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 light- ing that comply with Article 600.
Exception No. 6: Transformers for electric-discharge lighting that comply with Article 410.
Exception No. 7: Transformers used for power-limited fire alarm circuits that comply with Part 111 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 covers the installation of transformers dedicated to supplying power to a fire pump installation as modified by Article 695.
This article also covers the installation of transformers in hazardous (classified) locations as modified by Articles 501 through 504.
I. General Provisions
450.2 Definition. For the purpose of this article, the fol lowing definition shall apply.
Transformer. 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 450.3(A), (B), or (C).
As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single- phase transformers operating as a unit.
FPN No. 1: See 240.4, 240.21, 240.100, and 240.101 for overcurrent protection of conductors.
FPN No. 2: Nonlinear loads can increase heat in a trans- former without operating its overcurrent protective device.
(A) Transformers Over 600 Volts, Nominal. Overcurrent protection shall be provided in accordance with Table 450.3(A).
(B) Transformers 600 Volts, Nominal, or Less. Overcur- rent protection shall be provided in accordance with Table 450.3(B).
Exception: Where the transformer is installed as a motor-control circuit transformer in accordance with
430.72(C)(1) through (C)(5).
(C) Voltage Transformers. Voltage transformers installed
indoors or enclosed shall be protected with primary fuses.
FPN: For protection of instrument circuits including volt- age transformers, see 408.52.
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. Where this calculation does not correspond to a standard rating of a fuse or nonadjust- able circuit breaker and the rated input current is 9 amperes or more, the next higher standard rating described in 240.6 shall be permitted. 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 Figure 450.4.