TradesHow this works
The breaker exists to protect the wire. It sizes to the load, rounds up to a standard breaker size, and never goes above what the conductor can safely carry. This tool runs those rules for you. It applies the 125% bump for continuous loads, respects the small-conductor cap on 14, 12, and 10 gauge, and tells you if the wire you picked is actually too small for the load.
If it warns that the conductor is undersized, size the wire first with the Wire Size engine, then come back. A bigger breaker is never the fix for a wire that cannot carry the load.
Common questions
Does the breaker protect the wire or the device?
The wire. The breaker is there so the conductor never carries more current than it can safely handle. That is why you cannot just put a bigger breaker on to get more capacity. The breaker is capped by what the wire can carry.
What is the 125% continuous rule?
A load that runs for three hours or more is continuous, and the circuit has to be sized for 125% of it. So a 40 amp continuous load is sized as 50 amps, which usually means the next breaker up.
Why is 12 gauge always on a 20 amp breaker?
Small conductors, 14, 12, and 10 gauge, are capped by NEC 240.4(D) at 15, 20, and 30 amps no matter what the ampacity table says. It is a safety rule specific to small wire.
Sources: NEC 240.6(A) standard sizes, 240.4(D) small-conductor limits, 210.19/215.2 (2023) · NEC Table 310.16 ampacity (2023). Engine version 1.0.0. Risk tier 2.