Breaker Size Calculator

Overcurrent protection sizing starts from the load current, not from the wire. For a single-phase load, current equals watts divided by the product of voltage and power factor, so a 2,400 watt resistive load at 120 volts draws 20.0 amps while the same wattage in an inductive load at 0.85 power factor draws 23.5 amps. NEC 210.20(A) then requires the overcurrent device to be rated for at least 125 percent of any continuous load, defined in Article 100 as a load expected to operate for three hours or more. The 25 percent cushion keeps the breaker and its terminations below the temperature at which connections and insulation degrade under sustained loading, which is why a 16 amp continuous load requires a 20 amp breaker. Motor branch circuits follow a different logic in Article 430. Inrush current on a squirrel-cage induction motor commonly reaches six to eight times full-load current for a fraction of a second, so NEC Table 430.52 permits an inverse-time breaker at up to 250 percent of full-load current to ride through starting without nuisance tripping. Standard device ratings are fixed by NEC 240.6(A) at 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, and 100 amps and upward, so results are rounded to the next available size. Two caveats dominate practice: the breaker must never exceed the ampacity of the conductor it protects, and ambient temperature, raceway fill, and long conductor runs each require separate derating that this arithmetic does not include.

Fuses came first. Thomas Edison patented a fusible-link protective device in 1890, and for the next three decades branch circuit protection in buildings meant renewable or cartridge fuses in a panel or fuse block. The resettable alternative arrived in 1924, when Hugo Stotz, working with Brown, Boveri and Cie in Mannheim, patented a compact thermal-magnetic breaker intended as a direct replacement for the household fuse. The design paired a bimetallic strip that responds slowly to sustained overload with an electromagnetic trip that responds almost instantly to a short circuit, and that two-mechanism arrangement still defines the shape of a modern residential breaker trip curve. Miniature circuit breakers spread through North American residential construction after the Second World War, displacing fuse panels through the 1950s and 1960s partly because they could not be defeated by the common practice of substituting an oversized fuse or a coin behind it. Standardized frame ratings, now codified in NEC 240.6, developed alongside so that conductor ampacity tables and device ratings could be matched in a single step. The 125 percent continuous-load rule and the separate motor rules of Article 430 grew out of decades of field experience with heat damage at terminations and with nuisance tripping on motor starts. Both continue to be revised on the three-year NEC cycle as newer loads such as electric vehicle supply equipment change the mix of sustained demand on branch circuits.