Voltage Drop Calculator

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Calculate voltage drop in a wire run from gauge, length, current and material. Check whether the drop stays within the recommended 3% for copper or aluminum, single or three phase.

A
m
V
Source Load
Voltage drop –
Voltage at load –
Within 3%? –
Resistance of run (per leg) –
Single-phase counts both conductors (2 × length); three-phase uses √3 × length. Resistivity assumes copper or aluminum at about 20 °C. The 3% guideline is a common branch-circuit recommendation.

About Voltage Drop Calculator

The voltage drop calculator determines how much voltage is lost along a wire run. Enter the AWG wire size, the load current, the one-way length, the source voltage, the conductor material and the phase configuration, and it returns the drop in volts and percent, the voltage remaining at the load and the resistance of the run.

The calculation accounts for the full circuit path: single-phase runs count both conductors, doubling the one-way length, while three-phase runs multiply the length by √3. Conductor resistivity assumes copper or aluminum at roughly 20 °C, with aluminum dropping noticeably more than copper of the same gauge.

The result is checked against the common 3% guideline for branch circuits, answering the practical question directly: is this wire gauge adequate for this distance and load? Use this free calculator to size feeders to sheds, pumps, EV chargers and long lighting runs before pulling cable.

How to use Voltage Drop Calculator

  1. Select the wire size in AWG and the conductor material — copper or aluminum.
  2. Enter the load current in amps and the source voltage.
  3. Enter the one-way length of the run in metric or imperial units.
  4. Choose single-phase or three-phase.
  5. Read the voltage drop, the percentage, the voltage at the load and whether the run stays within 3%.

Frequently asked questions

A widely used guideline recommends keeping branch-circuit voltage drop within about 3%, with 5% for the combined feeder and branch. The calculator flags whether your run meets the 3% figure.

Current flows out on one conductor and back on the other, so the electrical path is twice the physical distance. Three-phase circuits instead use √3 times the one-way length in the drop formula.

Drop = current × the resistance of the conductor path, where resistance comes from the wire’s gauge, material and total length. Halving the gauge number roughly doubles the copper cross-section and halves the drop.

Aluminum has about 60% higher resistivity than copper, so an aluminum conductor of the same AWG drops considerably more voltage. Aluminum feeders typically need to be about two gauge sizes larger.

Motors run hot and struggle to start, lights dim, chargers slow down and energy is wasted heating the cable. Fix an excessive drop by choosing a larger wire gauge or shortening the run.

Yes — conductor resistance rises with temperature. The calculator assumes roughly 20 °C, so hot environments or heavily loaded conductors will drop slightly more than estimated.
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