(a) If the utility is engaged in more than one utility service, such as water, electric and gas, 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 may be designated and classified as “common utility plant”.

(b) The book amount of utility plant designated as common plant shall be included in account 118.1, Common Utility Plant, and if applicable in part to water department, shall be segregated and accounted for in subaccounts as water plant is accounted for in accounts 101 to 107, inclusive, and water plant adjustments in accounts 114 and 116; any amounts classifiable as common plant acquisition adjustments or common plant adjustments shall be subject to disposition as provided in paragraph C and B of accounts 114 and 116, respectively, or amounts classified in those accounts. The original cost of common utility plant in service shall be classified according to detailed utility plant accounts appropriate for the property.

(c) The utility shall be prepared to show at any time and to report to the commission annually, or more frequently, if required, and by utility 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 accumulated provision 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, and taxes 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 departments using the common utility plant shall be supported in such manner as to reflect readily the basis of allocation used. Provisions in this system of accounts applicable to water plant and operations shall extend also to the accounts for common utility plant and operations.