[RE-wrenches] Wire Sizing - Inverter Outputs with Generator Pass Through

[Chris Sparadeo]

Jason,

This is a great question. I was waiting for responses, because I’ve thought
this as well.

Typically I have sized load conductors to either Sol Ark output or gen
output, depending on which was higher, but you are absolutely right that
with grid assist programmed, this number could be higher. Either OCPD or
internal settings could work to avoid upsizing conductors to meet potential
combined amapacity of gen + inverter. However, I’m not crystal clear on how
those settings work for Sol Ark. I know there was a discussion a few weeks
back where Jharen from Sol Ark said it’s possible to derate AC
input/output, but it sounded like the conversation went off line.

Any insight would be appreciated.

Best,

Chris

On Fri, Jun 13, 2025 at 7:36 AM Jason Szumlanski via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> I think there was a previous thread on this, but I can't seem to find it.
>
> I'm wondering how everyone is handling wire sizing on inverter output
> circuits for off-grid systems when there is a generator on the grid input.
> Let's use the Sol-Ark 15K as an example.
>
> Without a generator, it's pretty simple. 80 amp over current protection
> with #4 copper minimum.
>
> With a 20 kW generator that has 80A rated current and 100A overcurrent
> protection, the entire generator current can pass through, so now we are
> talking about a minimum of #3 copper. But is that really the case? Since
> the inverter can be programmed to do grid assist, is it necessary to size
> the conductors for 180A?
>
> It gets trickier in parallel installations. If you have two inverters and
> the same generator, in theory the generator input and output is split
> evenly between the two inverters, so do you size the output conductors for
> 80A inverter output, 130A for inverter plus half of the generator, or do
> you still need to assume 180A?
>
> Assuming the conductors are the same length from the generator to the
> load, in theory the generator pass through should be equal on each
> inverter, but is that really the case? How do we know how each inverter
> will use the generator input and pass through current, especially in a grid
> assist scenario?
>
> I have been erring on the side of caution and simply using 200A conductors
> on the output side of both single and parallel inverter installations. The
> inverter output wire cost, complexity, and difficulty is high.
>
> The inverter manufacturers have not made it clear in their documentation
> how grid pass through and grid assist works.
>
> This is one of the reasons that I have started specifying the Midnite AIO.
> With a 100A load breaker, the output load conductor requirement is #2
> copper per the manual. I assume it is not specified as #3 because it is a
> 100% continuous duty breaker. Regardless, #2 saves a lot of cost and is way
> easier to work with than 3/0 copper.
>
> I am getting ready to specify a parallel EG4 Flexboss21 system using the
> GridBoss to combine them and do load shedding. This seems to be another
> good alternative where the inverter output circuit only needs to be 90A
> rated. The GridBoss can accept a 125A generator breaker, eliminating the
> requirement for pass through all together. This seems like a pretty well
> thought out approach.
>
>
> Jason Szumlanski
> Florida Solar Design Group
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