(a) Unless otherwise authorized by order of the commission, separate records shall be maintained for the book cost of property comprising each electric plant owned or operated. The term plant as here used covers each separate generating station, substation, transformer station and transmission line, and the distribution system located in each city, village, or town.

(b) In like manner, expenses of operation and maintenance (accounts 701-734 shall be separately recorded for each plant as defined above, except transmission lines located wholly within a single city, village, or town, and distribution systems, provided that when a large number of small plants of the same nature are located so that two or more are operated as a unit under circumstances which make impracticable the segregation of such expenses, they may be kept for the group instead of for each plant.

(c) Expenses of operation and maintenance of the distribution, street lighting, and signal system property and transmission lines located wholly within a single city, village, or town, and the accounting, collecting, and sales activities (accounts 731-774) shall be kept separately for each city, village, or town, except that, with the approval of the commission, expenses for two or more such territories which are contiguous and are operated as a unit, may be combined.