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Wire Gauge Calculator

Load current and wire run length to a recommended wire gauge, keeping drop under 3%.

Your numbers
Your circuit

Recommended gauge

Enter your numbers above.

The right gauge covers two separate rules

A wire has to be thick enough for two unrelated reasons: it must carry the circuit's amperage without overheating (ampacity), and it must be thick enough over the run's length to keep voltage drop reasonable. Long runs often need to size up for drop even when the shorter ampacity table would allow a thinner wire.

This calculator works through the standard AWG ampacity table for your conductor material, starting from the thinnest gauge that's rated to carry your load's current, then checks whether that gauge also keeps voltage drop under the common 3% guideline over your specific run length. If a thin gauge passes ampacity but fails the drop check, it moves up to the next size until both pass.

Copper and aluminum have meaningfully different resistance per gauge — aluminum needs roughly two gauge sizes thicker than copper to carry the same current with similar voltage drop, which is why aluminum branch circuit wiring isn't simply a drop-in swap for copper at the same size.

This tool estimates a starting point for planning. The actual wire size for any circuit must also account for the specific NEC ampacity table for the insulation type, ambient temperature, and number of conductors bundled together, and should be confirmed by a licensed electrician.

Where these numbers come from

These are planning estimates only. Electrical work must meet the National Electrical Code and your local code, and should be installed or inspected by a licensed electrician. This is not a substitute for a permit or a licensed electrician's judgment.

Frequently asked questions

Why would I need a thicker wire than the ampacity table requires?

Ampacity tables only ensure the wire won't overheat carrying that current. On a long run, even an ampacity-safe wire can drop enough voltage to dim lights or slow a motor, so the recommended gauge here also checks the 3% voltage drop guideline and upsizes if needed.

Is aluminum wire cheaper to use than copper?

Often yes for the raw material, but aluminum requires a larger gauge for the same ampacity and drop performance, plus special anti-oxidant compound and compatible connectors, which can close much of the cost gap and adds installation considerations best handled by a licensed electrician.

Does this account for multiple circuits in the same conduit?

No, this is for a single circuit's sizing based on ampacity and drop. Bundling several current-carrying conductors together requires ampacity derating per NEC tables, which is a separate calculation — see the Conduit Fill Calculator for the raceway-capacity side of that question.

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