Reverse Connecting 150KVA Transformer

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chevyx92

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VA BCH, VA
Customer is moving into building that a has 1200A 120/208V 3PH 4W service. They have various 480V equipment that needs power such a welders, press brakes, band saws, compressor, CNC Laser. Question is, am I allowed by code to reverse feed a 150 KVA WYE-DELTA transformer(which is listed for reverse connecting) to feed a 225A 480V panel which will be ungrounded? I know ungrounded systems need ground detecting equipment installed and the panel must be label as ungrounded system.
 
Yes it's permitted with, as you've stated, a ground detection system. Why not use a Delta-Wye transformer?
 
infinity said:
Yes it's permitted with, as you've stated, a ground detection system. Why not use a Delta-Wye transformer?
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Time constraints for customer. A 150 KVA 208V Delta-277/480V Wye step-up transformer is 6 week lead time vs. reverse connecting a standard step down 150 KVA which is a stock item.
 
chevyx92 said:
Customer is moving into building that a has 1200A 120/208V 3PH 4W service. They have various 480V equipment that needs power such a welders, press brakes, band saws, compressor, CNC Laser.
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How many of those items can be configured for 480 or 208-240 volts fairly easily? I'd change incoming voltage taps on as many of them as possible.
 
If I recall going in reverse you need to up the kva, Acme transformer, used to have a chart to help.
Just looked it up, you're ok it's on 1k and less.
 
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chevyx92 said:
Customer is moving into building that a has 1200A 120/208V 3PH 4W service. They have various 480V equipment that needs power such a welders, press brakes, band saws, compressor, CNC Laser. Question is, am I allowed by code to reverse feed a 150 KVA WYE-DELTA transformer(which is listed for reverse connecting) to feed a 225A 480V panel which will be ungrounded? I know ungrounded systems need ground detecting equipment installed and the panel must be label as ungrounded system.
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I can't tell you whether it is allowed per your code, me being a furriner here and all that.
What I can say is that the voltage ratio the transformer is designed for does not usually match exactly the turns ratio. For step down, the secondary will include some turns to compensate for regulation. That is, there will be some more than the voltage ratio to get designed output voltage at rated output.

For reverse connection that means that the step up will produce reduced output voltage. There may be taps that will allow you to compensate.
 
Zig-zag

Zig-zag

You might want to consider adding a grounding transformer to the 480V side. This allows using the 208Y/120V-480V transformer but gives you a 480Y/277V secondary, although with limited fault current capability. Addressed the MOV problem. You can build a zig-zag bank from three small single-phase transformers. Acme Transformer catalog shows how.

You can also build a 208V delta to 480V wye transformer bank from three individual transformers. May not be any more available than what you are planning.
 
beanland said:
You can also build a 208V delta to 480V wye transformer bank from three individual transformers. May not be any more available than what you are planning.
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Which would require 208x277 volt transformers to build it with - probably not something on the self at most supply houses.
 
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