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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