Ungrounded System & 250.20(B)

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I have seen, and touched, several 1500 and 2000kVA 480Y/277V transformers that were ungrounded (their X0 terminal was floating). They were installed in a paper (converting) mill in the early 70's. The customer wanted 'run-of-the-mill' transformers. All 4-wire loads were fed by individual 480-480Y/277V.

The current owner (a meat processor) is considering changing the un-grounded system to high-resistance ground instead.
 
jim dungar said:
I have seen, and touched, several 1500 and 2000kVA 480Y/277V transformers that were ungrounded (their X0 terminal was floating). They were installed in a paper (converting) mill in the early 70's. The customer wanted 'run-of-the-mill' transformers. All 4-wire loads were fed by individual 480-480Y/277V.

The current owner (a meat processor) is considering changing the un-grounded system to high-resistance ground instead.
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Are these at least bonded at the source?
 
It sounds like your system is indeed correct. As long as the center point of the 480 wye is not connected to anything and the neutral point is not brought out to be used, it will look like an ungrounded delta system and should be treated like one. In accordance with the 2008 code, it is to have ground detectors installed. Also, it could be corner grounded in the same manner as a delta system.

If your customer wants a high resistance grounded system, this should be relatively easy to convert. :)
 
jim dungar said:
I have seen, and touched, several 1500 and 2000kVA 480Y/277V transformers that were ungrounded (their X0 terminal was floating)...
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Jim, don't they have to connect the center point of the wye to get the 277V, assuming they have a common conductor?

Otherwise this would simply be a 480Y if nothing is connected to the center point, is this correct?
 
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tryinghard said:
Jim, don't they have to connect the center point of the wye to get the 277V, assuming they have a common conductor?

Otherwise this would simply be a 480Y if nothing is connected to the center point, is this correct?
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On their substation transformers they have absolutely no connection to the X0. 277V loads are fed from smaller 480-480Y/277V transformers.
 
charlie said:
It sounds like your system is indeed correct. As long as the center point of the 480 wye is not connected to anything and the neutral point is not brought out to be used, it will look like an ungrounded delta system and should be treated like one. In accordance with the 2008 code, it is to have ground detectors installed. Also, it could be corner grounded in the same manner as a delta system.

If your customer wants a high resistance grounded system, this should be relatively easy to convert. :)
Click to expand...

Thanks, sure helps to have other opinions to validate
 
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