⚡ Voltage Drop Calculator
Long wire runs waste voltage as heat, leaving less for the load at the far end — dim lights and sluggish motors are the classic symptoms. Enter the current in amps, the one-way distance in feet and the copper wire gauge to see the drop.
The result is shown in volts and as a percentage of a 120V circuit. Electricians aim for under 3% on branch circuits and under 5% total.
How to use this calculator
Enter the current in amps, the one-way wire distance in feet, and pick the copper wire gauge. The calculator returns the voltage drop in volts, the percentage of a 120V circuit, and a verdict.
- Type the Current in amps the circuit will carry.
- Type the One-way distance in feet from panel to load.
- Pick the Wire gauge from the list.
- Read the drop in volts, the percentage, and the verdict.
Frequently asked questions
What does the Voltage Drop Calculator compute?
It estimates how many volts are lost as heat along a copper wire run. You enter current, one-way distance and wire gauge, and it returns the drop in volts, the drop as a percentage of a 120V circuit, and whether the result is acceptable.
What is the formula?
Voltage drop equals 2 times current times resistance per foot times one-way distance. The factor of 2 accounts for the round trip out and back along the pair of conductors.
Can you give a worked example?
20 amps over 100 feet of 12 AWG copper drops 6.35 volts, which is 5.3 percent of 120V, so the verdict is too high and a larger gauge is advised.
What percentage is acceptable?
Electricians generally aim for under 3 percent on branch circuits and under 5 percent total from service to load. Above 5 percent you should upsize the wire.