(a) If a municipal corporation offers retail electric utility service in a municipality having a population of more than 151,000 that is located in a county having a population of more than 2.5 million, the commission shall singly certificate areas in the municipality's boundaries in which more than one electric utility provides electric utility service.
(b) In singly certificating an area under Subsection (a), the commission shall preserve the right of an electric utility to serve the customers the electric utility was serving on June 17, 1983. This subsection does not apply to a customer at least partially served by a nominal 69,000 volts system who gave notice of termination to the utility servicing that customer before June 17, 1983.