Changes That Matter: Transformers

There are many types of transformers with just as many different uses. Transformers can be installed within electrical equipment, as separate individual transformers, as polyphase banks of two or more single-phase transformers operating as one unit, as exposed coils, or completely enclosed with or without ventilating openings. Transformers can be grounded or ungrounded, indoors or outdoors, dry-type or liquid-filled, less flammable or nonflammable, and oil-insulated or askarel-insulated.

National Electrical Code (NEC) Article 450 covers some transformers; however, 450.1’s scope statement contains eight exceptions where Article 450 does not cover transformers. For example, Article 450 does not require compliance of current transformers (CTs); transformers located inside of other electrical equipment; transformers that are integral parts of X-ray, high-frequency and electrostatic-coating apparatuses; Class 2 and Class 3 transformers; and transformers for sign, outline lighting and electric-discharge lighting. Other NEC articles cover these transformers.

Section 450.10 underwent a significant change in the 2014 NEC. For grounding dry-type transformer enclosures, a new subsection and a new exception was added as 450.10(A). It states, “where separate equipment grounding conductors and supply-side bonding jumpers are installed, a terminal bar for all grounding and bonding conductor 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.”

In other words, the NEC no longer permits the common practice of using the vent area on the bottom of the transformer as a point of connection for bonding and grounding connections. In addition, the NEC no longer permits using individual lugs with fender washers in the venting slats, because a terminal bar must be installed for multiple points of connection.

The new exception 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 methods in 250.8 and shall be bonded to the enclosure, if of metal.”

This exception permits single lugs and various other bonding and grounding devices to be used for transformers with wire leads for connections (e.g., where a potted transformer is used and the connections within the transformer are leads in the transformer enclosure).

Section 240.21(C)(2) covers transformer secondary conductors not longer than 10 feet. This section states that the ampacity of the secondary conductors must be “(a) not less than the combined calculated load on the circuits supplied by the secondary conductors, and (b) not less than the rating of the equipment containing an overcurrent device(s) supplied by the secondary conductors or not less than the rating of the overcurrent protective device at the termination of the secondary conductors.”

The bolded words were added to the 2014 NEC to ensure the transformer secondary conductors must either terminate at an overcurrent device or in equipment containing an overcurrent device.

An added exception states, “Where listed equipment, such as a surge protective device(s) [SPDs], is provided with specific instructions on minimum conductor sizing, the ampacity of the tap conductors supplying that equipment shall be permitted to be determined based on the manufacturer’s instructions.”

In other words, the conductors for the SPD, connected to the secondary side of a transformer, do not require sizing based on the overcurrent device rating and are permitted to be sized based on information provided by the SPD manufacturer. This same exception was added to 240.21(B)(1) for feeder taps that are not longer than 10 feet with the same reasoning as provided for transformer secondary conductors in 240.21(C)(1), Exception.

Finally, Section 450.5 provides requirements for grounding autotransformers using zigzag or T-connected transformers connected to three-phase, 3-wire ungrounded systems to create a three-phase, 4-wire distribution system or for providing neutral point for grounding purposes. The zigzag transformer, installed to create a new neutral, cannot be installed on the load side of any system grounding connection.

A new exception states, “an autotransformer with a wye configuration on its line side and a zigzag configuration on its load side that does not permit neutral or ground fault current to return over the line connection, shall be permitted on the load side of a system grounding connection.”

In simpler terms, under very limited conditions, the exception permits the zigzag transformer to create a neutral on the load side of an autotransformer.

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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