Circuit Breaker Calculator

This calculator starts from Actual Current Draw (Total Load in watts divided by Voltage), applies your Safety Factor to get Required Amps, then rounds up to the nearest standard breaker size from a table (15A through 100A). Voltage has the largest effect on Actual Current Draw and Required Amps: halving the voltage from 240V to 120V doubles the current draw for the same wattage, since amps equal watts divided by volts, and that swing is larger than a modest change in Total Load itself. Safety Factor is inert to Actual Current Draw -- it only applies to Required Amps, which is Actual Current Draw multiplied by Safety Factor as a percentage -- but it does move Recommended Breaker Size and Recommended Wire Gauge upward as it increases, since a higher safety margin raises the required amperage that has to be covered.

Recommended Wire Gauge is looked up from Recommended Breaker Size using a standard breaker-to-AWG table, so it moves in the opposite direction from breaker size: a larger breaker (more amps) always maps to a lower AWG number (thicker wire). This tool implements the 125% continuous-load rule from NFPA 70, the National Electrical Code (NEC), as a simplification, and its NEC 80% Rule check mirrors the NEC's guidance against loading a breaker past 80% of its rating on a continuous basis; it does not account for ambient temperature derating, multiple conductors in a raceway, or specific breaker manufacturer curves, so always confirm the final selection against your local code.