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Wire Gauge Ampacity Calculator (AWG)

Pick an AWG wire size and insulation rating to see its safe current-carrying capacity.

How it is calculated

This is a table lookup, not a formula. For the AWG size you choose (14 to 4/0), the tool reads the copper ampacity from a built-in NEC-style table in one of three columns, 60, 75 or 90 °C insulation rating. If the temperature is not 60 or 90, the 75 °C column is used. It also gives the conductor cross-section in circular mils and converts it to square millimetres with mm² = cmil × 0.0005067. Ampacity rises as the AWG number falls, and for a given size it rises with the insulation rating. A size outside the list returns no result. The tool does not apply derating for ambient temperature, bundling of conductors, conduit fill, continuous loads, voltage drop or cable length, and it covers copper only.

Worked example

You pick 10 AWG copper with a 75 °C insulation rating. In the built-in table the 10 AWG row reads 30 A at 60 °C, 35 A at 75 °C and 40 A at 90 °C, so the ampacity is 35 A. The wire is 10,380 circular mils, which the tool converts as 10,380 × 0.0005067 = 5.26 mm². Choosing the 60 °C column for the same wire would give 30 A, five amps less. The right column to use is the one that matches the lowest rating among the wire, the terminals and the devices it connects to.

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Common mistakes

  • Using the 90 °C column for sizing a circuit when the terminals or breaker are rated only for 60 or 75 °C; the usable figure is then limited by the lower rating.
  • Treating the table value as a guaranteed safe load without derating for a hot environment, many cables bundled together or a continuous load.
  • Reading a higher AWG number as a thicker wire; 14 AWG is thinner than 10 AWG, and 4/0 is the thickest size here.
  • Applying copper ampacities to aluminum conductors, which carry less current at the same size.
  • Ignoring voltage drop on a long run: a wire can be within its ampacity and still be too thin for the distance.

About this calculator

Ampacity is the maximum current a wire can carry continuously without exceeding its insulation's safe temperature rating, and in the U.S. it's governed by NEC Table 310.16, which lists safe amperage by copper (or aluminum) wire gauge (AWG) and insulation temperature class — 60°C, 75°C, or 90°C. A thicker wire (lower AWG number) and higher-rated insulation both allow more current; for example, 12 AWG copper is rated for 20A at 60°C but up to 30A at 90°C insulation, though the lower figure is typically what's used for circuit breaker sizing.

Electricians, electrical contractors, and DIYers wiring a panel, outlet circuit, or extension cord reference ampacity tables constantly to choose wire gauge that won't overheat under expected load, since undersized wire is a fire hazard and a common code violation.

This calculator looks up the NEC-rated ampacity for a chosen AWG size and insulation temperature rating, giving a quick reference without flipping through the full NEC table by hand.

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Comments (1)

  • Editorial team

    This table is ampacity only, and US code adds a separate cap on breaker size for the smallest copper sizes: 14, 12 and 10 AWG are normally protected at no more than 15, 20 and 30 A, even where a higher-temperature column shows more. So the 90 °C figure is mostly useful as a starting point for derating, not for choosing a breaker. Which part of a job do you use the table for most, a new circuit or checking an old one?

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