California Code, Public Utilities Code - PUC § 8054

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 the floor of the subway, manhole, chamber, or underground room is made of stone, concrete, brick, or other similar material not subject to decomposition.  This section does not apply to any 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.