A circuit breaker is a critical safety device that automatically disconnects power when electrical current exceeds a safe level, preventing fire, electrocution, and equipment damage. Unlike traditional fuses that must be replaced after each fault, breakers reset and can be used repeatedly. Circuit breakers operate on two principles: thermal protection (resistance from overheating) for gradual overloads, and magnetic protection (electromagnetic force) for sudden short circuits. Breaker sizing is governed by the National Electrical Code (NEC), which specifies that breakers must be selected to protect the smallest conductor in the circuit and sized appropriately for continuous vs. non-continuous loads.

The NEC Article 210 mandates that circuit designers apply a 125% multiplier to continuous loads (those operating 3 hours or longer) when selecting breaker ratings. This prevents nuisance tripping when typical loads cycle up to their rated values. For example, a 16-ampere continuous load requires a 20-ampere breaker (16 × 1.25 = 20), not a 16-ampere breaker. Wire gauge selection is equally critical—undersized wire from breaker to load creates heat and fire risk, while oversized wire wastes cost and space. The breaker must protect both the wire and the load, so wire gauge is always selected to match or exceed breaker capacity per NEC Table 310.15(B)(16). Residential circuits typically use 15-20A breakers with 14-12 AWG wire, while commercial and industrial systems scale to 200-400A breakers and larger conductors for industrial motors and equipment banks.