Voltage Drop Calculator

For a single-phase 2-wire run, voltage drop equals 2 times the one-way distance times the current times the wire resistance per foot. Resistance comes from the conductor's AWG size and material (copper or aluminum). The result is the volts lost in the wire, which this tool also shows as a percentage of your system voltage. For a balanced three-phase run, the leading 2 is replaced by the square root of 3 (about 1.732).

Current has to travel out to the load and back through the return conductor, so a single-phase circuit carries the load current over twice the one-way length. Enter only the panel-to-load distance and the calculator doubles it internally. For three-phase the factor is the square root of 3 instead, because the line-to-line geometry differs.

The National Electrical Code recommends keeping voltage drop at or below 3% on a branch circuit and at or below 5% for the feeder and branch combined (NEC 210.19 and 215.2 Informational Notes). These are recommendations for efficiency and performance, not hard code rules, so a run can exceed them and still pass inspection. This tool flags both thresholds so you can size conductors conservatively.

Aluminum has higher resistivity than copper, so for the same AWG size an aluminum conductor drops roughly 1.63 times more voltage over the same run. The calculator scales the copper resistance by that factor when you pick aluminum. To match copper's performance you generally need to step up one or two gauge sizes in aluminum.

You have four levers: use a thicker conductor (lower AWG number), shorten the run, raise the system voltage, or lower the current. Doubling the wire cross-section roughly halves the drop, and moving from 120 V to 240 V cuts the percentage drop in half for the same power. The gauge suggestion shows the thinnest size that keeps you under your chosen target percentage.

No. The resistance values are room-temperature DC figures from NEC Chapter 9, Table 8, which is the standard basis for residential and light commercial voltage-drop estimates. Hot conductors and the reactance of large AC feeders raise the real drop slightly, so treat results as a close planning estimate rather than a substitute for a licensed electrician on critical or high-current installations.