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#207516 - 11/03/12 12:20 AM Transfer switch: neutral  
pdh  Offline
Joined: Jan 2005
Posts: 354
I'm wanting to put in a small transfer switch at home for 3 circuits (2 120's, 1 240). Generator may or may not have neutral/ground bond (it has not even be selected yet). I expect to roll the neighbor's generator over before I buy my own.

Not being an electrician myself (but have worked with contractors for past jobs wiring data centers), I contacted two electrical home service businesses to do the work ... but they want to use switches that won't switch the neutral. I immediately see an issue with the unswitched neutral either creating a ground loop (if generators is bonded), or a neutral potential to ground either in the connected generator (that is not bonded), or on the blades of the inlet connection (when generator is not connected).

Are 3 pole switches much more costly than 2 pole switches? I really only need one switch.

The circuits will be on a subpanel with their own 15A breakers per circuit. I'm tempted to just have an inlet feed the subpanel directly, and connect a cord between an outlet from the main to the sub's inlet, and just remove that and plug in the generator to make the switch. That sure would "switch" the neutral.

They are also trying to upsell me on a 10 circuit panel designed just for generators that had a bunch of single pole switches. When I asked them if it switches the neutral, they say it's not needed.

Professional thoughts?

I haven't actually found a transfer switch I'd want. I just want 30A three pole but rated to disconnect neutral last, and connect neutral first (or at least concurrent).

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#207521 - 11/03/12 02:45 AM Re: Transfer switch: neutral [Re: pdh]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,124
The short answer is if the generator bonds the neutral (AKA an SDS), you use a 3 pole transfer switch.
You can use the 2 pole switch (not opening the neutral) if the generator does not bond the neutral.
You only want one bonding jumper in the system in either configuration on the switch.

Greg Fretwell

#207525 - 11/03/12 08:52 PM Re: Transfer switch: neutral [Re: gfretwell]  
pdh  Offline
Joined: Jan 2005
Posts: 354
Of course. You'd have that neutral-through-ground loop that parallels the neutral current the wrong way if neutral is connected in common.

I don't know if the generator will bond them or not. I figure if it doesn't, it can be made to, anyway (to be a SDS).

But do consumer grade generators do that? I'm curious why the local electricians said I didn't need to transfer the neutral.

I'm looking at getting a generator around 8K to handle refrigerator and freezer starts, and one that can make a reasonable waveform for a UPS for the computers (though I will be getting a new double conversion type that should work better). My next door neighbor has a 14K one which I think was formerly a construction site type.

Last edited by pdh; 11/03/12 08:53 PM.

#207528 - 11/03/12 11:16 PM Re: Transfer switch: neutral [Re: pdh]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,124
There are usually instructions of how to bond or lift the bond in the installation book.

It is probably not the worst of sins to have it bonded on both ends but it is a 250.6 violation, sharing circuit current on the EGC.

As far as I know Honda generators are the only ones that come without the bond ... but that is old info.

Personally I am not sure why you bond a truly portable generator. Without a ground rod, what are you bonding?

Greg Fretwell

#207530 - 11/04/12 03:25 AM Re: Transfer switch: neutral [Re: pdh]  
pdh  Offline
Joined: Jan 2005
Posts: 354
Utility power comes in on 3-wire and you split that to 4-wire at the entrance panel, and ground the bonding point where it splits. Maybe the same can be done with the generator. 3-wire from the generator, a 3-wire inlet, and bond it all in there so 4-wires go to the transfer switch. Still gotta switch the neutral.

In a better life, I'd have a big 100+ kVA, and the transfer switch would (or maybe could be) be the entrance panel (if it can also be the main cutoff). Now the bonding would be in the transfer switch, eliminating one issue.

#207531 - 11/04/12 10:49 AM Re: Transfer switch: neutral [Re: pdh]  
wire_twister  Offline
Joined: Jul 2007
Posts: 265
Georgia USA
I have installed a lot of gensets at cell tower sights and on dairy/poultry farms, and never use a 3 pole switch. We run a neutral wire from the incoming service, one from the load, and one from the genset and hook them all together in the transfer switch(isolated from ground) and we pull equipment grounds in all conduits and connect them together and bond to ground. The neutral in the genset is NOT bonded to the ground. This arrangement has been inspected and passed with power companies and AHJs all over the southeast.


Life is tough, Life is tougher when you are stupid

#207533 - 11/04/12 02:35 PM Re: Transfer switch: neutral [Re: pdh]  
pdh  Offline
Joined: Jan 2005
Posts: 354
Without the bond in the genset, you are not creating the loop. Your neutral is bonded to ground in the main. As long as a genset can be set that way, this can be done. The point I need from this is if that is always doable from with all gensets. I don't have any experience wiring up gensets so I don't know if that is common or rare for gensets like the high end at stores like Lowes, and work site portables in the 8K to 14K range. If every genset can be wired either way, then all I need to do is make sure the genset and transfer switch are properly matched.

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