250.21 (2017NEC)

I

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
If I have a 3phase, 3wire, delta-delta XFMR 13.8kV-480V, UNGROUNDED do I still need to provide a fault path back to the transformer, along with ground detectors as described in 250.21?


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don_resqcapt19 said:
The only difference in the grounding and bonding between a grounded system and an ungrounded system is that the ungrounded system will not have a main or system bonding jumper.
Ground detectors are required.
Click to expand...

Thanks Don.
So from XFMR secondary I would still run the (green) supply side bonding jumper to the SWGR MCB, and add ground detectors across each phase correct?


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250.30(B)(3) requires the supply side bonding jumper. 250.21(B) requires the ground detectors and must be connected as close as possible to the power source. The code does not specify the type of ground detector or what type of notification it must provide.
 
don_resqcapt19 said:
250.30(B)(3) requires the supply side bonding jumper. 250.21(B) requires the ground detectors and must be connected as close as possible to the power source. The code does not specify the type of ground detector or what type of notification it must provide.
Click to expand...

So there’s no physical grounding connection to the corner or middle point of the XFMR 2ndary windings, correct?


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Isaiah said:
So there’s no physical grounding connection to the corner or middle point of the XFMR 2ndary windings, correct?
Click to expand...
Not on an ungrounded system.

You could make it a corner grounded system if you wanted to, or if there is a midpoint tap you could make it a high leg grounded system. Neither of those require ground detectors.
 
don_resqcapt19 said:
Not on an ungrounded system.

You could make it a corner grounded system if you wanted to, or if there is a midpoint tap you could make it a high leg grounded system. Neither of those require ground detectors.
Click to expand...

Don, all downstream branch circuits/loads (from the MCC) such as motors, panel boards etc, would still require an EGC on an ungrounded system - correct?


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