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Voltage Drop Calculator
Voltage drop grows with length and current, and the NEC recommends keeping it at or below 3 percent on a branch circuit. This calculator shows the drop and the percent, with the full worked formula.
Worked example
Inputs: System = single-phase, Conductor = copper, Wire size (AWG/kcmil) = 12, Load current = 20 A, One-way length = 100 ft, Source voltage = 120 V.
- VD = 2 × K × I × L ÷ CM (K = 12.9 for copper, CM = 6,530 cmil for #12)
- = 2 × 12.9 × 20 × 100 ÷ 6,530
- = 7.90 V
- Percent of source = 7.90 ÷ 120 × 100 = 6.58%
The NEC recommends keeping voltage drop at or below 3% on a branch circuit (5% total with the feeder). Single-phase uses 2 (both conductors); three-phase uses √3.
When operators use this
Electricians size conductors for long runs so equipment sees enough voltage, and the exam expects you to know when a wire needs to go up a size.
Frequently asked questions
What is an acceptable voltage drop?
The NEC recommends 3 percent or less on a branch circuit and 5 percent total including the feeder. These are recommendations, not hard code limits, but exams test them as targets.
Why does aluminum need a bigger conductor?
Aluminum has higher resistance than copper for the same size, so for the same drop target it needs a larger conductor.
Is it free?
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