TRANSFORMERS AND

TRANSFORMER VAULTS (Including Secondary Ties) 450-1. Scope. This article covers the installation of all transformers. Exception No. 1: Current transformers. Exception No. 2: Dry-type transformers that constitute a component part of other apparatus and comply with the requirements for such apparatus. Exception No. 3: Transformers that are an integral part of an X-ray, high-frequency, or electrostatic-coating apparatus. Exception No. 4: Transformers used with Class 2 and Class 3 circuits that comply with Article 725. Exception No. 5: Transformers for sign and outline lighting that comply with Article 600. 70-444 NATIONAL ELECTRICAL CODE Exception No. 6: Transformers for electric-discharge lighting that comply with Article 410. Exception No. 7: Transformers used for power-limited fire protective signaling circuits that comply with Part C of Article 760. Exception No. 8: Transformers used for research, development, or testing, where effective arrangements are provided to safeguard persons from contacting energized parts. This article also covers the installation of transformers in hazardous (classified) locations as modified by Articles 501 through 504. A. General Provisions 450-2. Definitions. For the purpose of this article: Transformer: The word transformer is intended to mean an individual transformer, single- or polyphase, identified by a single nameplate, unless otherwise indicated in this article. 450-3. Overcurrent Protection. Overcurrent protection of transformers shall comply with (a), (b), (c), or (d) below. The secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device. If both breakers and fuses are utilized as the overcurrent device, the total of the device ratings shall not exceed that allowed for fuses. As used in this section, the word "transformer" shall mean a transformer or polyphase bank of two or more singlephase transformers operating asa unit. (FPN No. 1): See Sections 240-3, 240-21, and 240-100 for overcurrent protection of conductors. (FPN No. 2): Nonlinear currents caused by loads such as electric-discharge lighting, electronic computer/data processing, or similar equipment can increase heat in a transformer without operating its overcurrent protective device. (a) Transformers Over 600 Volts, Nominal. (1) Primary and Secondary. Each transformer over 600 volts, nominal, shall have primary and secondary protective devices rated or set to open at no more than the values of transformer rated currents as noted in Table 450-3(a)(1) Electronically-actuated fuses that may be set to open at a specific current shall be set in accordance with settings for circuit breakers. 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: As provided in (a)(2) below. (2) Supervised Installations. Where conditions of maintenance and supervision assure that only qualified persons will monitor and service the transformer installation, overcurrent protection as provided in (a)(2)a. shall be permitted. a. Primary. Each transformer over 600 volts, nominal, shall be protected by an individual overcurrent device on the primary side.