Stay Aware: Things to Know About Transformer Installations

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Transformer installations are common throughout residential, commercial and industrial applications. Most electricians, electrical contractors, electrical engineers and electrical inspectors are familiar with installation requirements and can state the general requirements without a problem. However, the past few National Electrical Code (NEC) cycles have introduced some changes that many designers and installers may be unaware of. A quick review of Article 450, Parts I and II, as well as a few of the pertinent sections within Article 250 on grounding and bonding, may help clarify these changes and make the reader more aware of the requirements.

Article 450 was fairly stable in the 2008 NEC and previous cycles with very few changes. In the 2011 NEC, a new Section 450.14 was added that stated “Transformers, other than Class 2 or Class 3 transformers, shall have a disconnecting means located either in sight of the transformer or in a remote location. Where located in a remote location, the disconnecting means shall be lockable, and the location shall be field marked on the transformer.”

The phrase “in sight of” is indicative of similar text located in Article 100 for the definition of “In Sight From” that states “where this Code specifies that one equipment shall be ‘in sight from,’ ‘within sight from,’ or ‘within sight of,’ and so forth, another equipment, the specified equipment is to be visible and not more than 15 m (50 ft) distant from the other.”

This requirement was necessary to ensure the person working on the transformer would know where the disconnecting means was located either in sight from or where located remote to be marked to aid in locating and disconnecting power to the transformer.

In the 2014 NEC, the second sentence in 450.14 was changed to reflect a reference to a new 110.25 that was inserted by the NEC Correlating Committee to ensure all references to lockable disconnecting means throughout the NEC were consistent. The new text in 110.25 stated, “Where a disconnecting means is required to be lockable open elsewhere in the Code, it shall be capable of being locked in the open position. The provisions for locking shall remain in place with or without the lock installed.”

An exception was added that permitted a cord-and-plug connecting locking means to not be required to remain in place without the lock installed. The purpose of this reference to 110.25 in 450.14 was to require a disconnecting means that was remotely located to have a permanent method of locking the disconnecting means open for safety purposes while a person is working on the transformer. The disconnecting means for the primary side of the transformer could be located any distance away from the transformer, even in a separate building. The location of the disconnecting means marked on the transformer will take the guesswork out of where the transformer is supplied.

Another change to Article 450 in the 2014 NEC involves grounding of dry-type transformer enclosures. Section 450.10(A) was added stating, “Where separate equipment grounding conductors and supply-side bonding conductors are installed, a terminal bar for all grounding and bonding connections shall be secured inside the transformer enclosure. The terminal bar shall be bonded to the enclosure in accordance with 250.12 and shall not be installed on or over any vented portion of the enclosure.”

Gone is the concept of installing individual listed pressure connectors with fender washers over the vent slats at the bottom the transformer. There are many transformer installations where pressure connectors and fender washers were installed without removing the paint from the point of connection. Section 250.12 states, “Nonconductive coatings, (such as paint, lacquer, and enamel) on equipment to be grounded shall be removed from threads and other contact surfaces to ensure good continuity or be connected by means of fittings designed so as to make such removal unnecessary.”

The bond bar and grounding cannot obstruct the air cooling or the venting of the enclosure. Proper cooling of a dry-type transformer is critical to its operation. A new exception was also added to 450.10(A) in the 2014 NEC that states, “Where a dry-type transformer is equipped with wire-type connections (leads), the grounding and bonding connections shall be permitted to be connected together using any of the methods in 250.8 and shall be bonded to the enclosure if of metal.”

The purpose of this exception is to permit smaller transformers, with leads provided by the manufacturer and located internal to the transformer housing plate, to be connected to the equipment grounding conductor using one of the eight different types of grounding connections permitted by 250.8(A).

Careful consideration of these changes will ensure fewer turndowns by the inspector and a better overall installation.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and

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