📏 How Far Can You Run 12 Gauge Wire?
Real distance limits for every common gauge — and why every site gives you a different number.
The short answer
On a fully loaded 20 amp, 120 V circuit, #12 copper stays under the 3% voltage-drop target for about 46 one-way feet. At 240 V the same wire goes about 91 feet. Lighter actual loads go further — a 20 A breaker feeding a 10 A load doubles the distance. Full table below; your exact case: Max Wire Length calculator.
Distance table (3% drop, copper, fully loaded)
| Wire | At amps | Max one-way @120 V | Max one-way @240 V |
|---|---|---|---|
| 14 AWG | 15 A | 38 ft | 76 ft |
| 12 AWG | 15 A | 61 ft | 121 ft |
| 12 AWG | 20 A | 46 ft | 91 ft |
| 10 AWG | 30 A | 48 ft | 97 ft |
| 8 AWG | 40 A | 58 ft | 115 ft |
| 6 AWG | 50 A | 73 ft | 146 ft |
Computed with our verified engine (K = 12.9 ohm·cmil/ft for copper). Distances are one-way — from panel to load; the round trip is already accounted for.
Why other sites say 50, 57, or 100 feet
Three honest reasons, no conspiracy. First — the amps assumption. Should you compute at the breaker's 20 A or the actual load's amps? Forums split 50/50 on this. Our table uses the breaker rating (worst case, safe for planning); if you know the real load, use it in the calculator and enjoy the extra distance. Second — the resistance constant. We use the field-standard K = 12.9, which is deliberately a bit conservative; some sites use the code's DC resistance table, which yields ~25% longer distances (that's the famous "57 feet"). Neither is wrong — ours leaves margin for real-world conditions. Third — the target. 3% is the common planning target, but it's a recommendation, not a hard law (full story here) — sites using 5% quote much longer runs.
What happens if you run too far?
Nothing dramatic — and that's the trap. The circuit works, but the far end runs at lower voltage: motors start hard and run hot (a compressor that trips on a 100 ft extension cord but not a short one is pure voltage drop), LEDs flicker, chargers slow down, heaters underperform. Long runs also waste real energy as heat in the wall.
Worked example
#12 copper, 20 A, 80 one-way feet, 120 V: drop = 2 × 12.9 × 20 × 80 ÷ 6,530 = 6.3 V = 5.3% — over both the 3% target and the 5% outer limit. Options: shorten the run, go up to #10 (drops 4.0 V = 3.3%), run it at 240 V if the load allows, or accept it for a non-critical load. The Voltage Drop calculator shows all of this instantly.
Related
Max Wire Length calculator · Voltage drop, explained · Ampacity chart