[RE-wrenches] Neutral run to critical loads sub-panel

[Jason Szumlanski]

I think it depends on how the backup load circuit ungrounded
conductors are extended. If they share the same raceway with the
inverter AC input and output conductors all the way to the backup
subpanel, and there is a neutral connection from the main panel to
inverter and inverter to subpanel, the intent of 300.3(B) would be
satisfied (assuming the neutral conductor is appropriately rated). But
if the backup load circuit ungrounded conductors are extended to the
subpanel in a separate raceway, there would need to be neutrals
extended as well in that same raceway. That's where using a wiring
gutter is often beneficial because you can often easily extend
ungrounded backup load conductors through the same raceway(s) to reach
a backup load subpanel.

One thing seems certain - you need a neutral to the subpanel, but it
can be a single neutral from the inverter output, or individual branch
circuit neutrals extended to the subpanel. One consideration would be
ensuring the neutral conductor is large enough to handle the total
load from the backup loads (while on grid), which may be greater than
the inverter rating, because the inverter only needs to serve the
largest backed up load. The inverter may be able to pass through a
larger utility feed than the inverter output rating in islanded mode.
That would most likely apply to the ungrounded conductors.

Anyway, I think it's better practice to just extend all of the backup
load neutrals as well. It's cleaner.

And you need a label indicating that there are pass-through loads (NEC
312.8(A)(3)). I see this missed VERY frequently.

Jason Szumlanski
Principal Solar Designer | Florida Solar Design Group
NABCEP Certified Solar Professional (PVIP)
Florida State Certified Solar Contractor CVC56956
Florida Certified Electrical Contractor EC13013208

On Wed, Jul 31, 2024 at 12:32 AM don energysolarnow.com via
RE-wrenches <re-wrenches at lists.re-wrenches.org> wrote:
>
> If all the 120 VAC loads have their neutrals land in the main meter panel then there is no need to run a neutral from there to the sub panel, as long as the inverter does have a neutral connection back to the meter panel.
> Especially when the inverter goes off grid it needs that neutral connection to the loads neutral in the meter panel.
>
> Don Barch
> Energy Solar
>
>
>
> On Mon, Jul 29, 2024 at 11:53?PM William Miller via RE-wrenches
> <re-wrenches at lists.re-wrenches.org> wrote:
> >
> > Friends:
> >
> > I found a grid-tie battery-backup system with a 120/240 VAC critical loads sub-panel that has no neutral run to it.  All of the 120 VAC loads have their neutrals land in the meter panel.
> >
> > I have always felt instinctively that the neutrals should land in the breaker panel that feeds the circuit.  I have looked through the NEC to find a passage that spells this out but I can seem to find one.
> >
> > Is this a code requirement?  Is this recommended practice?
> >
> > Thanks in advance.
> >
> > William Miller
> >
> > Miller Solar
> >
> > 17395 Oak Road, Atascadero, CA 93422
> >
> > 805-438-5600
> >
> > www.millersolar.com
>