Voltage Drop Calculator
Calculate the voltage drop of a wire run from the wire size, length, current and voltage, and check it against the common 3% guideline.
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About the Voltage Drop Calculator
Voltage drop is the loss of voltage along a wire because of its resistance. Too much drop wastes power and can make motors and lights perform poorly. The National Electrical Code recommends keeping voltage drop to about 3% on branch circuits and 5% total from the panel to the load. This calculator uses standard conductor resistance values for copper or aluminum wire.
How to use it
- Choose copper or aluminum, the wire size and the circuit phase.
- Enter the one-way length in feet, the current in amps and the source voltage.
- Press Calculate voltage drop to see volts lost, percent and the voltage at the load.
Voltage drop formula
Single phase: VD = 2 × R × L × I ÷ 1000. Three phase: VD = √3 × R × L × I ÷ 1000, where R is the wire resistance in ohms per 1,000 feet, L is the one-way length in feet and I is the current in amps.
Copper wire resistance (ohms per 1,000 ft, 75°C)
| AWG | Ohms per 1,000 ft |
|---|---|
| 14 | 3.14 |
| 12 | 1.98 |
| 10 | 1.24 |
| 8 | 0.778 |
| 6 | 0.491 |
| 4 | 0.308 |
Frequently asked questions
How do I calculate voltage drop?
Multiply 2 (single phase) by the wire resistance per 1,000 feet, the one-way length and the current, then divide by 1,000.
What is an acceptable voltage drop?
Common guidance is 3% or less on a branch circuit and 5% or less for feeder and branch combined.
What wire size do I need for a long run?
Use the calculator to try larger wire sizes until the voltage drop is within your target.
Does aluminum have more voltage drop than copper?
Yes. Aluminum has higher resistance, so for the same size it has more voltage drop.
Is this a code calculation?
It is a planning tool. Always follow the NEC and local codes and ask a licensed electrician.
Last reviewed: November 2026. Estimates for planning. Follow the National Electrical Code and have a licensed electrician verify wire sizing.