Single-Phase AC Voltage Drop Calculator
Single-phase voltage drop occurs in residential and light commercial circuits (120 V or 240 V).
What Is Voltage Drop?
When current flows through wire, some voltage is lost along the way. The longer the run or smaller the wire, the more you lose. Too much drop and the load sees low voltage: motors run hot or won't start, lights dim.
A commonly cited guideline is to keep branch circuit voltage drop below 3% and total feeder + branch drop below 5%. These are common practice, not a code requirement. Check the applicable NEC edition and a licensed electrician for code compliance.
Common Questions
What causes voltage drop?
Resistance in the wire. Longer runs, skinnier wire, more amps, and aluminum (vs copper) all make it worse.
How do I reduce voltage drop?
Bigger wire, shorter run, or less load. Sometimes splitting the circuit is the answer.
Is voltage drop an NEC requirement?
The NEC mentions 3% and 5% in informational notes, not as a hard requirement. Plenty of jurisdictions still expect you to stay within those numbers; check with your AHJ.