Grounding & Neutral connections question

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tom.bielectric

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New to the forum, hope you can help with my confusion regarding grounding & neutral connections: We installed a 2000A switchgear 480/277V 3 phase in a small building. That is our main service, grounds & neutrals are bonded together there. In that small building we also have a 75KVA transformer (3phase 120/208 secondary). We have the grounds and neutrals connected together at the transformer. That transformer feeds a 3 ph panel. Should the ground & neutrals be together or separate in that panel? To complicate things, there is a larger building that is fed from the switchgear.... 800A, 480/277 main breaker panel. The inspector said he wants the ground and neutrals separated in that panel. Then from that 800A panel, we have several transformers that feed sub panels. How should the ground/neutral connections be in the transformers, and panels? We tried to keep them separate in the transformers in that larger building, but got screwy voltage readings... then bonded them together at the transformers, and all readings are now OK. Can you give any advice? Thank you.
 
The Neutral and Ground (ing) connection for separately derived systems is addressed in Article 250.

How old are the transformers that had unsteady voltage readings?
 
-2000 amp need neutral to ground

-secondary of stepdown transformer - need neutral to ground either at transformer OR the main secondary transformer panel.... not both

-Separate building - no neutral to ground bond. However, you do need to provide a ground electrode system at that building location tied to the ground of the building panel.

-Separate building step down transformers - neutral to ground bond at secondary - all tied to the ground electrode system at the separate building.
 
tom.bielectric said:
For the transformers and the sub panels after the transformers... Why not tie the grounds and neutrals together in both?
Click to expand...
Creates a parallel path on grounding conductor for grounded neutral current. Only one grounded-to-grounding bond per system. Use to be a grounding conductor was not required, so there was no parallel path. Grounding conductor is now required with separately derived systems... called supply-side bonding jumper.
 
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