Voltage Drop Calculator
Voltage Drop Calculator
Check voltage drop over a wire run to see if you need to upsize the wire — for copper conductors.
Based on copper conductor resistance for a two-wire run. The NEC recommends keeping voltage drop under 3% on branch circuits (5% total). This is reference information, not a code ruling — consult a licensed electrician and follow local code.
Long wire run? Shop wire & cable and check the wire gauge calculator first. Browse outlets & switches.
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How to calculate voltage drop
Voltage drop = 2 × distance × (resistance per 1000 ft ÷ 1000) × amps. Keep it under 3% on branch circuits. Long runs — like a detached garage or outdoor lighting — often need the next thicker wire gauge to stay within limits. A 120 V, 15 A load on a 100 ft (200 ft round-trip) run of 12 AWG drops about 5.8 V (4.8%), so you’d upsize to 10 AWG.