Voltage Drop Calculator
Calculate the voltage drop across a wire run from its length, gauge and current — and check it against the 3% recommended limit.
Voltage drop
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Percent of system voltage —
Within NEC 3% guideline? —
Resistance values are typical for copper at room temperature; actual values vary slightly with temperature and conductor material.
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How it works
Vdrop = 2 × current × resistance-per-length × length (for single-phase round trip)Frequently Asked Questions
How do I calculate voltage drop?
Multiply current by the wire's resistance per unit length by twice the one-way distance (current travels out and back): a 20A load through 30m of 12 AWG copper wire (about 5.2 mΩ/m) drops roughly 6.2V — worth checking against the 3% guideline.
What is an acceptable voltage drop?
The NEC recommends keeping voltage drop under 3% for branch circuits (and under 5% combined with feeders) to avoid dim lighting, motor strain, and wasted energy. Long cable runs or undersized wire are the usual culprits.
How do I fix excessive voltage drop?
Use a thicker (lower-gauge-number) wire, shorten the run if possible, or split the load across multiple circuits. Doubling the wire's cross-sectional area roughly halves the voltage drop for the same current and length.