Voltage Drop Calculator

Estimate the voltage lost along a copper or aluminum wire run. See the drop in volts and percent, the voltage at the load, and the smallest wire that keeps drop under 3%.

Phase
Voltage drop
–
Percent drop–
Voltage at load–
Smallest size for ≤ 3%–

Voltage drop formula

Single phase: VD = 2 × K × I × L ÷ CM
Three phase: VD = 1.732 × K × I × L ÷ CM
K = 12.9 (copper) or 21.2 (aluminum) Ω·cmil/ft · L = one-way feet

Worked example

A 120 V circuit carrying 16 A over 100 ft of 12 AWG copper (6,530 circular mils) drops 2 × 12.9 × 16 × 100 ÷ 6,530 = 6.32 V, which is 5.3%. Moving up to 10 AWG (10,380 cmil) cuts it to 3.98 V (3.3%), and 8 AWG brings it to 2.5 V (2.1%).

Wire sizes and circular mils

SizeCircular milsSizeCircular mils
14 AWG4,1101/0 AWG105,600
12 AWG6,5302/0 AWG133,100
10 AWG10,3803/0 AWG167,800
8 AWG16,5104/0 AWG211,600
6 AWG26,240250 kcmil250,000
4 AWG41,740350 kcmil350,000
2 AWG66,360500 kcmil500,000

Ways to reduce voltage drop

  • Use a larger wire (a smaller AWG number).
  • Shorten the run by moving the panel or subpanel closer to the load.
  • Use a higher voltage, such as 240 V instead of 120 V, which halves the current for the same power.
  • Choose copper over aluminum for the same size, or upsize aluminum.

This is an estimating tool only; it does not check ampacity or code compliance. Electrical work should be done by a qualified, licensed electrician.

Last reviewed October 2026.

Frequently asked questions

What is voltage drop?

Voltage drop is the voltage lost along a wire because of its resistance. The longer the run and the higher the current, the more voltage is lost, so equipment at the far end gets less than the source voltage. Too much drop can make motors run hot and lights dim.

How much voltage drop is acceptable?

The National Electrical Code (NEC) recommends, in informational notes, no more than 3% drop on a branch circuit and 5% total for feeder plus branch circuit. These are recommendations for efficiency rather than strict rules, but many designers follow them.

What formula does the calculator use?

Single phase: VD = 2 × K × I × L ÷ CM. Three phase: VD = 1.732 × K × I × L ÷ CM. K is the resistance constant (12.9 for copper, 21.2 for aluminum), I is current in amps, L is the one-way length in feet and CM is the wire's area in circular mils.

Should I enter the one-way or round-trip length?

Enter the one-way distance from the source to the load. The formula already doubles it for single-phase circuits to account for current going out and back.

Does this choose the right wire size for safety?

No. This tool only estimates voltage drop. Wire must also be sized for ampacity, temperature rating, conduit fill and local code. Always have electrical work designed and checked by a licensed electrician.