Free voltage drop calculator · No signup
Copper or aluminum, single- or three-phase. Enter wire size, length, load, and system voltage to get voltage drop, percent drop, and voltage at the load — with a 3% / 5% flag.
Voltage drop is the loss along a conductor caused by its resistance carrying current. The standard estimate uses the circular-mil (K-factor) method: single-phase VD = (2 × K × I × L) ÷ CM, three-phase VD = (1.732 × K × I × L) ÷ CM — where K is resistivity (12.9 copper, 21.2 aluminum), I is amps, L is one-way feet, and CM is the conductor's circular-mil area.
The NEC doesn't mandate a limit, but its Informational Notes recommend ≤3% on a branch circuit or feeder and ≤5% total. Excessive drop dims lighting, overheats motors, and wastes energy as I²R heat. When a run is too long, the fix is to upsize the conductor (more circular mils = less drop).
This is the same check that drives feeder sizing on a job. Field PM keeps your panel schedules, equipment, and field documentation together in the job book — for final design on long or high-power-factor runs, verify against NEC Chapter 9 Table 9.
For single-phase: VD = (2 × K × I × L) ÷ CM. For three-phase: VD = (1.732 × K × I × L) ÷ CM. K is the conductor resistivity (12.9 for copper, 21.2 for aluminum), I is the load current in amps, L is the one-way length in feet, and CM is the conductor's circular-mil area.
Voltage drop is not a hard NEC requirement, but Informational Notes recommend a maximum of 3% on a branch circuit or feeder, and 5% total across feeder plus branch circuit, for reasonable efficiency. This calculator flags 3% and 5%.
Enter the one-way circuit length (the distance from the panel to the load). The formula already accounts for the return conductor with the "2 ×" factor on single-phase and "1.732 ×" (√3) on three-phase.
Aluminum has higher resistivity (K ≈ 21.2 vs 12.9 for copper), so for the same size and length it drops about 64% more voltage. That's why aluminum feeders are usually upsized one or two trade sizes versus copper.
It uses the standard circular-mil (K-factor) method, which is accurate for most building-wiring estimates. It assumes ~75°C copper/aluminum and unity power factor. For long runs, high power factor loads, or precise engineering, use NEC Chapter 9 Table 9 AC resistance/reactance values.
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