Voltage Drop Calculator

Where the formula comes from

A conductor has resistance proportional to its length and inversely proportional to its cross-sectional area. Expressed in the units the trade uses, that is V_drop = 2 × K × I × L ÷ CM, where K is the resistivity of the metal in ohm-circular mils per foot, I is the current in amps, L is the one-way length in feet and CM is the cross-section in circular mils. K is about 12.9 for copper and 21.2 for aluminum at normal operating temperature. The factor of 2 accounts for the return conductor — current has to come back. Three-phase circuits use √3 (about 1.732) instead of 2, because the phase currents partially cancel in the neutral.

Why 3% and 5%

Those figures are recommendations, not hard limits on safety. The common guidance is no more than 3% drop on a branch circuit and no more than 5% total from the service to the furthest outlet. The reasoning is performance rather than fire risk: motors lose torque and run hotter on low voltage, incandescent lamps dim noticeably, and electronic supplies draw more current to compensate, which makes the drop worse. Local codes differ on whether these are mandatory, so treat the check here as a design target and confirm against the rules that apply to you.

Aluminum is not just cheaper

Aluminum has roughly 61% the conductivity of copper, so an aluminum conductor needs to be about two AWG sizes larger to match a copper one for drop. It is lighter and cheaper per amp delivered, which is why utility feeders use it, but it also creeps under pressure and oxidises into a non-conductive film, so terminations need rated connectors and antioxidant compound. Mixing the two metals at a terminal without a rated connector is a classic cause of hot joints.

What this calculator does not include

This is a DC-resistance calculation with a fixed resistivity. It ignores conductor temperature rise, which raises resistance by about 0.4% per °C for copper, and it ignores AC effects — skin effect and proximity effect — that begin to matter above roughly 2/0 for 60 Hz power. It also assumes a power factor of 1; a motor load with a lagging power factor produces a different drop for the same real power. For a large feeder, a design calculation should use the impedance tables for the actual cable and installation method rather than a single K value.

Fixing a failing run

There are only four levers: shorten the run, raise the voltage, lower the current, or fatten the conductor. Raising voltage is the most effective when it is available — doubling the voltage quarters the percentage drop for the same delivered power, because current halves and the drop is proportional to current. That is why long runs to outbuildings are often fed at 240 V and stepped down locally rather than run at 120 V.

Frequently Asked Questions