Tenn. Code Ann. § 8-30-209

No preferred service employee shall be assigned to perform the majority of the duties and responsibilities of a position in a higher level classification than that of the position occupied by the employee, without the approval of the appointing authority. When an employee is so assigned, the duration of such assignment may not exceed ninety (90) days without the approval of the commissioner. The commissioner, in consultation with the commissioner of finance and administration, shall establish a procedure under which an employee who is assigned to perform the majority of the duties and responsibilities of a higher level classification for a period in excess of ninety (90) days shall receive additional compensation for such assignment.