Voltage Drop Chart per AWG (20 A, 100 ft, 120 V)

A quick-look reference for the 3% rule

This chart shows the voltage a conductor loses to its own resistance for one fixed scenario — 20 amps over a 100-foot one-way run on a 120 V single-phase copper circuit — across the common gauges, so you can see at a glance where a wire crosses the 3% guideline. Read it as a sanity check: if your circuit is close to these conditions, the gauge whose drop sits just under 3% (about 3.6 V on 120 V) is roughly what you need. Notice how quickly the loss falls as the wire gets thicker, because voltage drop is inversely proportional to cross-sectional area.

Your real circuit will rarely match the chart exactly, and three levers move the result. Distance is measured one-way (the formula already doubles it for the return path) and scales the drop linearly. Higher system voltage shrinks the percentage for the same absolute loss, which is why 240 V and 12 V behave so differently. And aluminum drops about 1.6 times as much as copper for the same gauge. For anything other than the reference case, compute your own figure with the Voltage Drop calculator, or size directly for a target with the Wire Size calculator.

The 3% branch-circuit figure is a widely used design guideline, not a hard NEC limit in most cases; your authority having jurisdiction may require a stricter target. Read the 3% voltage-drop rule explained for the full method.