Knowing how to size circuit breakers and select the right size and type of wire for any electrical project is key to a quality, safe design. Choosing the right components for a wiring system requires a good understanding of the National Electric Code (NEC) and your local building codes because your local authority having jurisdiction may have code requirements that supersede NEC's requirements. The NEC simply sets minimum standards for a safe wiring installation. Applying the NEC's rules and regulations in sizing circuit breakers and selecting the proper size conductors also requires the ability to work with basic algebraic formulas.
Make a list of all the loads that the circuit will supply. List the continuous duty loads and non-continuous duty loads separately. The NEC defines a continuous duty load as "a load where the maximum current is expected to continue for three hours or more (Article 100-Definitions)."
List the full load ampere (FLA) rating for all the continuous duty loads. The FLA can be found on the device's nomenclature tag. In some cases, as with baseboard heat units, there may be a wattage rating given instead of FLA. In those cases, the FLA can be calculated by dividing the unit's wattage rating by its voltage (watts/volts = FLA). For example, a 240-volt, 5,200-Watt heater would have an FLA of 21.7 amperes.
Calculate the minimum circuit breaker rating required for the continuous duty loads by multiplying the sum of all the FLA ratings by 1.25, per NEC Article 210.20.
Add the FLA rating for all the non-continuous duty loads to the minimum required circuit breaker rating previously calculated. The minimum rating for the circuit breaker is equal to 1.25, which is the sum of all FLA ratings for continuous duty loads plus the sum of all the FLA ratings for non-continuous duty loads.
Compare this computed minimum rating with the ratings given for standard size breakers in NEC Article 240.6. If the computed rating falls between two standard ratings, choose the next higher standard rating.
Add up all the FLA ratings for the non-continuous duty loads.
Add up all the FLA ratings for the continuous duty loads and multiply that sum by 1.25.
Apply NEC Article 240.6(A)(1), in which the minimum conductor ampacity equals 1.25 (the sum of FLA ratings for continuous duty loads plus the sum of FLA ratings for all non-continuous duty loads).
Compare this computed minimum ampacity rating with the standard ampacities given in NEC Table 310.16. If the rating falls between two standard ratings, choose the next larger conductor. For example, if it fell between 30 and 40 amperes, you would choose the larger wire, the AWG 8.