ATS Grounding and Main Switchboard Ground

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elecshop

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Electrical Engineer
I am working on an existing project that has a main distribution board (1200A service 480V) inside a building, powered from a utility transformer on a pad outside, through underground conduit runs. The conduits contain the three phases + neutral, without ground, as the transformer primary is grounded by the utility. Also, the main switchboard has an GEC that is established through a driven ground rod.

In my scope, I am adding a generator on site, and a service rated ATS at the building exterior wall, just oustide the mechanical room where the main switchboard is located. The generator is not a separately derived system as the ATS has a solid neutral configuration i.e. no neutral switching. Thus, the generator will not be grounded but the ATS will be. Hence, I am planning to splice the existing underground service conductors just beside the ATS, connect those incoming from the transformer to the ATS normal feed, and the other part to the ATS output. The problem is, the output of the ATS will have conduits that do not have a ground, if I am not planning to modify the conduits, but per NEC 250.122, there has to be an EGC between the ATS, which is now the main service disconnect, and the main switchboard, and the GEC existing at the main switchboard will have to be demolished. Thus, new EGC wires will have to be pulled between the ATS and the main switchboard thereby involving work inside the building which I am trying to avoid.

Is it possible to go around this by avoiding pulling EGC in the underground conduits, and keeping the GEC at the main switchboard? I could not yet find any code violation, but has anyone of you encountered this situation and seen and inspector's opinion on that?
 
It sounds like the ATS will become the service equipment and the main bonding jumper and the GEC connection must be made at the service equipment. When you do that, the conductors to the existing switchgear change from service conductors to feeder conductors and an EGC must be run with the feeder conductors.
 
Is the electric metering cubicle in the switchboard? You definitely don't want to connect the ATS upstream of that.

The main bonding jumper cannot be located downstream of the service disconnect.
 
Your wires probably aren't long enough to make up both connections on the new ATS anyway. Have you checked the locations of the lugs in the ATS? Typically one or both sets of terminations supply the ATS are near the top of the cabinet. I'd plan to run a new feeder in the existing conduit with a grounding conductor, assuming the existing conduit is sized for all the new wires.
 
Thank you guys for your answers. And you are right @Bwas , we probably need to replace the wires anyway. @electrofelon I'm not sure its probably rigid metallic conduit but it is pretty old so the resistivity test may not work. @JoeStillman the meter is at the utility transformer, upstream the ATS.
I appreciate your feedback.
 
Just to confirm, if it's not a separately derived system, the generator does not need to be grounded with a GEC, right? But an EGC will be pulled from the generator to the ATS.
 
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