(1) Every public utility shall carry a proper and adequate depreciation account. The Public Utility Commission shall ascertain and determine the proper and adequate rates of depreciation of the several classes of property of each public utility. The rates shall be such as will provide the amounts required over and above the expenses of maintenance, to keep such property in a state of efficiency corresponding to the progress of the industry. Each public utility shall conform its depreciation accounts to the rates so ascertained and determined by the commission. The commission may make changes in such rates of depreciation from time to time as the commission may find to be necessary.
(2) In the following cases the commission may allow in rates, directly or indirectly, amounts on the utility’s books of account which the commission finds represent undepreciated investment in a utility plant, including that which has been retired from service:
(a) When the retirement is due to ordinary wear and tear, casualties, acts of God, acts of governmental authority; or
(b) When the commission finds that the retirement is in the public interest.