Undersized wire on a long circuit run doesn’t just waste energy — it can cause dim lighting, sluggish motor performance, and in extreme cases, overheating. Our wire gauge calculator estimates the minimum copper wire gauge needed to keep voltage drop within a safe limit for your specific circuit.
The math
Round-trip length = One-way run length × 2
Voltage drop = Current × Resistance per foot × Round-trip length
Max allowed drop = System voltage × Drop percentage (3% general, 1% sensitive equipment)The calculator checks each standard AWG size (14, 12, 10, 8, 6, 4, 2…) against a resistance-per-1000ft table, starting from smaller gauges, and recommends the smallest (cheapest, most flexible) wire that still keeps the voltage drop under your chosen limit.
Example
A 20-amp, 120-volt circuit with a 50-foot one-way run, general-purpose wiring (3% max drop): 14 AWG produces a 5.05V drop, which fails the 3.6V maximum. Stepping up to 12 AWG produces a 3.176V drop, which passes — so 12 AWG is the recommended minimum gauge.
Why the run length gets doubled
Current has to travel out to the load and back to the source to complete a circuit, so both directions of wire contribute resistance. Doubling the one-way distance before calculating resistance is standard practice in voltage-drop calculations.
General purpose vs. sensitive equipment
General-purpose circuits (lighting, outlets, typical loads) commonly use a 3% maximum voltage-drop guideline. Voltage-sensitive electronics — things like precision instrumentation or certain communications equipment — benefit from a stricter 1% limit, since even small drops can measurably affect their performance.
Important caveat
This tool calculates voltage-drop-based sizing only — it does not replace official NEC ampacity tables, which define the maximum safe current-carrying capacity of a wire independent of voltage drop. Always cross-check against your local electrical code and consult a licensed electrician before any real installation.