Proper Sub-Panel Grounding with Existing Electrical System

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ElecEngbw

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I am working on a project where there is a sub-building fed electrically from a main building. The sub-building has existing electrical panels located inside currently feeding the general receptacles, lights, etc. I am looking into installing a new panel exterior to the building to power HVAC equipment that will be mounted to the exterior wall of the sub-building. This new panel feed will come from the main building electrical room. I am confused about how to properly ground the new panel according to NEC 250. I am running an equipment grounding conductor with the feeder conductors to the new panel. Is it better to tie the new panel ground bus to the existing sub-building panel ground bus, to a new ground rod at the exterior panel, or simply to the building steel? Or a combination of the three?

Thanks.
 
Thanks. I have read 250.50 several times just today and somehow missed that everytime. Thanks for pointing that out.

As for the new panel, I had not thought about 225.30 due to the new panel, along with all of it's loads, being outside of the building envelope. This new panel will not supply any loads inside this detached building. With that said, do you feel this is still a violation of 225.30?
 
(strange..my original post disappeared.... suicide I guess)..

If the new panel serves no loads in the detached building you may well have an AHJ decision ahead. You may be allowed to attached it to the building, but often these scenarios are solved by mounting your new panel on posts/pillars at the equipment and not on the building.
 
Thanks augie47. My thoughts were that I could use 225.30 (D) because the new panel will now provide power to the only HVAC equipment for the building. I will look into this further.

Thanks again.
 
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