California Code, Revenue and Taxation Code - RTC § 8652

(a) To propel an implement of husbandry, truck, or farm tractor used in agricultural operations off the highway and only incidentally operated upon a highway, for the purpose of moving between farms or parts of farms, which farms or parts of farms are in close proximity, and which implement of husbandry, truck or farm tractor is exempt from registration under the Vehicle Code;

(b) To propel any construction equipment while operated within the confines and limits of a construction project and only incidentally operated upon the highway and which construction equipment is exempt from vehicle registration pursuant to the Vehicle Code; or

(c) For a purpose other than the generation of power to propel a motor vehicle in this state.