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Old April 30th, 2018, 05:30 PM   #16
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jraef is offline
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Join Date: May 2006
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NEC Article 250.20 requires all AC premise wiring systems to be grounded, EXCEPT as denoted in 250.21. Then 250.21.A.1-4 list specific cases where they CAN be ungrounded, but then 250.21.B says IF you do that, you must add a "ground detection system" at the service point and use specific signage (which nobody does). The Ground Detection System will cost you more than just corner-grounding the transformer.

But as you know, in VFD world a corner grounded delta system might as well be ungrounded as far as the VFD is concerned, so be careful about recommending that. I just use the argument to get people to stop using UNGROUNDED delta and switch to (or create) a wye system if they are going to have any power electronics.
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