(a) If the Commission orders that a change shall not go into effect until final determination of the proceedings, it shall proceed to hear the matter as promptly as possible and shall make its determination within seven months from the date that it orders the investigation. If a company files for a change in rate design among classes of ratepayers, and the company has a rate case pending before the Commission, the Commission shall make its determination on the rate design change within seven months after the rate case is decided by the Commission. If the Commission fails to make its determination within the time periods set by this subsection, the changed rate schedules filed by the company shall become effective and final.

(b) The Commission, on its own motion, may order an investigation and hearing on the justness and reasonableness of existing rates of a company, subject to supervision under this chapter. The Commission shall proceed to hear the matter as promptly as possible and shall make every effort to make its determination within seven months from the date the proceeding was instituted. If the Commission does make its determination within such seven months then its final order shall be retroactive to the day that the proceedings were instituted and such final order shall contain a directive that the company, other than a common carrier of passengers by motor vehicle, shall repay to the persons from whom collected between the time the proceedings were instituted and the final order all sums which the Commission determines are in excess of the rates ultimately found to be just and reasonable. If the Commission does not make its determination within seven months of the institution of the proceedings then its final order when made shall be retroactive only to a date seven months after the institution of the proceedings and the final order shall contain a directive that the company shall repay to persons from whom collected between the date seven months after the institution of the proceedings and the determination thereof all sums which the Commission determines are in excess of the rates ultimately found to be just and reasonable. (Amended 1959, No. 329 (Adj. Sess.), § 39(b), eff. March 1, 1961; 1961, No. 263, § 4, eff. July 31, 1961; 1981, No. 226 (Adj. Sess.), § 5, eff. May 6, 1982; 1995, No. 182 (Adj. Sess.), § 17, eff. May 22, 1996; 2019, No. 31, § 20.)