480V 3Phase 3Wire Separately Derived System

[Cihan]

Please see the attached wiring diagram for more context: 480V 3Phase 3Wire Diagram


Background: I have a scenario where my site has no 277V neutral loads. In order to save money on unneccesary neutral conductors from both my 480V Utility provided XFMR and my 480V standby Generator, I have decided to have the neutrals for both sources bonded to their respective enclosures. I then bring 480V 3phase 3wire and my equipment grounding conductor from both the XFMR and the Generator to the transfer switch. I am pretty positive the utility transformer and generator will be Wye connected.

Question 1: Do I have a separately derived system?

Given what I have described and the attached wiring diagram, I believe I have a separately derived system as there is no direct connection to circuit conductors of any other electrical source other than thsoe established by grounding and bonding connections, per NEC Article 100 Definitons.

Question 2: In a wiring diagram as I have shown, is it possible to also switch the equipment grounding conductor brought from both the XFMR and the Generator?

I can't really think of any benefit to doing this, but the question keeps popping up in my head and figure it's worth asking.
I know of course that a neutral can be solidly connected or switched inside of a transfer switch. In my case I am not bringing any netrual conductors to the transfer switch.

 

[augie47]

As to the generator being a SDS it depends on the neutral being switched or not and you don't show the neutral connections on your diagram. 230.24 (C) would require your neutral be brought from the utility to your service disconnect (which could be your T/S if it's service rated).
The proper grounding of the generator would be dependent on the T/S switching the neutral; or not.

 

[Cihan]

As to the generator being a SDS it depends on the neutral being switched or not and you don't show the neutral connections on your diagram. 230.24 (C) would require your neutral be brought from the utility to your service disconnect (which could be your T/S if it's service rated).
The proper grounding of the generator would be dependent on the T/S switching the neutral; or not.

Hi Augie, thanks for your response but I'm not sure I quite understand your response.

As stated in the background section, I am not bringing the neutral from the XFMR or the Generator to the Transfer Switch because I have no 277V loads. As far as I know there is no code requirement that I need to bring the neutral from either the XFMR or the Generator to the Transfer Switch if I have no neutral (277V) related loads nor any future anticipated neutral related loads.

Also which code book year are you using with your 230.24C reference? I only have my 2017 code book on hand and 230.24C is about service conductor clearances.

 

[packersparky]

Hi Augie, thanks for your response but I'm not sure I quite understand your response.

As stated in the background section, I am not bringing the neutral from the XFMR or the Generator to the Transfer Switch because I have no 277V loads. As far as I know there is no code requirement that I need to bring the neutral from either the XFMR or the Generator to the Transfer Switch if I have no neutral (277V) related loads nor any future anticipated neutral related loads.

Also which code book year are you using with your 230.24C reference? I only have my 2017 code book on hand and 230.24C is about service conductor clearances.

The neutral is required to be installed to the service disconnect. Augie may have had a typo. The code reference is 250.24(C).

 

[Cihan]

The neutral is required to be installed to the service disconnect. Augie may have had a typo. The code reference is 250.24(C).

Thank you for the clarification.

From the transfer switch this ends up going to a Switchboard that I wanted to be 480V with no neutral bus. However this Switchboard is right next to the transfer switch. The transfer switch is 300 feet from the transformer and 150 feet from the generator.

I thought I would save money by not needing a neutral from either the transformer or the generator but it seems that won't be the case.

So If I take the neutral from the transformer and take the neutral from the generator and connect them to the transfer switch neutral bar and then ground the transfer switch neutral bar at that point, do I still need to provide an equipment grounding conductors from the transfer switch to the transformer or from the transfer switch to the generator?

 

[augie47]

Assuming the transformer is the service (POCO) supply and the transfer switch is service rated (has main OCO),the neutral will serve as the equipment ground from the service transformer.
Wiring to the generator will depend on its being an SDS or not which will depend on whether the neutral is switched by the T/S or not.

 

[ptonsparky]

The only cost difference is in how you handle the Gen & transfer switch.
The conductor to the bonded POCO transformer is the neutral. Not an EQ. Change the color and make sure it is at least minimum sized per Table 250.102(C)(1) NEC '20

 

[ron]

Just because you bring a neutral to the service disconnect (due to the code requirement) doesn't mean you need to bring a neutral from the generator. Leave the neutral at the generator if you don't need it.