Wire Size Calculator

A wire size calculator picks the right conductor gauge (AWG) for a circuit by checking both things that matter: it must safely carry the current (ampacity) and it must not drop too much voltage over the run (usually kept under 3%). It returns the larger of the two — the safe choice — and tells you which one governed.

Your measurements

Result

Long runs are where people undersize: a wire that's fine for the amps at 20 ft can drop too much voltage at 120 ft. This catches that.

How it is calculated

Ampacity: the smallest gauge whose rated current meets the load. Voltage drop: the smallest gauge keeping 2 × K × amps × distance ÷ circular-mils under the limit (K ≈ 12.9 copper, 21.2 aluminium; default 3%). The recommendation is the larger of the two. Simplified ampacities; no derating or continuous-load factor applied.

Steps

Enter the load current in amps.
Pick the voltage (120 or 240).
Enter the one-way distance of the run.
Choose copper or aluminium.
Read the recommended gauge and what governed it — ampacity or voltage drop.

Worked example

A 20 A, 120 V circuit run 50 ft in copper needs about 10 AWG — here voltage drop governs, not just the amps.

Common questions

What wire gauge do I need?

Big enough to carry the current AND keep voltage drop under ~3% over the run. The calculator returns the larger of those two requirements.

Why does distance matter?

Voltage drops along the wire; a long run may need a heavier gauge than the amperage alone would suggest.

Copper or aluminium?

Aluminium carries less current per gauge and must be sized up; the calculator adjusts for it. Use listed connectors for aluminium.

Is this code-compliant sizing?

It's a planning estimate. Real sizing applies temperature and conductor-count derating and the 125% rule for continuous loads — follow the NEC and use a licensed electrician.