Sec. 664.1. Computation of depreciation rates

(a) Depreciation percentage rates shall be computed in conformity with a group plan of accounting for depreciation and shall be such that the loss in service value of the property, except for losses excluded under the definition of depreciation, may be distributed under the straight-line method during the service life of the property. These percentage rates shall be based upon the estimated service values and service lives developed by a study of the company's history and experience and such engineering and other information as may be available with respective future conditions. Such percentage rates shall not include any allowance for loss in service value of property expected to be installed in the future. The percentage rates shall, for each primary account comprised of more than one class of property, produce a charge to operating expenses for that account equal to the sum of the amounts that would otherwise be chargeable for each of the various classes of property included in the account.

(b) In the event any composite percentage rate becomes no longer applicable, revised composite percentage rates shall be computed and filed with the Commission in accordance with subdivisions (a) and (d) of this section.

(c) The company shall keep such records of property and property retirements as will allow the determination of the service life of property which has been retired, or facilitate the determination of service life indications by mortality, turnover, or other appropriate methods. Such records will also allow the determination of the percentage of salvage value or net salvage value, as appropriate, for property retired from each class of depreciable plant. (See account 2321 regarding amortization of customer premises wiring.)

(d) Not later than six months following the date when a company becomes subject to this system of accounts, it shall have filed with this commission estimates of the composite annual percentage rates considered applicable to the original cost of each class of depreciable telecommunications plant owned or used by the company. (Note section 664.2 of this Part.) The estimates of the composite annual percentage rates so filed shall be accompanied by a statement subscribed to by an officer or other duly authorized representative of the company, showing the bases therefor and the methods employed in their computation.