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Voltage drop calculator

Enter the circuit. The result and every step update as you type.

System
Conductor material

For three-phase, enter the line-to-line voltage. Length is from the source to the load, not round trip.

Voltage drop--
Drop percent--
Voltage at load--
Area (cmil)--

The steps

    How this calculator works

    It uses the standard K-factor formulas:

    Single-phase: VD = 2 × K × I × L / CM Three-phase: VD = 1.732 × K × I × L / CM

    K is 12.9 for copper and 21.2 for aluminum (ohm-circular mils per foot at 75 °C). CM is the conductor area in circular mils from NEC Chapter 9, Table 8. L is the one-way length in feet. The percent is the drop divided by the system voltage.

    About the 3% figure

    The 3% comparison is a recommendation, not a requirement. In NEC 2020 and 2023 it comes from informational notes under 210.19 (branch circuits) and 215.2 (feeders), which suggest 3% for a branch circuit or feeder and 5% combined. Informational notes are not enforceable. Some special systems have mandatory limits in their own articles. Check your state's adopted edition.

    What it does not check

    Voltage drop is only one test. The conductor also needs enough ampacity for the load under NEC 310.16, with temperature correction, bundling adjustment and terminal ratings applied. See ampacity and derating. This tool uses the K-factor method and does not model AC reactance or power factor, which matter for large conductors and long feeders.

    Want to see the method in more depth? Read how to calculate voltage drop for the electrician exam.