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Large Generator Grounding 480/277 V #219569 08/29/18 12:55 PM
Joined: Mar 2003
Posts: 849
Y
Yoopersup Offline OP
Member
Grounding requirements & Sections.

SDS & Non SDS .

2017 / 2014 NEC & Related Books and Study Guides
Re: Large Generator Grounding 480/277 V [Re: Yoopersup] #219570 08/29/18 06:14 PM
Joined: Jul 2004
Posts: 9,383
G
gfretwell Offline
Member
The biggest difference between SDS and non SDS is whether you bond the neutral (transfer equipment switches the neutral). Otherwise I think you just create a 250.grounding electrode system based on the largest ungrounded conductor ... but that is just off the top of my head.

Last edited by gfretwell; 08/29/18 06:20 PM. Reason: clarification

Greg Fretwell
Re: Large Generator Grounding 480/277 V [Re: Yoopersup] #219571 08/29/18 08:08 PM
Joined: Jul 2004
Posts: 9,383
G
gfretwell Offline
Member
Looking at the code you get to 250.20 that talks about how you ground a SDS connected generator. If it is a non - SDS (unswitched neutral), there are no grounding requirements beyond the 250-122 EGC from the attached service.

From the handbook
Non SDS

[Linked Image]

SDS

[Linked Image]

If you do have an SDS it takes you to 250.30 that points you to 250.66 for the details of the GEC 250.52 for the electrode etc.


Greg Fretwell
Re: Large Generator Grounding 480/277 V [Re: Yoopersup] #219576 08/30/18 02:55 PM
Joined: Apr 2002
Posts: 7,105
HotLine1 Offline
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Greg:

Nice explanation!!


John
Re: Large Generator Grounding 480/277 V [Re: Yoopersup] #219578 08/31/18 02:00 AM
Joined: Jul 2004
Posts: 9,383
G
gfretwell Offline
Member
Thanks, we do lots of generators around here so it is a common question. I usually tell homeowners looking to have transfer equipment installed that, if this is a manual system, the listed panel interlock is usually the best way to go. The labor will be much less and the part, although ridiculous for what you get, is still pretty cheap.
The only thing they really need is a vacant double slot in the panel and moving some breakers around to get your backfed breaker next to the discionnect. You then have to be sure they lift the bonding jumper in the generator to get circuit current off the EGC. This will not be an SDS.


Greg Fretwell

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