California Code, Health and Safety Code - HSC § 114325

(a) Except on a mobile food facility that only utilizes the water for handwashing purposes, a water heater or an instantaneous heater capable of heating water to a minimum of 120°, interconnected with a potable water supply, shall be provided and shall operate independently of the vehicle engine. On a mobile food facility that only utilizes the water for handwashing purposes, a minimum one-half gallon-capacity water heater or an instantaneous water heater capable of heating water to a minimum of 100°, interconnected with a potable water supply, shall be provided and shall operate independently of the vehicle engine.

(b) A water heater with a minimum capacity of four gallons shall be provided for mobile food facilities that have a warewashing sink.

(c) A mobile food facility equipped with a three-gallon-capacity water heater that is in compliance with this section on January 1, 2014, is in compliance with this section after that date.