Breaker Size Calculator

A breaker size calculator picks the circuit breaker a load needs — applying the NEC's 125% rule for continuous loads — and tells you the minimum copper wire gauge that breaker must protect. It connects the three things that have to agree: the load, the breaker, and the wire.

Your measurements

Result

The 125% rule trips people up: a breaker should carry no more than 80% of its rating continuously, so a 16 A continuous load needs a 20 A breaker, not a 15 A. This builds that in and matches the wire to the breaker.

How it is calculated

Design current = load × 1.25 for continuous loads (else ×1.0). The breaker is the next standard size at or above the design current (15, 20, 25, 30 A…). Minimum wire is a simplified NEC copper pairing (15 A→14, 20 A→12, 30 A→10, 40 A→8…). The breaker must protect the conductor; derating, terminal ratings and load type can change the wire — follow the NEC.

Steps

Enter the load in amps, or in watts with the voltage.
Choose 120 V or 240 V.
Say whether it's a continuous load (on 3+ hours).
Read the recommended breaker size.
Use at least the minimum wire gauge shown — never less.

Worked example

A 16 A continuous load needs 16 × 1.25 = 20 A, so a 20 A breaker on 12 AWG copper.

Common questions

What size breaker do I need?

Size it to at least 125% of a continuous load, then round up to the next standard breaker. A 16 A continuous load needs a 20 A breaker.

What's the 80% / 125% rule?

A breaker shouldn't carry more than 80% of its rating continuously — equivalently, size continuous loads at 125%. It prevents nuisance trips and overheating.

Can I just put in a bigger breaker?

No — the breaker protects the wire. A bigger breaker on undersized wire is a fire risk. Upsize the wire first.

What wire goes with a 20 amp breaker?

12 AWG copper for a standard 20 A circuit; 14 AWG is only good to 15 A. The calculator pairs them.