Voltage Drop Calculator

Calculate voltage drop for copper and aluminum branch circuits and feeders, and check it against the common 3% / 5% guideline.

V
A

Voltage drop

7.46 V

  • Percent drop3.11 %
  • Voltage at the load232.5 V
  • RecommendationOver 3% — consider the next wire size

Estimates only. Round up when you order, and confirm structural details with your local building code.

How to use it

  1. Choose single- or three-phase.
  2. Enter source voltage, load current and one-way length.
  3. Pick conductor material and size.

The formula

Single-phase: VD = 2 × K × I × L ÷ circular mils

Three-phase: VD = 1.732 × K × I × L ÷ circular mils

K = 12.9 (copper), 21.2 (aluminum)

Worked example

SystemSingle-phase / DC
Source voltage240 V
Load current30 A
One-way circuit length100 ft
ConductorCopper
Wire size10 AWG

Result

Voltage drop7.46 V
Percent drop3.11 %
Voltage at the load232.5 V
RecommendationOver 3% — consider the next wire size

Tips before you order

  • The NEC informational notes recommend no more than 3% drop on a branch circuit and 5% total.
  • Long runs to detached garages and outbuildings are where voltage drop usually forces an upsize.

Questions

What wire size for 30 amps at 100 feet?

On 240 V, 10 AWG copper gives about a 3.1% drop; 8 AWG brings it under 2%.