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Wire Size Voltage Drop Calculator

Check voltage drop on any wire run and find the smallest AWG size that keeps drop under 3 percent, for copper or aluminum.

Voltage drop

Enter the circuit details. Results update live.

Voltage drop
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Enter the circuit details to check the drop.

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The voltage drop formula

Voltage drop equals 2 times K times current times one way length, divided by the wire's circular mil area. K is 12.9 for copper and 21.2 for aluminum. The factor of 2 accounts for the round trip out and back along the circuit.

Why 3 percent is the target

Electrical guidelines recommend keeping branch circuit drop under 3 percent so equipment gets close to its rated voltage. Low voltage makes motors run hot, lights dim and heaters underperform, especially on long runs.

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Wire Size Volt Drop FAQs

How far can I run 12 AWG on a 20 amp circuit?
About 60 feet one way at full 16 amp continuous load before hitting 3 percent drop at 120 volts. Longer runs need 10 AWG or larger.
Is voltage drop worse on 12V systems?
Much worse. A 2 volt drop is only 1.7 percent at 120 volts but nearly 17 percent at 12 volts. Low voltage systems need short runs or very large wire.
Does aluminum wire drop more voltage?
Yes, aluminum has higher resistance than copper, so it drops about 64 percent more voltage at the same size. That is why aluminum feeders are sized larger than copper.
Do I enter one way or round trip length?
Enter the one way distance from panel to load. The formula doubles it internally for the round trip, so do not double it yourself.
Can voltage drop cause a fire?
Not directly, but the lost voltage turns into heat along the wire, and undervoltage stresses motors. Keeping drop under 3 percent is both a performance and safety practice.
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