Cal. Pub. Util. Code § 1794

The commission or any commissioner or any party may, in any investigation or hearing before the commission, cause the deposition of witnesses residing within or without the state to be taken in the manner prescribed by law for like depositions in civil actions in the superior courts of this state under Title 4 (commencing with Section 2016.010) of Part 4 of the Code of Civil Procedure and to that end may compel the attendance of witnesses and the production of books, waybills, documents, papers, and accounts.