Sec. 664.5. Amortization accounting

(a) Amortization shall be computed on the straight-line method, i.e., equal annual amounts shall be applied, or other method as directed by the commission. The cost of each type of asset shall be amortized on the basis of the estimated life of that asset and shall not be written off in the accounting period in which the asset is acquired. A reasonable estimate of the useful life may be based on the upper or lower limits even though a fixed existence is not determinable. However, the period of amortization shall not exceed 40 years.

(b) In the event any estimated useful life becomes no longer applicable, a revised estimated useful life shall be determined in accordance with subdivision (a) of this section.

(c) Amortization charges shall be made monthly to the appropriate amortization expense accounts and corresponding credits shall be made to the appropriate amortization reserve accounts. Monthly charges shall be computed by the application of one-twelfth to the annual amortization amount.

(d) The company shall keep such records as will allow the determination of the useful life of the asset.