Wire Size Calculator (Ampacity)
Enter a load current and material, and this tool picks the smallest 75°C-rated conductor with enough ampacity to carry it — plus the small-conductor breaker cap where it applies.
Wire Size Calculator
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Terminations, ambient-temperature/bundling derating, and continuous loads (125% sizing) can require a larger wire than this base ampacity table shows.
Simplified starting point for planning. Does not apply every NEC adjustment/derating factor. Verify against the current NEC and your local AHJ before installing.
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How wire size is picked
Every conductor has a maximum continuous current rating — its ampacity — based on insulation temperature rating (this tool uses the common 75°C column) and material. The rule: pick the smallest size whose ampacity meets or exceeds the load.
| Size (Cu, 75°C) | Ampacity |
|---|---|
| 14 AWG | 20A |
| 12 AWG | 25A |
| 10 AWG | 35A |
| 8 AWG | 50A |
| 6 AWG | 65A |
Worked example: a 40A copper load skips 14, 12, and 10 AWG (20A, 25A, 35A — all under 40A) and lands on 8 AWG, rated 50A. Small conductors carry their own breaker caps regardless of table ampacity: 14 AWG copper is capped at a 15A breaker, 12 AWG at 20A, and 10 AWG at 30A (aluminum: 12 AWG at 15A, 10 AWG at 25A).
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Frequently asked questions
What is ampacity?
Ampacity is the maximum current a conductor can carry continuously without exceeding its insulation’s temperature rating. This tool uses the 75°C ampacity column, the most common rating for THHN/THWN-2 building wire.
Why does 14 AWG copper have a 15A breaker cap even though smaller loads exist?
The NEC caps overcurrent protection on small conductors (14, 12, and 10 AWG) regardless of their table ampacity, to protect the wire and its terminations. That is why 14 AWG copper tops out at a 15A breaker even though other factors might suggest more.
Does this account for derating or continuous loads?
No. Ambient temperature, conduit fill/bundling, and the 125% continuous-load rule can all push the required size up. This tool gives a base starting point only — verify with the current NEC and your local AHJ.
Is this wire size calculator free?
Yes — free, no sign-up. Enter your load current and material and read the recommended size instantly. Estimates only; confirm against the current NEC and your local AHJ.