[RE-wrenches] Redundant gensets: Must I switch the neutral in the manual transfer switch?

[Kent]

First, I'll say that I agree with William's discussion. Let me add 
another way of thinking about this:

Assume that, instead on two generators, your system is one backup 
generator and one utility company serving a commercial building. If the 
disconnect switch is your service connection, there will be a N-G bond 
located there and none will be allowed in the generator.

Kent Osterberg
Blue Mountain Solar


On 3/13/2024 2:19 PM, Mick Abraham via RE-wrenches wrote:
> Hi, Mechanix~
>
> My off grid project gig has redundant 120y208 generators that need a 
> 200a "break before make" manual transfer switch so the clients can 
> choose machine A or machine B. The switch has a lockable center-off 
> but I think the generators aren't set up for "lock out tag out". The 
> machines & the xfer switch will all be indoors and about 10 feet 
> apart. The AC power wires from each generator will be hard-wired to 
> the transfer switch & piped in EMT. I had planned for the transfer 
> switch to have a "solid neutral" bus bar (& I had planned to bond 
> Neutral to Ground in that box), but a consulting EE tells me the 
> neutral must be switched along with the three hots. The project is too 
> remote to have an AHJ, but the clients have requested a wet stamp from 
> the EE.
>
> I think his rationale is related to the N-G bond point location. When 
> I asked about his suggested location, the EE wrote: "The N-G bond will 
> be at each generator, so the neutral will be generated at the 
> generator providing power to the facilities."
>
> Mick resumes: The genset installation document says "The generator set 
> is shipped from the factory with the neutral and equipment ground not 
> bonded together." So...these gensets are not like those portables 
> where it's difficult to isolate the neutral. I've attached an image 
> that shows what the EE is advocating. The manual xfer switch that I 
> wish to use isn't made in "four poles" without going to the 400 amp 
> version and the 400a switch won't physically fit the available space.
>
> Can I make a credible case for the transfer switch being the "point of 
> first disconnect" instead of each generator taking turns at being the 
> POFD? If the transfer switch is also the bond point, can't the Neutral 
> from each generator happily meet at the neutral bus bar? OR: might the 
> EE have a concern about a loop (from the Neutral of one genset through 
> the bus bar then showing up on the terminal strip for the other 
> genset? Code citations would be appreciated along with your advice, 
> Wrenchies.
>
> During service intervals, both engines will be running for a 
> brief interval. Example: Gen A may be carrying the loads while the 
> mechanic manually runs Gen B as he completes an oil change. If I wire 
> it as the EE wishes, will both running machines play well safety-wise 
> if each genset has a N-G bond (with the equipment ground as their only 
> common connection)? How would the safety aspects change if both 
> machines are running but I've wired things per my original concept?
>
> Thanks as always, Mechanix. "The Wrench List is the Bomb!"
>
> Mick Abraham, Proprietor
> www.abrahamsolar.com <http://www.abrahamsolar.com>
>
> Landline: 970-731-4675
> Cell phone or for text messaging: 970-946-6584
> ᐧ
>
> _______________________________________________
>
>
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