30 Percent Rule

Q. I am looking for some clarification on the 30 percent rule for the monthly load testing of my generators. The generator in question is a 60 KW with an amp rating of 208.2. This generator does not have a KW output reading so we use the amp output for determining the 30 percent. ( 63 amps per leg at 208 voltage ) During one of the tests the amp readings where (65 , 40 , 110) which has one leg under 30 percent. If you look at the overall output of the generator it is 20.6 KW and I only need 18.1 KW to meet the 30 percent. Thank you for any help you could give me.

A. My opinion is that you are over the 30% at 20.6kw (even with one leg being severely out of balance…and under the average). As you know NFPA 110 doesn’t state how the kW is to be computed, but normal practice would be to “average” the legs.

Note that I cannot speak for the NFPA nor render an opinion on their behalf even though I am a member of the Technical Committee responsible for NFPA 110.