- 1). Decide whether the circuit breaker will be protecting continuous loads, noncontinuous loads or a combination of continuous and noncontinuous loads.
- 2). Make a list of all the continuous loads on the circuit and their full load ampere requirements or their Branch Circuit Selection Currents, using the larger of the two, found on the appliance's specifications plate. Appliances like electric baseboard heat units give a wattage rating instead of FLA or branch circuit selection current ratings and in those cases you must calculate the appliances' FLA rating by dividing its wattage rating by its operating voltage.
Convert from watts to amperes by dividing the wattage rating by the voltage rating, i.e., a 240-volt, 5,700-watt baseboard heater would have an FLA equal to 5,700/240 or 23.75 amperes (A). - 3). Add up all the FLA ratings for the continuous loads and multiply this sum by 1.25. Continuous loads are loads where the maximum current flows for three hours or more (National Electric Code, Article 100) i.e., baseboard heating units.
- 4). Add up the FLA ratings for all noncontinuous loads on the same circuit and then add this sum to the total computed load for the continuous duty loads. The National Electric Code states, " ...the rating of the overcurrent device shall not be less than the noncontinuous loads plus 125 percent of the continuous loads [Article 210.20(A)."] An over-current device may be either a circuit breaker or a fuse.
- 5). Compare the final computed amperage rating computed with the standard values for over-current protection devices given in Article 240.6(A) of the National Electric Code. If the computed value falls between two standard values, select the higher rated device. Although there are some circuit breakers designed to operate continuously at 100 percent of their rated amperage, the National Electric Code restricts most circuit breakers to serving continuous loads no greater than 80 percent of their ampacity.
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