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Voltage Drop Calculator

A voltage drop calculator tells you how much voltage a run of wire loses — in volts and as a percentage — from the gauge, current, distance and conductor material. Too much drop means dim lights, sluggish motors and wasted energy.

Your measurements

Result

Enter your wire and run and it shows the drop, the voltage actually reaching the load, and whether you're inside the usual 3% guideline.

How it is calculated

Voltage drop (single-phase) = 2 × K × current × one-way distance ÷ the wire's circular-mil area, where K ≈ 12.9 for copper and 21.2 for aluminium. Percentage = drop ÷ source voltage. The 3% branch-circuit figure is a recommended guideline, not a hard NEC limit.

Steps

Choose the wire gauge and conductor material.
Enter the current the circuit carries.
Enter the one-way distance of the run.
Enter the source voltage (120 or 240, typically).
Read the drop and percentage; if over ~3%, upsize the wire.

Worked example

15 A through 12 AWG copper over 50 ft at 120 V drops about 3.0 V — roughly 2.5%, just inside the guideline.

Common questions

What is an acceptable voltage drop?

About 3% on a branch circuit, and roughly 5% combined for feeder plus branch. The calculator flags the 3% line.

How do I reduce voltage drop?

Use a heavier (lower-AWG) wire, shorten the run, or raise the voltage. Heavier wire is the usual fix.

Does aluminium drop more voltage?

Yes — it's more resistive, so the same gauge drops more than copper. The calculator accounts for it.

Is the 3% a code requirement?

It's a recommendation in the NEC (a fine-print note), not a strict rule — but exceeding it causes real performance problems.