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

[Jason Szumlanski]

Yes, I'm hoping to get some feedback here, as Sol-Ark tech support has a
new answer every time you ask them.

I don't love the thought of buying a MID for an off-grid project, but the
GridBoss approach seems well thought out. A single generator connection for
all paralleled inverters is a nice feature as long as you can accept 200A
maximum combined load and three inverters, which should be sufficient for
most off-grid homes. The savings in wire size and other distribution
components makes the MID a palatable expense, especially in a triple
inverter scenario. It also improves serviceability in some ways (although
the MID itself is another potential failure point).

Next, if we could just get someone to make a 400A MID, we would be in
dreamland territory for me.

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 Tue, Jun 17, 2025, 6:36 PM Chris Sparadeo <sparadeo.chris at gmail.com>
wrote:

> 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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