Automatic Transfer Switch

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utilguy

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We're installing an ATS between two 120/208 transformer services to an IT network bldg - one from utility, other from a university dist system (which is essentially another feeder from the utility). Typically the services from the transformers would have the neutral grounded at the transformer. There will be a service disconnect between each transformer and the ATS. Do I need to treat the univsersity one as a separately derived system and use a 4 pole switch, or can I treat it like two utility feeds, use a 3 pole and do the bonding in the service disconnects?
thanks
 
You can do it either way. My preference is always to use a 4-pole transfer switch. It makes things easier to visualize for me. Not a great reason, I know, but I did call it a preference. :cool:
 
charlie b said:
You can do it either way. My preference is always to use a 4-pole transfer switch. It makes things easier to visualize for me. Not a great reason, I know, but I did call it a preference. :cool:
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Like Charlie said ether way. Most that feed electronic equipment like to see the four pole. Cant say why but it is preferred.
 
Charlie and ceb58, I don't see how the OP can use anything but a switched neutral transfer switch. The main bonding (from the utility) and the system bonding jumper (from university) put the neutral and ground in parallel with each other.

Rick
 
250.24 Grounding Service-Supplied Alternating-Current Systems.
(2) Outdoor Transformer.

I may be picturing this wrong but I do not see how the neutral grounded at the transformers effects using a three pole or four pole transfer switch.

If they ran the equipment ground from the transformer to the service disconnect and infect created a parallel grounded conductor I would ask the equipment ground be removed. Maybe this is not an outside supply to the building and I am picturing this wrong.
 
Thanks David - It would be two outdoor transformers - one utility owned, one customer owned. I have not planned to install an equipment ground from either transformer to the service equipment. Does that fit what you are saying?
 
Irreguardless of where the system bonding jumper is placed, (either in the transformer or the first disconnecting means), you still have the main bonding jumper from the service supply and the system bonding jumper from the transformer serving the common transfer switch.

Its the same idea as having a generator set up as a SDS where the system bonding jumper is at the gen set, you would have to switch the neutral. With the transformer, you can't set it up as a non SDS, you have to install a system bonding jumper thereby requiring a switched neutral transfer switch.

Rick
 
utilguy said:
I think my questions revolved around whether it would be an SDS or not, and you cleared that up.
Bill
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To back up my statement that the university transformer is a SDS, look at definitions in article 100.(SDS). Its power is derived from a source of energy other than a service. It is fed from a feeder circuit. Since its a SDS, article 250.30 apply.

Rick
 
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