Voltage Drop Calculator
Enter the wire size, the current and how long the run is. The calculator shows the voltage lost along the wire, the percent drop and the voltage that reaches the load.
How to use this Voltage Drop Calculator
- Choose the wire size and whether it is copper or aluminum.
- Choose single-phase or DC, or three-phase.
- Enter the current the circuit carries in amps.
- Enter the one-way distance from the source to the load in feet.
- Enter the supply voltage and your drop limit, then press Calculate.
How to calculate voltage drop
The calculator uses the common circular mil formula for conductors.
- Voltage drop = factor x K x current x length / circular mils
- Factor = 2 for single-phase or DC, 1.732 for three-phase
- K = 12.9 for copper, 21.2 for aluminum (ohm-cmil per foot)
- Percent drop = voltage drop / supply voltage x 100
Worked example
A 12 AWG copper circuit carries 20 amps over 100 feet one way at 120 V single-phase. The wire is 6,530 circular mils. The drop is 2 x 12.9 x 20 x 100 / 6,530 = 7.90 V, which is 6.58% of 120 V. The load sees 112.10 V. That is over a 3% limit, so the run needs a larger wire.
Changing to 8 AWG copper (16,510 circular mils) lowers the drop to 3.13 V, or 2.60%, which is within the limit. Doubling the length doubles the drop, and halving the current halves it. Long runs to a shed, a pump or an EV charger are where voltage drop matters most. Use the Wire Size Calculator to find the smallest wire for a target drop. Note that the wire must also be large enough to carry the current safely, and that is a separate check.
Assumptions & limits
- The K values of 12.9 and 21.2 are common planning constants for copper and aluminum at about 75 C. Check the National Electrical Code and your local rules, and use a licensed electrician for real installations.
- The wire sizes and circular mil values are those commonly published for AWG sizes. Confirm them against the table you use.
- The calculation is for a steady load and ignores the power factor, temperature and conduit effects.
- It checks voltage drop only. It does not check that the wire can carry the current safely.
- The 3 percent limit is a common design goal, not a rule that applies everywhere.
Frequently asked questions
How do I calculate voltage drop?
Multiply 2 (or 1.732 for three-phase) by K, the current and the one-way length, then divide by the circular mils of the wire.
What is an acceptable voltage drop?
Many designers aim for about 3 percent on a branch circuit and 5 percent overall. Check your local rules.
Does wire length mean one way or round trip?
Enter the one-way length. The formula already counts the return path with the factor of 2.
Why use a larger wire?
A larger wire has less resistance, so less voltage is lost over the same distance.
Is aluminum worse than copper?
It has higher resistance for the same size, so the drop is greater unless you use a larger wire.
Does this check wire safety?
No. Use the Wire Size Calculator and the code tables to check the current rating too.
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Last updated: November 2026