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Voltage Drop Calculator

Estimate the voltage lost along a copper or aluminum wire run and compare it with the common 3% guideline; a licensed electrician must verify the result.

Enter a number from 1 to 1000

Enter a number from 0 to 1000

Enter a number from 0 to 10000

Distance from the panel or source to the load, not the round trip. The formula doubles it for you.

Voltage drop
3.16 V (2.63%)
116.8 Vvoltage at load
6,530circular mils
20 Atypical max amps
Formula2 × 12.9 × 16 A × 50 ft ÷ 6,530
Voltage drop3.16 V
Percent of source voltage · within the 3% guideline2.63%
Voltage at the load116.84 V

Estimate at 75 °C conductor resistance. The NEC and your local code govern, and a licensed electrician must verify any wiring before it is installed.

See breakdown

How the voltage drop calculation works

  1. Voltage drop = 2 × K × amps × length ÷ circular mils

    K is the resistance of one circular-mil foot of wire: about 12.9 ohms for copper and 21.2 for aluminum. Use 2 for single-phase or DC circuits and √3 (1.732) for three-phase.

  2. Use the one-way length

    Enter the distance from the panel to the load. The 2 in the formula already counts the trip out and back on the two conductors.

  3. Divide by the source voltage for the percent

    3.16 V lost on a 120 V circuit is 2.63%. The voltage left at the load is 120 − 3.16 = 116.84 V.

  4. Keep branch circuits near 3% and the total near 5%

    These figures come from informational notes in the NEC. They are recommendations for performance, not hard rules, but most designers follow them.

Worked example. A 120 V single-phase circuit feeds 16 A over a 50 ft run of 12 AWG copper (6,530 circular mils). Voltage drop = 2 × 12.9 × 16 × 50 ÷ 6,530 = 3.16 V, which is 2.63% and leaves 116.84 V at the load.

Voltage Drop FAQ

What is the formula for voltage drop?

For single-phase or DC circuits, VD = 2 × K × I × L ÷ CM, where K is 12.9 for copper or 21.2 for aluminum, I is amps, L is the one-way length in feet and CM is the wire's circular mils. For three-phase, replace 2 with 1.732.

What is an acceptable voltage drop?

NEC informational notes suggest no more than 3% on a branch circuit and 5% for the feeder and branch circuit combined. Motors, LED drivers and long outdoor runs often need the lower figure.

How do I reduce voltage drop?

Use a larger wire, shorten the run, or raise the voltage. Moving a 30 A load from 120 V to 240 V halves the current and cuts the percentage drop to a quarter.

Does this work for a DC voltage drop?

Yes. Pick single-phase AC or DC and enter the DC voltage. Low-voltage DC systems such as 12 V solar or RV wiring lose a much larger percentage over the same distance, so they need heavier wire.

Is this calculator enough to size wire for a permit?

No. It is a planning estimate. The NEC, your local code and the inspector govern, and a licensed electrician must check conductor ampacity, temperature ratings, conduit fill and breaker size.

Sources: NFPA 70, National Electrical Code 2023 (free read-only access)

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