Voltage Drop Calculator
Estimate the voltage lost over a copper wire run from its gauge, length, and current draw.
Example
A 100 ft run of 12 AWG copper (0.4018 ohms/1000 ft) carrying 15 A on a 120 V circuit: Vdrop = 2 x 100 x 15 x 0.0004018 = 1.21 V, about 1.0%, leaving 118.8 V at the load.
How it works
For a two-conductor run, Vdrop = 2 x length(ft) x current(A) x R, where R is the wire resistance per foot (AWG ohms-per-1000ft divided by 1000). Voltage drop percent = Vdrop / source voltage x 100.
Good to know
This Voltage Drop Calculator estimates how much voltage is lost along a copper wire run based on four inputs: source voltage, current in amps, the one-way distance to the load, and the wire's AWG gauge. It returns the drop in volts, the drop as a percentage of source voltage, the voltage actually reaching the load, and a quick OK/High/Too much status flag. It's aimed at DIY electricians, electric vehicle and solar installers, RV and boat wiring, and anyone sizing a long run for lights, pumps, or outbuildings.
You'd reach for it whenever a circuit travels a long distance or carries heavy current, because resistance accumulates with both length and load. Common situations include running power to a detached garage or shed, wiring a well pump, planning a low-voltage landscape lighting loop, or checking whether a 12 AWG run is too thin for the distance. Enter your one-way length only; the tool automatically doubles it to account for both the outbound and return conductors.
Read the result against the status flag, which mirrors NEC guidance: at or below 3% is OK, 3 to 5% is High, and above 5% is Too much. The "voltage at load" figure is often the most practical number, since it tells you what the device will actually see at the far end. If the percentage is too high, you can lower it by choosing a thicker wire (a smaller AWG number), shortening the run, or reducing the current draw.
One caveat: this calculator assumes copper conductors at a standard reference temperature and uses DC-style resistance, so it does not factor in aluminum wire, conductor heating under sustained load, conduit fill, or AC power-factor and reactance effects on large feeders. Treat the output as a planning estimate and confirm final wire sizing and ampacity against code tables or a licensed electrician for anything that matters.
Frequently asked questions
Why is the length multiplied by 2?
Current flows out to the load and back through the return conductor, so the total wire the current travels is twice the one-way run length. The formula uses 2 x length to account for both conductors.
What voltage drop percentage is acceptable?
The NEC recommends a maximum of 3% drop on a branch circuit and 5% total including the feeder. Higher drops waste energy and can cause dimming lights, slow motors, or overheating.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser. Your inputs never leave your device, and it works offline once loaded.
Is this calculator free?
Yes, completely free with no sign-up and no limits.
People also ask
How do I reduce voltage drop on a long wire run?
You can lower voltage drop by using a thicker conductor (a lower AWG number), shortening the run distance, or reducing the current load. Increasing the source voltage also reduces the drop as a percentage. Upsizing the wire is the most common fix for long runs.
What happens if voltage drop is too high?
Excessive voltage drop means devices receive less voltage than rated, which can cause dim or flickering lights, motors that run hot or stall, electronics that misbehave, and wasted energy as heat in the wire. Persistent overheating in undersized conductors can also be a fire risk.
Does this calculator work for aluminum wire?
No. This tool uses copper conductor resistance values. Aluminum has roughly 60 to 65% higher resistance for the same gauge, so it would show a larger drop than copper of the same size and is not modeled here.
Is voltage drop different for AC and DC circuits?
For typical branch circuits the resistive calculation used here applies reasonably well to both. On large AC feeders, conductor reactance and power factor can add to the drop, so detailed AC sizing may use impedance values rather than resistance alone.
How does wire length affect voltage drop?
Voltage drop is directly proportional to length: doubling the run roughly doubles the drop, all else equal. Because current flows out and back, the calculator multiplies your one-way length by two to cover both conductors.
Why does higher current increase voltage drop?
Voltage drop equals current multiplied by resistance, so for a fixed wire the drop rises in direct proportion to the amperage. A run that is fine at a low load can exceed acceptable limits when the current increases.
What AWG wire size should I use for a 100-foot run?
It depends on the voltage, current, and acceptable drop, not distance alone. Enter your values and adjust the AWG until the status reads OK; for a 120 V, 15 A, 100 ft copper run, 12 AWG typically stays around 1%.
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