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

[Jason Szumlanski]

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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