Breaker Size Calculator

Circuit breaker sizing is fundamentally based on Ohm's Law and safety standards dictated by electrical codes. To find the total current demand of a circuit, the calculator sums the individual power draws (in Watts or Amps) of all connected appliances. For AC circuits, it accounts for single-phase or three-phase systems, system voltage, and the power factor (Pf). Once the continuous and non-continuous loads are established, the NEC typically requires applying a 125% multiplier to continuous loads (loads running for three hours or more) to prevent nuisance tripping and thermal fatigue. The calculated amperage is then matched up to the next standard commercial breaker size available on the market.

Imagine you are setting up a dedicated 120V single-phase workshop circuit (VS = 120V) running a continuous load. You have two primary equipment items: a heavy-duty table saw drawing 1,440 Watts and a dust collection system drawing 960 Watts, resulting in a total connected load of 2,400 Watts. First, calculate the total operating current by dividing power by voltage: 2,400W / 120V = 20 Amps. Because this is designated as a continuous load, you apply the 125% safety multiplier: 20 Amps * 1.25 = 25 Amps. Therefore, the calculator determines that a 25-Amp circuit breaker paired with appropriate 10 AWG copper wiring is required to safely power this setup without overheating.

Always factor in the 80% rule for continuous loads, meaning a breaker should never operate above 80% of its rated capacity for extended periods. Ensure your conductor (wire) gauge is rated to handle at least the amperage of the breaker you choose; never install a larger breaker on a wire gauge that is too thin, as this creates a severe fire hazard. Finally, always consult local electrical codes and obtain necessary permits before modifying your main electrical panel.