[RE-wrenches] N-G bond for supply side connection Midnite Power

[Zeke Yewdall]

I remember going back and forth with two different Boulder County
inspectors on this issue around 2007 -- one made me install the
neutral-ground bond in the supply side disconnect switch.  The next one
inspected that and failed me and made me remove it. Then the first one came
back and failed me again and made me put it back.

My understanding more recently has been that a supply side PV disconnect is
not really a separate service and therefore should not have a separate
nuetral-ground bond.  It is more like when you have two 200A breakers fed
by a 320A meter socket -- it is a single service with two outputs.  But...
we end up with a separate physical box, rather than both outputs being in
the same physical box like that example.  It still has to be within 6 feet
of the main service to that second PV disconnect, but is a separate box.
Per Tom's replay, looks like 2023 NEC clarified that if it's a separate
enclosure, then it requires a separate neutral-ground bond, and that would
make sense for the PV disconnect requiring a separate bond.  So... if you
are on 2023 NEC, then having that make sense.  If on 2020 or earlier...
seems like it's still ambiguous.

What having multiple neutral ground bonds does in practice (other than some
bad things with lighting surges) is cause neutral current to flow in the
equipment grounds, which is generally not good, because equipment grounds
may be much smaller than neutral conductors.

What I have been doing, if possible, on the two inverters backing up a full
200A subpanel situation like you describe is putting a 200A 8 space
feedthrough panel on the exterior of the house.  This panel gets the
nuetral-ground bond as the first point of disconnect from the grid.  Then
two 100A breakers feed the two inverters AC-in terminals.  And the feed
through terminals of the panel go to the grid source for a manual bypass
switch.  Then you are essentially connecting the PV system per 705.12(B)(3)
rather than 705.12(A) like a supply side connection.  Have to mark the
feedthrough panel that no additional loads are allowed (fill the other two
spaces with plug on surge suppressors).  The complication is that if you
had the bypass switch in inverter bypass, sending the grid to your 200A
load panel, the inverters could still be drawing from the grid to charge
batteries, or selling PV to the grid, and potentially overloading that
feedthrough pane (at least per code... I can't think of how it could
actually overload the panel, as if it drew more than 200A total between
inverters and the feed through lugs going to the loads while charging
batteries, the main 200A breaker would trip to protect the bus.  If it sold
back 200A from solar while the inverters were bypassed from the load (not
that the inverters could, but if they did sell the full amount the breakers
were capable of), that 200A could either go fully to the load, or fully to
the grid, or somewhere in between, but would never be more than the 200A
rating of the bus.
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