TRANSFORMERS AND TRANSFORMER VAULTS 70-445

Where fuses are used, their continuous current rating shall not exceed 250 percent of the rated primary current of the transformer. Where circuit breakers or electronically-actuated fuses are used, they shall be set at not more than 300 percent of the rated primary current of the transformer. Table 450-3(a)(1). Transformers Over 600 Volts Maximum Rating or Setting for Overcurrent Device Primary Secondary Over 600 Volts Over 600 Volts 600 Volts or Below Transformer Circuit Circuit Circuit Breaker Rated Breaker Fuse Breaker Fuse Setting or Impedance Setting Rating Setting Rating Fuse Rating Not more than 6% 600% 300% 300% 250% 125% More than 6% and not more than 10% 400% 300% 250% 225% 125% Exception No. 1: Where the required fuse rating or circuit breaker setting does not correspond to a standard rating or setting, the next higher standard rating or setting shall be permitted. Exception No. 2: An individual overcurrent device shall not be required where the primary circuit overcurrent device provides the protection specified in this section. Exception No. 3: As provided in (a)(2)b below: b. Primary and Secondary. A transformer over 600 volts, nominal, having an overcurrent device on the secondary side rated or set to open at not more than the values noted in Table 450-3(a)(2)b, or a transformer equipped with a coordinated thermal overload protection by the manufacturer, shall not be required to have an individual overcurrent device in the primary connection, provided the primary feeder overcurrent device is rated or set to open at not more than the values noted in Table 450-3(a)(2)b. Table 450-3(a)(2)b. Transformers Over 600 Volts in Supervised Locations Maximum Rating or Setting for Overcurrent Device Primary Secondary Over 600 Volts Over 600 Volts 600 Volts or Below Transformer Circuit Circuit Circuit Breaker Rated Breaker Fuse Breaker Fuse Setting or Impedance Setting Rating Setting Rating Fuse Rating Not more than 6% 600% 300% 300% 250% 250% More than 6% and not more than 10% 400% 300% 250% 225% 250% 70-446 NATIONAL ELECTRICAL CODE (b) Transformers 600 Volts, Nominal, or Less. Overcurrent protection of transformers rated 600 volts, nominal, or less, shall comply with (1) or (2) below. (1) Primary. Each transformer 600 volts, nominal, or less, shall be protected by an individual overcurrent device on the primary side, rated or set at not more than 125 percent of the rated primary current of the transformer. Exception No. 1: Where the rated primary current of a transformer is 9 amperes or more and 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, the next higher standard rating described in Section 240-6 shall be permitted. Where the rated primary current is less than 9 amperes, an overcurrent device rated or set at not more than 167 percent of the primary current shall be permitted. Where the rated primary current is less than 2 amperes, an overcurrent device rated or set at not more than 300 percent shall be permitted. Exception No. 2: An individual overcurrent device shall not be required where the primary circuit overcurrent device provides the protection specified in this section. Exception No. 3: As provided in (b)(2) below. (2) Primary and Secondary. A transformer 600 volts, nominal, or less, having an overcurrent device on the secondary side rated or set at not more than 125 percent of the rated secondary current of the transformer shall not be required to have an individual overcurrent device on the primary side if the primary feeder overcurrent device is rated or set at'a current value not more than 250 percent of the rated primary current of the transformer. A transformer 600 volts, nominal, or less, equipped with coordinated thermal overload protection by the manufacturer and arranged to interrupt the primary current, shall not be required to have an individual overcurrent device on the primary side if the primary feeder overcurrent device is rated or set at a current value not more than six times the rated current of the transformer for transformers having not more than 6 percent impedance, and not more than four times the rated current of the transformer for transformers having more than 6 percent but not more than 10 percent impedance. Exception: Where the rated secondary current of a transformer is 9 amperes or more and 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, the next higher standard rating described in Section 240-6 shall be permitted. Where the rated secondary current is less than 9 amperes, an overcurrent device rated or set at not more than 167 percent of the rated secondary current shall be permitted. (c) Voltage Transformers. Voltage transformers installed indoors or enclosed shall be protected with primary fuses.