Voltage Drop Chart

A #12 copper branch circuit at 20A/120V can run about 45 feet before it hits the 3% voltage-drop limit, and roughly 90 feet at 240V since the allowable drop doubles with the voltage. NEC does not mandate a limit, but the Informational Notes at 210.19(A) (branch circuits) and 215.2(A) (feeders) recommend keeping branch-circuit drop under 3% and combined feeder-plus-branch drop under 5% for reasonable efficiency. To size any run yourself, use the K-factor method: VD = (2 × K × I × L) / CM, where K is 12.9 for copper, I is amps, L is one-way length in feet, and CM is the conductor's circular mils (6,530 for #12), then solve for the length that keeps VD at or below 3% of your system voltage. See the chart on this page for max lengths at 3% by AWG, load, and voltage so you can pick the run without doing the math.

For a single-phase circuit, voltage drop = (2 x K x I x L) / CM, where K is the conductor resistivity constant (12.9 for copper, 21.2 for aluminum), I is the load current in amps, L is the one-way run length in feet, and CM is the conductor area in circular mils. For three-phase, replace the 2 with 1.732 (the square root of 3). Percent drop = voltage drop divided by system voltage, times 100.

The NEC recommends a maximum of 3% voltage drop on a branch circuit and 5% total for feeder plus branch circuits combined (NEC 210.19(A) and 215.2 informational notes). On a 120V circuit, 3% is 3.6V and 5% is 6.0V. These are recommendations, not enforceable requirements, but most inspectors and engineers treat 3% as the practical target. For sensitive electronics, aim for 2% or less.

On a 20A 120V copper circuit at the 3% limit, #12 AWG copper reaches about 45 feet one-way. At 240V the same wire and load reaches about 91 feet, because doubling the voltage halves the percent drop. Aluminum #12 reaches roughly 27 feet at 120V. See the copper and aluminum charts on this page for every common wire size and circuit.

Twice as far. Percent voltage drop is inversely proportional to system voltage, so at the same wire size and current, a 240V circuit has half the percent drop of a 120V circuit. That means you can run the wire about twice the distance before hitting the 3% limit. This is why long runs to detached garages, well pumps, and EV chargers are almost always wired at 240V.

Yes. Aluminum has a K-factor of 21.2 versus 12.9 for copper, so an aluminum conductor of the same size drops about 64% more voltage than copper. To match copper's run distance you generally go up about two wire sizes in aluminum. Aluminum is still common for feeders and services because it is cheaper per amp of capacity, but you must account for the higher drop on long runs.

The charts on this page are limited by voltage drop only, at the 3% recommendation. Ampacity (NEC 310.16) is a separate, independent check: a wire can pass ampacity and still fail voltage drop on a long run, or vice versa. Always size the conductor to satisfy both. Use the wire size calculator for the ampacity side with derating and terminal temperature limits.