(a) For the purpose of applying the system of accounts prescribed herein, utilities are divided into two classes as follows:

Utilities having annual electric operating revenues of more than $150,000 but not more than $1,000,000. Such utilities shall keep the accounts prescribed herein which are applicable to their operations.

Utilities having annual electric operating revenues of more than $25,000 but not more than $150,000. Such utilities shall keep all the accounts prescribed herein which are applicable to their operations, except those accounts in which the letter D does not appear in the prefix.

(b) The class to which any utility belongs shall be determined by the average of its annual electric operating revenues for the three years next preceding the effective date of this system of accounts. If the average of its electric operating revenues for the three years ended December 31st of any year following the effective date of this system of accounts is greater than the maximum specified for the class within which the utility has theretofore been included, such utility shall for the succeeding year keep the accounts prescribed for such higher class.

(c) Any utility, which because of its revenues falls within a given class, may, at its option, adopt the system of accounts prescribed for a higher class. Notice of such action shall be promptly filed with the commission