Voltage Drop Calculator

How to use the voltage drop calculator

Enter the current the circuit carries in amps, the one-way distance from the panel to the load in feet, the wire gauge, and the supply voltage — 120 V or 240 V — and you get the voltage lost along the run, the percentage that loss represents, the voltage that actually reaches the load, and a verdict on whether the run is sized well. The defaults model a common case: 20 A over 100 ft of 12-gauge copper. Copper is assumed throughout, which is what most branch wiring is.

Wire is not a perfect conductor; it has resistance, and every foot of it bleeds off a little voltage as the current pushes through. The thicker the wire, the lower the resistance per foot, so the gauge selector is the main lever you have. Twelve-gauge copper is 1.93 ohms per 1000 ft; step down to 10-gauge and that drops, step up to 14-gauge and it climbs. Picking the gauge here lets you see, before you pull any wire, whether a given size will hold the voltage where it needs to be.

This is the “why does my shed, garage, or detached-building outlet read low or run my tools weakly” tool. A circuit can be perfectly safe and still deliver soft voltage at the far end if the run is long and the wire is thin. Motors run hot and lose torque on low voltage, lights dim, and electronics misbehave — and the cause is invisible at the panel, where the voltage reads fine. Plugging the actual run length and load into the tool turns that mystery into a number you can act on.

Two things drive the loss, and both are in the math: length and current. Double the distance and you double the drop; double the amps and you double it again. That is why a long run to an outbuilding is the classic problem case — the very situation where you most want a healthy outlet is the one where the wire is working hardest against you. The verdict reads the percentage against the standard targets: at or under 3% is good, 3 to 5% is acceptable, and above 5% means the wire is too thin for the run and you should size it up.

Use it to plan a run before you buy wire, or to diagnose one that is already misbehaving. If the verdict says to upsize, move up a gauge or two and watch the drop fall — thicker copper is the fix. One caution worth stating up front: this assumes copper. Aluminum wire has roughly 60% higher resistance for the same gauge, so if you are running aluminum, size up a gauge or two beyond what the copper number suggests to land in the same range.

The formula

Voltage drop is the current times the round-trip resistance of the wire; the percentage compares that loss to the supply voltage you started with:

voltage drop = 2 × distance × current × (ohms per 1000 ft ÷ 1000)
% drop = voltage drop ÷ supply voltage × 100

Worked example with the defaults — 20 A over 100 ft of 12-gauge copper at 1.93 ohms per 1000 ft: 2 × 100 ft × 20 A × (1.93 ÷ 1000) = 7.72 V dropped, and 7.72 ÷ 120 × 100 = about 6.4%. That is over the 5% limit, so the verdict says to upsize the wire — 12-gauge is too thin for this run.

Step up to 8-gauge copper (0.764 ohms per 1000 ft) and the same run drops just 3.06 V, about 2.5%, comfortably within spec. The ×2 in the formula is the round trip: current flows out to the load and back along the second conductor, so the wire’s resistance counts twice over the one-way distance you entered.

Frequently asked questions