Wire Ampacity Calculator

Choose an AWG wire size, conductor material, and insulation temperature rating to look up its NEC base ampacity rating.

How to Use the Wire Ampacity Calculator

Pick an AWG wire size, toggle copper or aluminum, and select the insulation temperature rating — 60°C, 75°C, or 90°C — and the ampacity rating in amps appears immediately below.

Ampacity is the maximum current a conductor can carry continuously without exceeding its insulation's temperature rating, and it isn't calculated from wire geometry the way resistance and voltage drop are — it's set by code, in a table, because it depends on how heat actually builds up and dissipates in a real installation, not just on how much copper is in the wire.

These figures are the National Electrical Code Table 310.16 base ampacity values — copper or aluminum, no more than 3 current-carrying conductors bundled together, 30°C ambient. No ambient-temperature correction and no conductor-bundling adjustment factor has been applied. A real installation with a hotter ambient, more than three conductors in the same raceway, or other code-specific conditions needs those correction factors applied on top of this number, and every figure here should be checked against the current NEC edition before being used for anything safety-critical. This tool doesn't replace a licensed electrician's sizing calculation.

12 AWG copper, 60°C:  20 A
12 AWG copper, 90°C:  30 A
4/0 AWG copper, 90°C: 260 A

Notice the rating climbs with the insulation temperature at a fixed wire size — 90°C-rated insulation lets 12 AWG copper carry 30 amps instead of the 60°C column's 20 amps, because the conductor is allowed to run hotter before its insulation is at risk. That doesn't mean an installer gets to use the 90°C column freely, though: the actual allowable ampacity for a circuit is also capped by the temperature rating of the terminals and equipment the wire connects to, which are very often only rated for 60°C or 75°C regardless of what the wire itself could handle.

Aluminum consistently needs a larger gauge than copper for the same ampacity — 12 AWG copper carries 20 A at the 60°C rating, but 12 AWG aluminum only reaches 15 A at that same rating and temperature, a real cut for switching metals at an identical wire size. That gap is exactly why aluminum branch circuits and feeders typically get sized a step or two larger than the copper equivalent would need.