Labor Code § 7625
CHAPTER 1. Scope of Chapter and General Provisions
The following steam boilers are not subject to this part:
(a) Boilers under the jurisdiction or inspection of the United States Government, and all other boilers operated by employers not subject to Division 4 of this code.
(b) Boilers on which the pressure does not exceed 15 pounds per square inch.
(c) Automobile boilers and boilers on road motor vehicles.