Tex. Util. Code § 164.005
(b) If the ordinance or resolution authorizing or the deed of trust or trust indenture securing the revenue bonds of a utility system provides for the subsequent issuance of additional bonds or the creation of a contractual obligation described by Section 164.004 and provides that the payments to be made for the security or payment of the subsequent bonds or contractual obligation are to be on a parity with the previously issued bonds or bonds then to be issued, the political subdivision may, subject to any conditions contained in that ordinance, resolution, deed of trust, or trust indenture, authorize, issue, and sell additional bonds or incur the contractual obligation in a different series payable from the entire revenue of the utility system on a parity with the previously issued bonds or bonds then to be issued and secured by a lien on the revenue of the system on a parity with the lien securing the previously issued bonds or bonds then to be issued. This subsection applies without regard to whether the previously issued bonds:
Acts 1997, 75th Leg., ch. 166, Sec. 1, eff. Sept. 1, 1997.