Free tool · Electrical
Voltage Drop Calculator
Choose the conductor size and material, then enter the load current, the one-way length and the source voltage. The calculator shows the voltage drop in volts, as a percent of the source, and the voltage left at the load.
The numbers stay in your browser: nothing is saved or sent anywhere.
Voltage drop
— V
Percent of source
—
Voltage at the load
— V
The formulas
Single phase (two wires): drop = 2 × K × current × one-way length ÷ circular mils.
Three phase: drop = 1.732 × K × current × one-way length ÷ circular mils.
K is the resistivity constant of the metal: about 12.9 for copper and 21.2 for aluminum at the usual working temperature. Circular mils come from the conductor size, and the calculator works them out from the AWG definition.
A worked example
A 20 amp load sits 100 ft from a 120 V single-phase source. On 12 AWG copper the drop is 7.9 V, which is 6.6 percent.
On 10 AWG copper it falls to 5.0 V, or 4.1 percent. On 8 AWG copper it is 3.1 V, or 2.6 percent. Going up a size or two is often the cheapest fix on a long run.
What percent is acceptable
A common guideline is to keep the drop to about 3 percent on a branch circuit and about 5 percent from the service to the farthest load. The National Electrical Code lists these as informational recommendations, not requirements, and your project specifications or local rules may set their own limit. Equipment such as motors can also have its own limit.
What this does not do
This is a resistance-only estimate. It ignores reactance, power factor, conductor temperature and the effect of conduit material, so it is most accurate for smaller conductors and resistive loads. It does not check ampacity, which is a separate question. For large conductors, motors or critical loads, run a full calculation.
FAQ
Is the length one-way or round trip?
Enter the one-way length from the source to the load. The two-wire formula already includes the return path.
Why is three phase different?
In a balanced three-phase circuit the current is shared across the phases, so the factor is 1.732 instead of 2.
How do I reduce voltage drop?
Use a larger conductor, shorten the run, raise the voltage, or move the source closer to the load.
Is anything I type saved?
No. The calculation runs in your browser and nothing is stored or sent to us.
Pricing the whole electrical job? PoPs Estimating measures conduit and wire from your plans and keeps the quantities tied to the bid.
Questions?
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