National Electrical Installation Standards

Standards as High as Your Own

Monday, February 4, 2019


I thought I knew how to wire transformers until a few days ago. There are some guys that say you don't need to bond the transformer to the building grounding electrode system when you aren't using the neutral on the secondary. Say when you have a 480 volt 3 phase 3 wire primary and a 208 volt 3 phase 3 wire secondary to some piece of manufacturing equipment or A/C unit. I have looked through the code book and asked several other electricians and haven't got any real substantial answers or code references. I talked to one person that mentioned the exception in 250.121. I tried to follow it through the code book and I still don't think I understand it. Please help me understand this and find me a code reference so that I can go back to the people asking me these questions and give them some answers. I hope I have explained my issue well enough and my terminology isn't to messed up. I have been reading Code question of the day for a couple of code cycles now and it has been a big help. Thank you for the forum.




Hey Vern thanks for your question and the kind words. The same thing happens to me a lot, thought I knew something then realized I didn't.

In your example the 208Y/120 volt transformer secondary needs to be grounded, even though the neutral conductor is not used, as stated in 250.20(B)(1). Separately Derived Systems, as that term is defined in Article 100 and which the transformer in your example very likely is, must be installed based on 250.30. The building grounding electrode system must be used as the grounding electrode for the transformer as stated in 250.30(A)(4). A grounding electrode conductor must be installed as stated in 250.30(A)(5) and be connected as stated in 250.68(C).

Any metal water piping systems and exposed structural metal building frames, in the area served, need to be bonded to the grounded conductor as stated in 250.104(D). That NEC wording probably needs to be improved because in your example there really is no grounded conductor other than the termination point (XO) - even though the system is grounded.

The exception to 250.121 allows a wire type equipment grounding conductor to also be used as a grounding electrode conductor if that conductor complies with the rules for both types of conductors.

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