Air Compressor Breaker & Wire Size Calculator (NEC)

A compressor that trips its breaker on startup, or one wired too thin, is almost always a sizing problem. Enter the motor's rated horsepower and your supply voltage and this tool returns the full-load current, the minimum copper wire gauge, and a recommended breaker size using standard NEC Article 430 motor rules.

Use the motor's rated running horsepower from the nameplate, not the inflated "peak" or "SP" horsepower on the box. If your nameplate lists amps directly, size the breaker at the next standard size above 250 percent of that figure.

How the numbers are worked out

The full-load current comes from the NEC motor tables (430.248 for single-phase motors). From there:

• Wire size is picked so the conductor carries at least 125 percent of the full-load current, read from the 60°C copper column (NEC 430.22 and Table 310.16).
• Breaker size is the next standard size at or above 250 percent of the full-load current, the ceiling for an inverse-time breaker under NEC 430.52. The larger breaker lets the motor pull its brief starting surge without nuisance tripping.

On a motor circuit the breaker protects the wiring against a short circuit, while the compressor's own thermal overload protects the motor. That is why the breaker can be rated higher than the wire's everyday ampacity, which looks wrong for a normal lighting or receptacle circuit but is correct here.

This is a planning estimate for copper conductors in a typical run at 30°C ambient. Long runs need a voltage-drop check, and the final install must follow your local code. Have a licensed electrician verify and connect any hard-wired circuit.