How the voltage drop calculator works
Every wire has resistance, so some voltage is lost along its length. The loss grows with current and distance and shrinks with a thicker wire.
Three-phase: VD = 1.732 × K × I × L ÷ CM
K is the resistivity of the conductor (about 12.9 ohm-cmil/ft for copper and 21.2 for aluminum at operating temperature), I is current in amps, L is one-way length in feet and CM is the wire's cross-section in circular mils from NEC Chapter 9, Table 8. The factor 2 accounts for the current going out and back. The NEC's informational notes recommend no more than 3% drop on a branch circuit and 5% in total.
Worked example
A 15 A load at the end of a 100 ft run of 12 AWG copper on a 120 V circuit drops 2 × 12.9 × 15 × 100 ÷ 6,530 = 5.93 V, or 4.94%, leaving 114.1 V at the load. Upsizing to 10 AWG cuts the drop to 3.73 V (3.1%), and 8 AWG to 2.34 V (1.95%).
Frequently asked questions
What is an acceptable voltage drop?
The NEC suggests 3% for branch circuits and 5% for feeder plus branch combined. These are recommendations rather than requirements in most cases.
Does wire length mean one way or round trip?
One way. The formula includes the return path.
Why does aluminum drop more voltage?
Aluminum has about 64% more resistance than copper of the same size.
Does voltage drop damage equipment?
Low voltage makes motors run hot and lights dim, and can shorten equipment life.
Updated October 2026. Results are estimates for education, not financial or medical advice. All math runs in your browser; nothing you type is sent anywhere.