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Monday, October 20, 2014
Question:
Re: CQD answer published Tuesday, October 7, 2014
Charlie,
To clarify I would be running utility fed power (service) as well as generator power (feeder) to the buildings. The purpose for this is the size of my generator would only be sized to handle partial load of the buildings. A generator panel and a service panel at each building. Since this set up would feed multiple buildings I chose to have only one ATS rather than multiple transfer switches at each building. This then could potential create a parallel path between the two separate panels because of the common grounding electrode conductor to a common grounding electrode? I hope this clarifies a little,
Thanks, Zach
A
Answer:
Hey Zach thanks for the additional clarification. If I understand your planned installation each building will be supplied by a service and a feeder to two separate panelboards. The service will come directly from the utility transformer and the feeder will be supplied from the automatic transfer switch (ATS) which is supplied from the service and generator.
All the grounding electrodes at each building will need to be bonded together as stated in 250.50. The grounded neutral conductor from the service will be connected to the grounding electrode conductor at the building as stated in 250.24(A) but the grounded neutral conductor from the feeder will not be connected to the grounding electrode conductor as stated in 250.24(A)(5). The feeder equipment grounding conductor will also be connected to a grounding electrode conductor at the building. So you are correct, neutral current for loads supplied from the service will have a multiple paths back to the utility transformer. One being the service grounded neutral conductor and the other is through the main bonding jumper to the grounding electrode conductor which is connected to the feeder equipment grounding conductor back to the ATS an ultimately to the utility transformer. It could be argued that this is a violation of 250.6.
One possible solution is to install service equipment such as a panelboard or some other distribution equipment at a location outside the buildings. Grounding of the grounded conductor would occur there but not be extended downstream, essentially single point grounding on the load side of the service point. All conductors supplied from this distribution equipment would be feeders with their grounded neutral conductors "isolated" from panelboards in the buildings. One feeder would supply each building directly and another feeder would supply the ATS then to each building for the second panelboard. I hope you can follow what I have described as there are a lot of words to digest.
250.24, 250.50, 250.32, 250.6