Wire Gauge (AWG) Calculator
Find the copper wire gauge for a circuit from the amps, one-way run length and allowed voltage drop — the larger of the ampacity size and the voltage-drop size.
How it works
required circular mils = 2 × K × amps × one-way length ÷ (allowed % × volts) · K ≈ 12.9 (copper), 21.2 (aluminum) · then take the larger of the ampacity size and this size
Two things size a conductor: it must carry the current without overheating (ampacity, set by the breaker) and it must not lose too much voltage over a long run. The voltage-drop side uses the classic single-phase formula — the 2 accounts for the current traveling out and back — solved for the circular-mil area that keeps drop under your target. Whichever requirement calls for the bigger wire wins, which is why long runs often jump a size or two above what the amps alone would need.
Worked example
A 20 A circuit, 50 ft one way, 120 V copper, 3% max drop: allowed drop = 0.03 × 120 = 3.6 V, so required area = 2 × 12.9 × 20 × 50 ÷ 3.6 ≈ 7,167 cmil. That is 10 AWG (10,380 cmil). Ampacity alone would allow 12 AWG (20 A), but the run pushes it to 10 AWG — actual drop then falls to about 2.5 V, or 2.1%.
Sources: NEC 310.16 ampacity (Wikipedia: AWG)
Frequently asked questions
Why does distance change the wire size?
Resistance is proportional to length, so a long run drops more voltage at the same current. Codes generally recommend keeping branch-circuit voltage drop under 3% (and total under 5%), which on long runs forces a larger conductor than the amperage by itself would require.
Is 3% voltage drop a hard code rule?
It is a recommendation in NEC informational notes, not a strict mandate for most branch circuits. Sensitive loads, motors and long feeders still benefit from staying under it, so most electricians treat 3% as the working target.
Can I use aluminum instead of copper?
Yes, but aluminum has about 60% of copper’s conductivity, so it needs a larger size for the same current and drop. It is common and economical for large feeders and service entrances when terminations are rated for it.
Related calculators
Educational estimates only — electrical work must follow the National Electrical Code (NEC) and local amendments, and be done or inspected by a licensed electrician.