What is the 125% rule?

For continuous loads (running 3+ hours), the circuit breaker and wire must be rated for 125% of the continuous load. For example, a 16A continuous load requires a 20A breaker and wire rated for 20A.

Copper is a better conductor but more expensive. Aluminum is cheaper and lighter but requires a larger gauge for the same current. Always use anti-oxidant compound with aluminum.

Which temperature column should I use?

Most residential wiring (NM-B/Romex) is limited to the 60°C column, even if the wire itself is rated 90°C. THHN/THWN in conduit can often use the 75°C column if the terminations are also rated 75°C.

⚡ Wire Gauge Ampacity Calculator

Determine the correct wire size based on NEC Table 310.16. Ensure safety and compliance.

Circuit Current (Amps)

Conductor Material

Temperature Rating

💡 Expert Tips

Voltage Drop Matters. For runs over 100 feet, you often need to upsize the wire to prevent voltage drop, even if the ampacity is sufficient.

Breaker Sizing. The wire must always be rated equal to or greater than the breaker size. You can put 12 AWG wire on a 15A breaker, but NEVER put 14 AWG wire on a 20A breaker.

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Nick Evans

Licensed Electrician

"The most common mistake I see is DIYers using the 90°C column for Romex. You can't do that. This calculator defaults to the safer options."

⚡ Wire Gauge Ampacity Calculator

Minimum Wire Size

💡 Expert Tips

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