Electricians

Voltage Drop Calculator

Calculate voltage drop across a wire run to ensure code compliance and proper performance.

Distance from panel to load (formula doubles for return)
3.95V
Voltage Drop
3.29%
Percent Drop
116.0V
Receiving Voltage
Within 5% - Marginal
NEC Recommendation

Formula: VD = (2 × K × I × L) / CM where K=12.9 (copper), I=20A, L=50ft. NEC recommends no more than 3% drop on branch circuits and 5% total from service entrance to load.

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What is voltage drop?

Voltage drop is the loss of voltage that happens as electricity travels through wire. Think of it like water pressure dropping the further you get from the pump. Some drop is normal, but too much and your equipment will not run properly. Motors run hot, lights dim, and sensitive electronics can malfunction. The NEC recommends keeping voltage drop under 3 percent for branch circuits and under 5 percent total.

When voltage drop becomes a problem

Short runs inside a house are rarely an issue. Voltage drop becomes a real concern on long runs. Running power 200 feet to a detached garage or shop, powering a well pump at the edge of your property, or feeding a barn from the main panel are all situations where you need to check voltage drop. If you do not size the wire correctly, you will have problems from day one.

How to fix excessive voltage drop

The fix is almost always using a larger wire gauge. Going from 12 gauge to 10 gauge on a long run can cut your voltage drop significantly. For very long runs, you might need to jump up two or three sizes from what the amperage alone would require. The other option is running a higher voltage. A 240 volt circuit has half the voltage drop of a 120 volt circuit at the same wattage.

Common Questions

What causes lights to dim in my house?

Dimming lights usually mean voltage drop from undersized wiring, a long wire run, or too many loads on one circuit. If it happens when a big appliance kicks on, that circuit is working too hard.

How far can you run 12 gauge wire?

On a 20 amp circuit at 120 volts, 12 gauge wire can run about 50 feet before you hit 3 percent voltage drop. Go longer than that and you need to bump up to 10 gauge.

Does voltage drop waste electricity?

Yes. The lost voltage turns into heat in the wire. Motors and equipment also draw more amps to compensate, which wastes even more energy and shortens equipment life.

What is acceptable voltage drop?

The NEC recommends no more than 3 percent on a branch circuit and 5 percent total from the panel to the outlet. Most inspectors hold you to those numbers.

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