Wire Size Calculator - AWG by Current, Length & Voltage Drop
Wire Size Calculator
Find the AWG wire size for your load and run length
Two Limits Checked
The wire must both carry the current safely and keep the voltage drop within your limit; the thicker of the two answers wins.
Heat Loss Shown
See the power the wire wastes as heat at your load.
Metric or Imperial
Enter the run in metres or feet; results show AWG and mm².
Full Privacy
Everything is calculated in your browser.
How Wire Size Is Chosen
A wire has resistance, so it drops some voltage and turns some power into heat. The current goes out and back, so the length that counts is twice the one-way run: drop = current × resistivity × 2 × length ÷ cross-section. Copper is taken as 1.724 × 10⁻⁸ Ω·m and aluminium as 2.82 × 10⁻⁸ Ω·m at 20 °C. For 15 A over 20 m at 12 V and a 3% limit, a 2 AWG copper wire drops about 2.6%, while a 3 AWG wire would drop about 3.2%.
The current ratings used are the conservative 60 °C column of the US National Electrical Code for building wire, which suits many general cases but not all. Insulation type, temperature, bundling, conduit fill and local rules can change the answer, and motors, long runs and high loads need specific design. Treat this as a planning aid and have a qualified person confirm any mains wiring.
Key Takeaways
- Copper and aluminium: Aluminium has higher resistance, so it needs a thicker wire.
- Sizes 18 to 4/0 AWG: Covers everything from hobby wiring to heavy feeds.
- Planning aid: Have mains wiring checked by a qualified person.