Section 12 - Common Utility PlantA. If the utility is engaged in more than one utility service such as electric, gas, and water, and any of its utility plant is used in common for several utility services or for other purposes to such an extent and in such manner that it is impracticable to segregate it by utility services currently in the accounts, such property, with the approval of the Department, may be designated and classified as "common utility plant."B. Utility plant designated as common utility plant shall be classified according to the detailed utility plant account appropriate for the property.C. The utility shall be prepared to show at any time and to report to the Department annually, or more frequently, if required, and by gas plant accounts (301 to 399) the following: (1) the book cost of common utility plant, (2) the allocation of such cost to the respective departments using the common utility plant, and (3) the basis of the allocation.D. The reserve for depreciation and amortization of the utility shall be segregated so as to show the amount applicable to the property classified as common utility plant.E. The expenses of operation, maintenance, rents, depreciation and amortization of common utility plant shall be recorded in the accounts prescribed herein but designated as common expenses, and the allocation of such expenses to the department using the common utility plant shall be supported in such manner as to reflect readily the basis of allocation used.