Connecting low voltage transformers

[K_Mark]

I have run into an interesting situation. I am helping re-design an office building into a dormitory. This requires about a 3,000-amp switchboard at 120/208, 3 phase. We are stepping down from a 277/480 volt system. Instead of using one large dry-type transformer, we were looking at using three smaller dry-type transformers. Has anyone ever seen this done? What does the code say about it? How would you connect them? On the secondary side I figured you could just take the feeder from each transformer to the switch board. But I might be wrong about that. I was thinking that each transformer should have it's own feeder on the primary side. Any ideas or comments would be appreciated.

 

[jim dungar]

Are you going to have three seperate switchboards? Or, are you intending to parallel the three transformers to feed one switchboard?

 

[K_Mark]

Going to the same switchboard.

 

[jim dungar]

I am having a hard time visualizing any benefit from paralleling transformers. Most smaller "off-the-shelf" transformers are probably not built with consistent enough impedances to reduce load splitting problems.

I would be interested in why you are thinking of this method instead of a standard 1000KVA unit?

 

[peter d]

Using this approach presents a nightmare. It renders compliance with 300.3(B) virtually impossible.

 

[bob]

I assume that the building has several floors. If so, I would keep the 277/480 service and install a 120/208 transformer on each floor.
What do you have except Lts and receptacles?

 

[LarryFine]

K, without knowing more details, I'd suggest each transformer have its own feeder and OCP, and have each feed a separate panel.

 

[K_Mark]

Thanks everyone. It wasn't my project and I just wanted to get some other people's perspectives. The gentleman I am working with saw a riser diagram from the existing building and wanted to copy what they had done. But there is more to it than he realized. I appreciate all the input.