I found a relevant recommendation:
"Engine generators for Tier III and IV sites" (my situation) "shall not have a limitation on consecutive hours of operation when loaded to 'N' demand."
I note that it doesn't say "loaded to 90% of 'N'". So I'd interpret this to mean that the Continuous, or de-rated Prime rating of the genset, must be greater than total load.
I'll still say that THD or motor load could make this recommendation a moot point.
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