentage of the supply voltage, so at 240 V you are allowed 7.2 V of drop against 3.6 V at 120 V. The conductor's resistance has not changed, so twice the allowable drop means twice the run at the same current.
This is the single strongest argument for running 240 V to a detached garage or workshop rather than 120 V. The same conductor reaches twice as far, and for the same power delivered the current halves as well — which doubles the distance again. Two hundred and forty volts gets you roughly four times the reach for the same wire and the same load in watts.
Reading the chart
Find your load current in the left column and read across to your conductor size. The figure is the maximum one-way distance — panel to load — that stays inside 3% drop at 120 V or 240 V single phase, copper.
Three-phase runs go further for the same conductor, because √3 is smaller than 2 — multiply the figures here by 2/√3, about 1.155.
Aluminium goes roughly 62% as far as copper at the same size, being about 1.6 times the resistance.
Two situations to be careful about
Motors. Starting current is several times running current, so the drop during start-up is several times what this chart shows. A motor that will not start at the end of a long run is usually a voltage-drop problem, and the running-current calculation will not have predicted it.
Low voltage. At 12 V, 3% is 0.36 V. Landscape lighting and solar runs hit the limit at remarkably short distances, and the same absolute drop that is trivial at 240 V is fatal at 12 V. Low-voltage runs are almost always sized on voltage drop rather than ampacity.
Common questions
How far can I run 12 AWG on a 20 amp circuit?
About 47 feet at 120 V, or 93 feet at 240 V, before exceeding 3% voltage drop.
How far can I run 10 AWG at 30 amps?
About 50 feet at 120 V and 99 feet at 240 V for 3% drop.
Should I run 120 V or 240 V to a detached garage?
240 V, almost always. It reaches roughly four times as far for the same conductor and the same load in watts — twice from the larger allowable drop and twice again from the halved current.
Does three-phase go further than single phase?
Yes, about 15.5% further for the same conductor and current, because the multiplier is √3 (1.732) rather than 2.