TRANSFORMERS AND TRANSFORMER VAULTS 70-449

main system shall be permitted, but shall not exceed values that would be more than the short-time current rating of the grounding autotransformer or any series connected devices in the neutral connection thereto. (c) Ground Reference for Damping Transitory Overvoltages. A grounding autotransformer used to limit transitory overvoltages shall be of suitable rating and connected in accordance with (a)(1) above. 450-6. Secondary Ties. A secondary tie is a circuit operating at 600 volts, nominal, or less, between phases that connects two power sources or power supply points, such as the secondaries of two transformers. The tie shall be permitted to consist of one or more conductors per phase. As used in this section, the word "transformer" means a transformer or a bank of transformers operating as a unit. (a) Tie Circuits. Tie circuits shall be provided with overcurrent protection at each end as required in Article 240. Exception: Under the conditions described in (a)(1) and (a)(2) below, the overcurrent protection shall be permitted to be in accordance with (a)(3) below. (1) Loads at Transformer Supply Points Only. Where all loads are connected at the transformer supply points at each end of the tie and overcurrent protection is not provided in accordance with Article 240, the rated ampacity of the tie shall not be less than 67 percent of the rated secondary current of the largest transformer connected to the secondary tie system. (2) Loads Connected Between Transformer Supply Points. Where load is connected to the tie at any point between transformer supply points and overcurrent protection is not provided in accordance with Article 240, the rated ampacity of the tie shall not be less than 100 percent of the rated secondary current of the largest transformer connected to the secondary tie system. Exception: As otherwise provided in (a)(4) below. (3) Tie Circuit Protection. Under the conditions described in (a)(1) and (a)(2) above, both ends of each tie conductor shall be equipped with a protective device that will open at a predetermined temperature of the tie conductor under short-circuit conditions. This protection shall consist of one of the following: (1) a fusible link cable connector, terminal, or lug, commonly known as a limiter, each being of a size corresponding with that of the conductor and of construction and characteristics according to the operating voltage and the type of insulation on the tie conductors or (2) automatic circuit breakers actuated by devices having comparable currenttime characteristics. (4) Interconnection of Phase Conductors Between Transformer Supply Points. Where the tie consists of more than one conductor per phase, the conductors of each phase shall be interconnected in order to establish a load supply point, and the protection specified in (a)(3) above shall be provided in each tie conductor at this point. Exception: Loads shall be permitted to be connected to the individual conductors of a paralleled conductor tie without interconnecting the conductors of each phase and without the protection specified in (a)(3) above at load connection points, provided the tie conductors of each phase have 70-450 NATIONAL ELECTRICAL CODE a combined capacity of not less than 133 percent of the rated secondary current of the largest transformer connected to the secondary tie system; the total load of such taps does not exceed the rated secondary current of the largest transformer; and the loads are equally divided on each phase and on the individual conductors of each phase as far as practicable. (5) Tie Circuit Control. Where the operating voltage exceeds 150 volts to ground, secondary ties provided with limiters shall have a switch at each end that, when open, will deenergize the associated tie conductors and limiters. The current rating of the switch shall not be less than the rated current of the conductors connected to the switch. It shall be capable of opening its rated current, and it shall be constructed so that it will not open under the magnetic forces resulting from short-circuit current. (b) Overcurrent Protection for Secondary Connections. Where secondary ties are used, an overcurrent device rated or set at not more than 250 percent of the rated secondary current of the transformers shall be provided in the secondary connections of each transformer. In addition, an automatic circuit breaker actuated by a reverse-current relay set to open the circuit at not more than the rated secondary current of the transformer shall be provided in the secondary connection of each transformer. 450-7. Parallel Operation. Transformers shall be permitted to be operated in parallel and switched as a unit provided that the overcurrent protection for each transformer meets the requirements of Section 450-3 (a)(1) or (b)(2). 450-8. Guarding. Transformers shall be guarded as specified in (a) through (d) below. (a) Mechanical Protection. Appropriate provisions shall be made to minimize the possibility of damage to transformers from external causes where the transformers are exposed to physical damage. (b) Case or Enclosure. Dry-type transformers shall be provided with a noncombustible moisture-resistant case or enclosure that will provide reasonable protection against the accidental insertion of foreign objects. (c) Exposed Energized Parts. Switches or other equipment operating at 600 volts, nominal, or less, and serving only equipment within a transformer enclosure shall be permitted to be installed in the transformer enclosure if accessible to qualified persons only. All energized parts shall be guarded in accordance with Sections 110-17 and 110-34. (d) Voltage Warning. The operating voltage of exposed live parts of transformer installations shall be indicated by signs or visible markings on the equipment or structures. 450-9. Ventilation. The ventilation shall be adequate to dispose of the transformer full-load losses without creating temperature rise that is in excess of the transformer rating. (FPN): See General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers, ANSI/IEEE C57:12.00-1987, and General Requirements for Dry-Type Distribution and Power Transformers, ANSI/IEEE C57.12.01-1979.