Tenn. Code Ann. § 7-82-607: Vacancies in board

(a) Except as provided in subsection (b), the office of any commissioner, other than a commissioner at large, shall become vacant if the incumbent ceases to reside in or be a district customer in the county from which the commissioner was elected.

(b) In any county having a population of not less than twenty-seven thousand nine hundred (27,900) nor more than twenty-seven thousand nine hundred twenty (27,920), according to the 1980 federal census or any subsequent federal census, the office of any commissioner, other than a commissioner at large, shall become vacant if the incumbent ceases to reside in the county from which the incumbent was elected.