Article 450, Part I provides the general requirements for transformers. In the 2020 revision, Part 2 was titled “Specific Provisions Applicable to Different Types of Transformers,” with the 2023 revision, Part 2 is titled “Installation.” Part III is still about transformer vaults.
In the 2020 revision, the scope [Sec. 450.1] said Art. 450 applies to all transformers and there were eight Exceptions followed by a couple of short paragraphs that said Art. 450 applies to transformers dedicated to fire pumps and transformers in hazardous locations. Obviously, this needed to be cleaned up, so the 2023 revision deletes those two paragraphs. There’s also a style change that makes it more readable. Instead of using Exceptions it uses an enumerated list.
Overcurrent protections vary [Sec. 450.3]:
- If the transformer is over 1,000V nominal, use Table 450.3(A).
- If the transformer is not over 1,000V nominal, use Table 450.3(B) unless the transformer is a motor control circuit transformer.
- If the transformer is a potential transformer (PT) that’s indoors or enclosed, protect it with primary fuses.
If you have an autotransformer, you’ll need to apply the provisions of Sec. 450.4 and Sec. 450.5. In the latter, there are requirements for grounding the autotransformer. The NEC can be confusing in its use of the word “grounding,” often using that term when it means bonding. But transformers are by definition separately derived sources (there’s no direct connection between the supply side and the load side, only induction between the two), and separately derived sources are actually grounded.
Why is it called an autotransformer? This has nothing to do with any automatic operation versus what any other transformer does. We get “auto” from the Greek word for “self,” and it is another way of saying “one” (as in oneself). An autotransformer has just one coil.
If you have a circuit operating at no more than 1,000V nominal and it connects two power sources or power supply points (e.g., the secondaries of two transformers), you have a secondary tie. Such a tie must adhere to the requirements of Sec. 450.6. This isn’t the same thing a transformers operating in parallel (and switched as a unit); those fall under Sec. 450.7.
Transformers must be guarded per the four methods listed in Sec. 450.8(A) through (D). For example, you must provide adequate mechanical protection to eliminate or minimize damage from external causes. Some common ways of achieving this include the use of bollards, guard rails, and elevated mounting. Sometimes, simply choosing a “sheltered” location will suffice. For example, if you can place it such that a support I-beam is between the transformer and the lift truck path but the transformer is still reasonably accessible, then you reduce the chance a lift truck will back up into the transformer and smash the transformer enclosure with its counterweight.
Make sure that bonding strap inside the transformer is connected, and make sure all non current-carrying metallic parts (e.g., fences and guards) are bonded [Sec. 450.10]. A disconnect must be provided either within sight of the transformer or in a remote location [Sec. 450.14]. If the latter, it must be lockable and its field location must be marked on the transformer.
