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How to Size a Generator Output Circuit Breaker

William Kinsey

Generator output circuit breakers are a very important safety feature. These breakers are designed to disconnect the load if it tries to pull too much current from the generator, which would put the generator in an overload condition. The National Electric Code (NEC) has not established strict rules for sizing generator output circuit breakers. Controlling the generator's output is an effort to prevent generator damage and avoid possible safety and fire hazards. Also, by limiting the generator output, you can make the generator last a lot longer.

  1. Read through the generator specifications and identify the rated current, rated KW, and the lag factor of the generator. Write each of these items down on a sheet of paper.

  2. Divide the rated current, in amps, by the power factor. The rated current of the generator can be found in the generator specifications or on the label on the generator. This value should not have to be calculated. For example, a generator with a rated current of 50 amps and a power factor of 0.8 has a true current value of 50/0.8 or 62.5 amps. This would portend to using a 60 to 70 amp breaker. Verify that the breaker size does not exceed 115 percent of the generator's maximum rated current.

  3. Use additional electronic protection to safeguard the generator under special load conditions. This type of protection utilizes and electronic monitoring circuit. Electronic protection can analyze the generator's current operating conditions and cross reference that condition with previous manufacturer results. If the generator output exceeds those results, then the generator will automatically shutoff. This feature is standard with large generators over 10KW.