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

[Mick Abraham]

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

Landline: 970-731-4675
Cell phone or for text messaging: 970-946-6584
ᐧ
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20240313/15a98eae/attachment.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: EE image from Mick.png
Type: image/png
Size: 71383 bytes
Desc: not available
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20240313/15a98eae/attachment.png>