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#74650 - 01/30/07 07:38 AM Transfer switch  
guschash  Offline
Joined: Jan 2007
Posts: 12
I will doing my first residentical transfer switch hook-up. Do I tie the neutrals together at the transfer switch enclosure.

[This message has been edited by guschash (edited 01-30-2007).]

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#74651 - 01/30/07 08:05 AM Re: Transfer switch  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,205
Chesapeake, VA
Yes. There needs to be exactly one ground/neutral connection in the entire system, and the ATS is generally the place to do it. (250.30(A)(1) Exception 1 permits this.

NEC 250.30 will tell you what you need to know. Well, if you can decipher it at least- it's one of the most confusing codes to try to read. Why can't the NEC just use plain english?

[This message has been edited by SteveFehr (edited 01-30-2007).]

#74652 - 01/30/07 11:15 AM Re: Transfer switch  
ShockMe77  Offline
Joined: Jun 2005
Posts: 821
Rahway, New Jersey
I haven't done many of these but the ones I have done have NOT been seperately dervied systems since the grounded conductor was NOT switched.

#74653 - 01/30/07 07:31 PM Re: Transfer switch  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,205
Chesapeake, VA
It was my understanding that generators and UPSs are are always a seperately derived source, regardless of how the neutral is handled.

Quoth NEC 2005:
Separately Derived System. A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

250.30(A)(1) Exception No.1: For separately derived systems that are dual fed (double ended) in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single system bonding jumper connection to the tie point of the grounded circuit conductors from each power source shall be permitted

[This message has been edited by SteveFehr (edited 01-30-2007).]

#74654 - 01/30/07 07:54 PM Re: Transfer switch  
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
It is up to the installer to decide if it will be a separately derived system or not.

If you switch the neutral and put a bonding jumper at the generator it will be an SDS.

If you do not switch the neutral and do not install a bonding jumper at the generator than it will not be an SDS.

Both ways have advantages and disadvantages.

Primarily non-SDS is cheaper, SDS works better with GFP mains.

Bob Badger
Construction & Maintenance Electrician

#74655 - 01/30/07 08:02 PM Re: Transfer switch  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,077
You also need to check the generator. Some come with the neutral bonded, others, particularly Honda, don't.

Greg Fretwell

#74656 - 01/31/07 07:24 AM Re: Transfer switch  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Interesting that you mention Honda, with this being a foreign make. By far the majority of generators on the U.K. market are sold with an unbonded neutral, as I believe are most in Europe. Perhaps this accounts for it?

#74657 - 01/31/07 07:57 AM Re: Transfer switch  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,205
Chesapeake, VA
That's odd, now that you mention it- I'd think it would be the other way around, as PME & shared earth/neutral is illegal in the US but more prevalent in Europe.

Boy that THAT throw me for a loop the first time I saw it! 3W WYE bus duct? Not very common in the US [Linked Image]

[This message has been edited by SteveFehr (edited 01-31-2007).]

#74658 - 01/31/07 07:30 PM Re: Transfer switch  
guschash  Offline
Joined: Jan 2007
Posts: 12
This is a Honda, so if I don't tie neutrals together, how do the loads that will be running now on the generator get there neutal. From the generator?

#74659 - 01/31/07 08:57 PM Re: Transfer switch  
Roger  Offline
Joined: May 2002
Posts: 1,716
Guschash, here is a diagram from Ed MacLaren that shows a proper hook up for an unswitched neutral transfer switch.

Notice the generator neutral is not bonded to the generator housing.

[Linked Image]


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