Cal. Health & Safety Code § 41701.6

Neither the state board nor any district shall impose a discharge requirement on emissions of visible smoke from any diesel auxiliary engine or generator used exclusively to operate a drinking water system which is more stringent than the Ringelmann 2 limit, as published by the United States Bureau of Mines on January 1, 1995, when operated under emergency circumstances, or operated not more than 30 minutes each week, or two hours each month, under nonemergency circumstances.