California Code, Public Utilities Code - PUC § 1209

Personal service shall be made in accordance with the provisions of the Code of Civil Procedure or by depositing a copy of the order to show cause certified under seal of the commission with a copy of the petition or order on the commission's own motion attached thereto or made a part thereof in the United States mail, enclosed in a sealed envelope, registered, with postage prepaid, addressed to the party to be served.