Resolución 2535: adopta el Código Eléctrico Nacional, versión 1993. (Sustituye el Anejo A del Reglamento 4435)

(d) Fire Pump Installations. Where the transformer is dedicated to supplying power to a fire pump installation, it shall not require secondary overcurrent protection. Primary overcurrent protection shall be provided in accordance with Section 450-3(a) or 450-3(b). The primary rating or setting shall be sufficient to carry the equivalent of the transformer secondary current sum of the locked-rotor currents of the fire pump motor(s) and associated fire pump accessory equipment indefinitely. 450-4. Autotransformers 600 Volts, Nominal, or Less. (a) Overcurrent Protection. Each autotransformer 600 volts, nominal, or less, shall be protected by an individual overcurrent device installed in series with each ungrounded input conductor. Such overcurrent device shall be rated or set at not more than 125 percent of the rated full-load input current of the autotransformer. An overcurrent device shall not be installed in series with the shunt winding (the winding common to both the input and the output circuits) of the autotransformer between Points A and B as shown in Diagram 450-4. leese A ellelle Shunt winding(s) B Diagram 450-4 Exception: Where the rated input current of an autotransformer 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 input current is less than 9 amperes, an overcurrent device rated or set at not more than 167 percent of the input current shall be permitted. (b) Transformer Field-Connected as an Autotransformer. A transformer field-connected as an autotransformer shall be identified for use at elevated voltage. (FPN): For information on permitted uses of autotransformers, see Section 210-9. 70-448 NATIONAL ELECTRICAL CODE 450-5. Grounding Autotransformers. Grounding autotransformers covered in this section are zig-zag or T-connected transformers connected to 3-phase, 3-wire ungrounded systems for the purpose of creating a 3-phase, 4-wire distribution system or to provide a neutral reference for grounding purposes. Such transformers shall have a continuous per phase current rating and a continuous neutral current rating. (FPN): The phase current in a grounding autotransformer is one-third the neutral current. (a) Three-Phase, 4-Wire System. A grounding autotransformer used to create a 3-phase, 4-wire distribution system from a 3-phase, 3-wire ungrounded system shall conform to the following: (1) Connections. The transformer shall be directly connected to the ungrounded phase conductors and shall not be switched or provided with overcurrent protection that is independent of the main switch and common-trip overcurrent protection for the 3-phase, 4-wire system. (2) Overcurrent Protection. An overcurrent sensing device shall be provided that will cause the main switch or common-trip overcurrent protection referred to in (a)(1) above to open if the load on the autotransformer reaches or exceeds 125 percent of its continuous current per phase or neutral rating. Delayed tripping for temporary overcurrents sensed at the autotransformer overcurrent device shall be permitted for the purpose of allowing proper operation of branch or feeder protective devices on the 4-wire system. (3) Transformer Fault Sensing. A fault sensing system that will cause the opening of a main switch or common-trip overcurrent device for the 3-phase, 4-wire system shall be provided to guard against single-phasing or internal faults. (FPN): This can be accomplished by the use of two subtractive-connected donut-type current transformers installed to sense and signal when an unbalance occurs in the line current to the autotransformer of 50 percent or more of rated current. (4) Rating. The autotransformer shall have a continuous neutral current rating sufficient to handle the maximum possible neutral unbalanced load current of the 4-wire system. (b) Ground Reference for Fault Protection Devices. A grounding autotransformer used to make available a specified magnitude of groundfault current for operation of a ground responsive protective device on a 3-phase, 3-wire ungrounded system shall conform to the following requirements: (1) Rating. The autotransformer shall have a continuous neutral current rating sufficient for the specified ground-fault current. (2) Overcurrent Protection. An overcurrent protective device of adequate short-circuit rating that will open simultaneously all ungrounded conductors when it operates shall be applied in the grounding autotransformer branch circuit and rated or set at a current not exceeding 125 percent of the autotransformer continuous per phase current rating or 42 percent of the continuous current rating of any series connected devices in the autotransformer neutral connection. Delayed tripping for temporary overcurrents to permit the proper operation of ground responsive tripping devices on the