California Code, Public Utilities Code - PUC § 8053

No person shall build or rebuild in any subway, manhole, chamber, or underground room, used or to be used to contain, encase, cover, or conduct any wire, cable, or appliance to conduct or handle electricity, any opening which is at the surface of the ground, within the distance of three feet at any point from the rail of any railway or streetcar track.  This section does not apply where satisfactory proof is submitted to the commission that it is impracticable or physically impossible to comply with this law in the space or location so designated by the proper municipal authorities.