Voltage Drop Calculator

Calculator

Voltage drop calculator

Work out the drop before you buy the wire. Enter the current, the one-way length and the supply voltage, and the tool shows the smallest wire that meets your target, with the assumptions used.

Inputs

Three-phase is not in this version.
For example 120 or 240 for AC, 12 or 24 for DC.
The tool counts the return conductor for you. Enter the distance from source to load only.
Allowed drop
3% and 5% are informational guidance for branch and total drop, not mandatory limits unless your authority adopts them.
What to find
Enter the allowable ampacity for the wire size shown, after derating for temperature and conduit fill. The tool does not hold ampacity tables.

Smallest size that meets the target

12 AWG

2.96 V
Percent drop2.47 %
Voltage at load117.04 V
Allowed drop3%

Within 3% target

Assumptions used
  • Formula: V = 2 × K × I × L ÷ CM, single-phase and DC (the 2 counts the return conductor).
  • K = 12.9 ohm·CM/ft (copper, 75 °C basis).
  • I = 15 A, one-way L = 50 ft, V = 120 V.
  • CM for 12 AWG = 6,530.
  • Not checked here: temperature derating, conduit fill, power factor, and code compliance.

How the number is made

For DC and single-phase circuits the drop is V = 2 × K × I × L ÷ CM. I is the load current in amps, L is the one-way length in feet, CM is the conductor area in circular mils, and K is 12.9 for copper or 21.2 for aluminum. The 2 counts the return conductor. Details and sources are on the method page.

Voltage drop and ampacity are two different checks

The drop check protects the equipment at the far end: the motor starts, the lights stay bright, the fridge runs. The ampacity check protects the wire against heating. Either one can set the wire size. The ampacity guide explains which one governs.

Frequently asked questions

What is voltage drop?

Voltage drop is the voltage lost along the wire because of its resistance. A longer run or a smaller wire loses more voltage before the load.

Is 3% a legal limit?

No. The 3% branch and 5% total figures are informational guidance in the National Electrical Code (NEC §210.19 informational note). A local authority may adopt them as requirements, so check yours.

Does the length include the return wire?

Enter the one-way distance from the source to the load. The formula counts the return conductor (the 2 in V = 2 × K × I × L ÷ CM).

Why does another calculator give a different answer?

Tools differ in target, length convention, and temperature basis. This tool shows its formula, constants and inputs in the assumptions panel so you can compare.

Does this check ampacity?

It checks ampacity only against a value you enter from your code book, after derating. The tool does not hold ampacity tables.

Is the result code compliant?

This tool does not check code compliance. Verify your design with the authority having jurisdiction and a licensed electrician.

Learn more: how to calculate voltage drop, ampacity vs voltage drop, how this calculator works.