2 step down xfrmrs + generator

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Npstewart

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Hi everyone, I said I would re-post this riser diagram when I was finished so here it is. Im not 100% finished, I have some loose ends to tie up with the generator. Most notably, im not sure whether I need a 4 pole or 3 pole transfer switch. Ill look forward to any comments, thanks! :)

Nick
 
You can usually just make a new post with the text "bump" if you want to bring the previous thread to someone's attention.

No need to start a new thread.
 
Npstewart said:
Hi everyone, I said I would re-post this riser diagram when I was finished so here it is. Im not 100% finished, I have some loose ends to tie up with the generator. Most notably, im not sure whether I need a 4 pole or 3 pole transfer switch. Ill look forward to any comments, thanks! :)

Nick
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You have shown in riser 20kW generator. Can you please give the complete generator description?
 
Something looks funny about what you show as 'Existing Meter and Disco'. You have an existing 1200A 480V (y?) service yet I see no CT's How does that meter work. No GFP?

The system designations are unclear. 480y/277? 208y/120?
 
Unless there is an absolute reason why you mnust use a four pole switch, use a three pole with a solid neutral.

In my opinion there is no electrical reason why you need a four pole switch for this installation; a three pole will do just fine. Others who are more code savvy may have a legislatory reason, however.

I'm assuming there is a T1 (and T2, for that matter) secondary side neutral / ground bond (for the SDS), not just to the transformer frame.
 
chris kennedy said:
Something looks funny about what you show as 'Existing Meter and Disco'. You have an existing 1200A 480V (y?) service yet I see no CT's How does that meter work. No GFP?

The system designations are unclear. 480y/277? 208y/120?
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Agree, Ground Fault Protection?

Also, the equipment bonding jumper between "T1" & "L" should be a #4 AWG 250.32(A)(2).
 
chris kennedy: Thats a good point. this was originally a mall so I am unclear how they WERE metering it. I had the electrical contractor draw me a riser however he left the meter off, ill be looking into the metering type.

OTT2:
I appreciate the code citing, I will call that out as a #4, thanks! :)

dbuckley: The question of 3 pole or 4 pole has been bothering me all day. Ill go with a 3 pole unless another person has an opinion on this. Ive been reading a lot of articles on this and most just listed the pros and cons of each. However, these were for larger generators.

Thanks again everyone, any other suggestions im glad to hear.
 
In general, this post is not a discussion or simple question, it is being used to design this system, of which no real details are provided or known. The OP indicates they work for a design firm, so then I would have to question under who's direct responsibility does this design belong; why are they not the one providing the guidance to the OP.
 
I didn't realize all posts on here had to be a simple question. Im not using anyone to design the system, im just looking for suggestions to make sure everything is 100% as good as it could be. Do you think I could design that entire diagram based on the few comments I received on this post? Someone told be to call out the ground size connecting the ground rods, another person told me to use a 3 pole transfer switch.

I see people on asking the most basic questions on here that work for design firms and they have every right to and this is the first time ive seen someone question a thread.
 
You seem to show building steel, water pipe AND a ground rod as the grounding electrode for your separately derived systems. This seems like overkill. 250.30(7) requires the electrode to be the "nearest" of either structural metal or metal water pipe as specified in 250.52.
 
Npstewart said:
dbuckley: The question of 3 pole or 4 pole has been bothering me all day. Ill go with a 3 pole
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Just to add some clarity to this; the only common scenario where a four pole breaker is required is where you have one generator backing up multiple services: Each service has its own N/G bond, and if you add a single genset to two or more services then you need to interconnect all the grounds (nothing wrong with that) and all the neutrals. And it is this interconnecting of neutrals that causes problems.

Essentially, due to the presence of multiple N/G bonds, you have neutral current flowing deliberately through the ground wiring. So you need a four pole switch after the service entrance and after the N/G bond, and then switch all four poles to either utility power or to generator power. In this situation the generator has a N/G bond.

In the simpler situation of one generator backing up one supply, the single N/G bond is all that's needed, and there are no consequent nasty currents on the ground wiring.
 
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