Cal. Public Utilities Code § 13122

Money set aside and placed in any such reserve fund or special trust account shall remain therein until from time to time expended for the purposes thereof, and shall not be used for any other purpose whatsoever, except for temporary investment thereof as provided in this chapter; provided, however, (1) That money may be paid or transferred from any such fund or account in furtherance of the purpose of its establishment, to any other such fund or account established for a like purpose in connection with the same issue of bonds, to such extent as may be required or permitted under the terms of the resolution authorizing that issue; and (2) That all money remaining in any such fund or account in excess of the amount required to make provision for the payment in full of the principal of, and interest on, the bonds with respect to which it was established, including premiums, if any, due upon the redemption of any thereof, may be returned and transferred to the revenue fund out of which such reserve fund was established.