California Code, Public Utilities Code - PUC § 8031

No person shall run, place, erect, or maintain vertically on any pole any wire or cable used to conduct electricity, without causing such wire or cable to be at all times wholly incased in a casing equal in durability and insulating efficiency to a wooded casing not less than one and one-half inches thick.

This section does not apply to vertical signal wires or cables on poles where no other such wires or cables are maintained, and which are outside the corporate limits of a city, nor to wires or cables run vertically on iron poles or structures where both pole or structure and conduit are securely grounded.