100. STANDARD NOMINAL VOLTAGE AND PERMISSIBLE VOLTAGE VARIATION Each utility shall adopt a standard nominal voltage, or standard nominal voltages, as may be required by its distribution system for its entire constant voltage service, or for each of the several districts into which the system may be divided, and shall file with the Commission a statement as to the standard nominal voltages adopted. The voltage maintained at the utility's point of service to the customer, shall be reasonably constant and shall not vary more than ten (10%) per cent plus or minus of the nominal voltage adopted. A greater variation of voltage than specified above may be allowed when service is supplied directly from a transmission line, or in case of emergency service. In all cases, the best voltage regulation shall be provided that is practicable under the circumstances.105. OTHER ALLOWABLE VARIATIONS Variations in the voltage in excess of those specified caused by (1) the operation of power apparatus on customer's premises, which necessarily requires large starting currents, (2) the action of the elements, and (3) infrequent and unavoidable fluctuations of short duration due to line and/or station operation, shall not be considered a violation of this rule.110. CLASSIFICATION OF ELECTRIC POWER SUPPLY CIRCUITS1. A secondary electric distribution circuit is defined as an electric power supply circuit in which the voltage existing between line conductors does not exceed a nominal 750 volts, and which is principally provided as a utilization circuit.2. A primary electric distribution circuit is defined as an electric power supply circuit in which the voltage existing between line conductors exceeds a nominal 750 volts but does not exceed a nominal 35,000 volts.3. An electric transmission circuit is defined as an electric power supply circuit in which the voltage existing between line conductors exceeds a nominal 35,000 volts.* *Rule 25. C., initiated by Order of the Commission in Docket U-3868, effective August 19, 1980. Rule 25.C. is now known as Rule 25.110. (renumbering 2011).