Ground (3) Single Phase 4160V - 480V Transformers Bank Connected in Delta

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zemingduan

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Philadelphia,PA
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Electrical Designer
We have (3) outdoor single phase 4160V - 480V transformers connected in delta and form the transformer bank. The (3) transformers are not utility transformer. They have another onsite transformer that step down from 13.2KV utility service to 4.16KV. I guess they are previously used for manufacture factory. The electrician propose to reuse the transformer bank to supply a 1200A 480V 3ph 3w disconnect located inside the building. And they can use 480V delta - 480/277V wye transformer in the future as needed.

1655398020328.png 1655398716040.jpeg

I think we should ground and bond the ungrounded separately derived system per 250.30(B). We will need to make the grounding electrode connection to connect the transformers metal enclosures to the building grounding electrode system at the transformer bank for outdoor source per 250.30(C). We will need to have supply-side bonding jumper to connect the transformers metal enclosures to the 1200A disconnect inside the building. Then done! Please correct me if I am wrong.

But how are the grounding electrode connection and supply-bonding jumper connections made at the 3-transformers bank in reality? I need a grounding & bonding detail showing these connection. I only know how it is done for a transformer that has all three phases windings in one enclosure. Please help. Thanks!
 
zbang said:
Pick any one of the 480v leads and ground that, making a corner-grounded system.
Click to expand...
I think this is an ungrounded separately derived system. 250.30(B)(1) states connecting the metal enclosure of the transformers instead of the ungrounded conductors to the grounding electrodes system. See below.

(1) Grounding Electrode Conductor. A grounding electrode
conductor, sized in accordance with 250.66 for the largest
derived ungrounded conductor(s) or set of derived ungroun‐
ded conductors, shall be used to connect the metal enclosures
of the derived system to the grounding electrode as specified in
250.30(A)(5) or (6), as applicable.
 
Most of the installs from that era that I have seen have a grounding electrode to the transformer at most (some had nothing) but I think your are correct that under present Codes you would need to follow 2590.30(B) and (C).

If the 240 transformer has a neutral then 250.20(B) and 250.30(A) would come into play for that transformer.
 
To make my questions more specific:

1) Do you guys agree with me regarding the grounding and bonding of the 3-transformer bank.

2) How are the grounding electrodes connection and supply-side boding jumper connection made at the transformers. After all they are three separate transformers. Does these transformers typically have grounding terminals or grounding bar for these connections to be made. Typically are the grounding terminal/bar of each transformer bonded together in some ways?

Thanks!
 
zemingduan said:
To make my questions more specific:

1) Do you guys agree with me regarding the grounding and bonding of the 3-transformer bank.

2) How are the grounding electrodes connection and supply-side boding jumper connection made at the transformers. After all they are three separate transformers. Does these transformers typically have grounding terminals or grounding bar for these connections to be made. Typically are the grounding terminal/bar of each transformer bonded together in some ways?

Thanks!
Click to expand...
There is a nut welded to the tank during production. A grounding lug screws into that nut for your tank grounding.

PENN-UNION HGSE-020 TFMR GRND CLAMP | Gordon Electric Supply, Inc.

PENN-UNION HGSE-020 TRANSFORMER GROUND CLAMP
www.gordonelectricsupply.com www.gordonelectricsupply.com

notice the nuts welded on the bottom part of the tank.
3D8660CA-2E46-4E4C-8598-278EB713DFDB.jpeg
 
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