California Code, Corporations Code - CORP § 14316

(a) Water mains to be installed below the frost level or otherwise protected to prevent freezing and shall not have less than 30 inches of cover over the top of the pipe in public streets or alleys except where such depth is rendered impossible by underground obstructions or rocky or hardpan conditions.

(b) The size, design, material, and installation of service pipes shall conform to the reasonable requirements of the mutual water company;  provided, however, that the minimum size of the pipe shall be 3/4 inch or greater nominal size.  Except where services are not intended for use during freezing weather and arrangements are made to drain service pipes prior to freezing weather, and except at terminations in connection with the meter or water consumer's piping, all service pipes shall be laid at a depth sufficient to prevent freezing.