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Voltage Drop Calculator

Calculate voltage drop, percentage drop and voltage at the load for any wire gauge, length, current and voltage, copper or aluminum, single or three phase.

Wire size

Conductor

Current (A)

One-way length (ft)

Source voltage (V)

System

How to use it

Choose the gauge and material.
Enter current, one-way length and source voltage.
Keep branch circuits under about 3% and feeder + branch under about 5% (NEC informational notes).

How it works

Single-phase / DC: Vdrop = 2 × L × I × R ÷ 1000, with R in ohms per 1,000 ft (NEC Chapter 9, Table 8, DC resistance at 75 °C). Three-phase: Vdrop = 1.732 × L × I × R ÷ 1000. AC reactance in large conductors adds a little more drop.

FAQ

What is an acceptable voltage drop?

The NEC suggests 3% for branch circuits and 5% total including the feeder; these are recommendations, not requirements, in most cases.

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