Cal. Public Utilities Code § 8051

No person shall build or rebuild 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, unless such subway, manhole, chamber, or underground room has an inside measurement of not less than four feet at the maximum points between the side walls thereof, and between the end walls thereof, and not less than five feet at all points between the floor and the top or ceiling thereof, or if circular in shape, at least four feet diameter inside measurement and not less than five feet at all points between the floor and ceiling thereof. This section does not apply to any such subway, manhole, chamber, or underground room, within which it is not intended or required that any human being perform work or labor or be employed. 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 within the space or location so designated by the proper municipal authorities.