Wire Size & Ampacity Calculator

Match your load current to a conductor whose ampacity equals or exceeds it in the correct temperature column of NEC Table 310.16. This tool does that for copper — enter the amps, pick the insulation temperature rating, and it returns the smallest gauge that carries the load.

Ampacity is the maximum current, in amps, a conductor can carry continuously without exceeding its temperature rating. It depends on the conductor size, the insulation's temperature rating, and the installation conditions like ambient heat and how many conductors share a raceway.

Use the column that matches the lowest temperature rating in the circuit. Most modern building wire (THHN/THWN-2) is rated 90°C, but breakers and lugs are commonly rated only 75°C, and on small breakers the 60°C rule can apply. Conductors are generally sized to the termination rating, even though the 90°C column can be used for derating math.

The NEC defines a continuous load as one expected to run at its maximum for 3 hours or more. For those loads the conductor and overcurrent device must be sized at 125% of the load current to account for sustained heating. Toggle the continuous option and the tool multiplies your current by 1.25 before sizing.

No. This calculator uses the copper columns of NEC Table 310.16 only. Aluminum and copper-clad aluminum conductors have lower ampacity for the same gauge and use a separate set of values, so do not apply these results to aluminum.

Because the weakest link sets the limit. Even with 90°C conductors, if your breaker or lug terminations are rated 75°C, the circuit must be sized to that 75°C ampacity. The 90°C value is mainly used as a starting point for derating calculations, not as the final allowable current at the terminals.

This tool gives base ampacity only. It does not apply the ambient-temperature correction or the conduit-fill (more-than-3-current-carrying-conductors) adjustment from NEC 310.15, nor does it size for voltage drop. Long runs and hot or crowded conduits often require a larger conductor than the base value suggests.

Yes, the calculator is completely free with no signup required. Your inputs are saved locally in your browser so they persist between visits — nothing is uploaded to a server. Clearing your browser data will erase your saved values.