Leo1
Member
- Location
- Los Alamos, NM
To be NEC compliant with grounding a service it requires at least one additional grounding connection be made at the outdoor transformer from the grounded service conductor to a grounding electrode, per NEC - 250.24(A)(2) & NEC - 250.30(C), at the source location provided a parallel path for the ground conductor is not established per NEC - 250.30(A)(1) exep. 2. One proper way to achieve the two grounds (source and first disconnect) is:
* Ground neutral conductor to grounding electrode at the first disconnect (whether it is outside or inside)
* Ground neutral conductor to grounding electrode and the outside transformer case at the transformer. However, to eliminate a ground loop between transformer and the first disconnect do not install an equipment grounding conductor between the first disconnect and the outside transformer; the raceway cannot be metal, e.g., use PVC and do not include an equipment grounding conductor between the outside transformer and the first disconnect - only install phase conductors and the neutral. Now events at the transformer immediately go to ground without effects on the interior wiring, likewise events inside immediately go the first disconnect ground connection and back to the transformer via the neutral and parallel grounds are not created between transformer and first disconnect. Correct?
If my description above is correct then would the same logic apply to an outside generator that creates a separately derived system? However, if the outside generator feeds an outside located automatic transfer switch (similar to transformer above) the the separately derived generator neutral is tied to a grounding electrode same as done for the transformer but an equipment grounding conductor is required between the generator an ATS because the first disconnect on the outside of the building is out of the circuit now thus no parallel grounds are created. Correct?
* Ground neutral conductor to grounding electrode at the first disconnect (whether it is outside or inside)
* Ground neutral conductor to grounding electrode and the outside transformer case at the transformer. However, to eliminate a ground loop between transformer and the first disconnect do not install an equipment grounding conductor between the first disconnect and the outside transformer; the raceway cannot be metal, e.g., use PVC and do not include an equipment grounding conductor between the outside transformer and the first disconnect - only install phase conductors and the neutral. Now events at the transformer immediately go to ground without effects on the interior wiring, likewise events inside immediately go the first disconnect ground connection and back to the transformer via the neutral and parallel grounds are not created between transformer and first disconnect. Correct?
If my description above is correct then would the same logic apply to an outside generator that creates a separately derived system? However, if the outside generator feeds an outside located automatic transfer switch (similar to transformer above) the the separately derived generator neutral is tied to a grounding electrode same as done for the transformer but an equipment grounding conductor is required between the generator an ATS because the first disconnect on the outside of the building is out of the circuit now thus no parallel grounds are created. Correct?