450.10(A) Transformer Grounding. Dry-Type Transformer Enclosures.
Code Change Summary: Code language was added to provide specific rules on grounding inside the transformer enclosure.
A wealth of red tags has been written based on bonding and grounding violations behind the transformer enclosure. The grounding and bonding rules in Article 250 can be very confusing.
Now, specific code language has been added to address some of these bonding configurations.
450.10(A) now requires a single terminal bar for all grounding and bonding conductor connections when separate equipment grounding conductors and supply-side bonding jumpers are installed.
Also, since dry type transformer enclosures are not specifically listed for grounding and bonding purposes, the grounding and bonding terminal bar used cannot be secured over the transformer vents.
The transformer will usually have a perforated metal bottom that allows air flow. Securing the grounding terminal bar over compromised metal such as perforated metal, will not create the best bonding or grounding connection. Only solid portions of the metal transformer enclosure should be used for mounting the terminal bar.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).
2011 Code Language:
450.10 Grounding. Where grounded, exposed non–current carrying metal parts of transformer installations, including fences, guards, and so forth, shall be grounded and bonded under the conditions and in the manner specified for electrical equipment and other exposed metal parts in Parts V, VI, and VII of Article 250.
2014 Code Language:
450.10 Grounding
(A) Dry-Type Transformer Enclosures. 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.
Exception: 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.