A 30-amp breaker does not operate safely with a 12-gauge wire. The minimum wire size that is allowable for use with a 30-amp breaker is 10 gauge.
A smaller gauge of wire indicates a larger diameter and the ability to carry more current safely. Wire amperage is a measure of the amount of current a wire can handle without overheating and presenting a fire hazard.
Twelve-gauge wire is acceptable with 20-amp or smaller circuit breakers. It supplies power to outlets, lights and small kitchen appliances. Thirty-amp breakers protect circuits for larger appliances, such as 220-volt air conditioners, ovens and water heaters.