Installing new inverter/panels to existing grid tied system

quique · June 2022

I have 12 panels feeding a 3kW Fronius uni ig plus inverter. I want to increase my capacity and I just happen to have another inverter just like it and Id have to buy the 12 new panels. My question is how to interconnect if at all possible.

In the picture there is the original fronius 3kW up top. A dc disconnect at the bottom where the solar panel string cables come into the dc disconnect. Then an AC disconnect to the right of the inverter where the AC is fed through and then goes down into the mains panel.

Id install the 2nd inverter where the red rectangle is drawn. Now here is where I was thinking that the 2 new dc cables come down from the new 2nd string and my first question is;

Q1: Should I use a new DC disconnect box for the new string of panels? Or can I wire the dc cables to the same dc disconnect terminals?

And then the second question is, from that new inverter, the ac cables come out and I can just run them "pass though" the 1st inverter and into the AC disconnect box and so once again;

Q2: Can I feed those AC cables from the 2nd inverter into the same AC disconnect or do I need a new one?

Image: https://us.v-cdn.net/6024911/uploads/editor/jo/eovkc2fvtzqb.jpg

solar_dave · June 2022

I know here I have 2 5000watt inverters, the DC is split inside a single disconnect but there are enough lugs to do that and so is the Ac which goes to a sub panel with separate breakers before it is combined to a 6 gauge line back to a single disconnect and then the main breaker in the home panel.

DC disconnect, inverters, and AC junction:

AC combiner:

DC disconnect

AC disconnect and solar utility meter:

the main breaker panel

Image: https://us.v-cdn.net/6024911/uploads/editor/mp/g8avpwqoalrq.jpg

quique · June 2022

Ok in my case, the +/- from the first array come into the DC disconnect which has 3 terminals. What I did was:

Pv+ direct connect on Terminal1 to Inverter+
Pv- switched on Terminal2 to Inverter-

So I have a free Terminal3 on my dc disconnect. Is you dc Disconnect also a 3 terminal box?

or did you just patch the PV+ to Inv+ without using up a terminal on your dc disconnect, therefore having one terminal per PV- ?

Image: https://us.v-cdn.net/6024911/uploads/editor/5e/g0u8ip478vnd.jpg

The AC disconnect harder to understand because I can see combining the neutrals (white) but you still have 2 live lines from each inverter and you cant combine those, so here is mine, im assuming your box has 4 terminals then?

Image: https://us.v-cdn.net/6024911/uploads/editor/vk/h1a6hwsr4t77.jpg

solar_dave · June 2022

My DC disconnect has wiring for 2 strings of panels the positive is on the switch.

The AC is a standard AC sub panel with a 240 breaker for each inverter the power then runs to an AC disconnect at the main panel required by fire department and then runs to main breaker panel with a single 240 breaker.

quique · June 2022

Yeah sorry, I miswrote there, the PV+ is switched and the PV- is direct. Ok so you have your PV strings each on a separate terminal, so you have 4 terminals:

Terminal 1: PV1 +
Terminal 2: PV2 +
Terminal 3: PV1 -
Terminal 4: PV2 -

Like that?

DanS26 · June 2022

First of all those Fronius inverters have the DC disconnects built in......so get rid of that external DC disconnect that you did not need in the first place. Someone sold you equipment you did not need.

Now just wire the AC outputs in parallel in a combiner box (new equipment) with the proper disconnect (ie 15 or 20 amp breakers) if allowed by your AHJ. Your grid tied equipment needs to be adequate to accept additional production.

Finally you need to disclose your additional production to your POCO and OK to your interconnect agreement.

quique · September 2022

Ok so I can do away with this blue marked box and then use the AC disconnect (3 lugs) to wire L1 together, L2 together and the Neutral wire together as well?

Installing new inverter/panels to existing grid tied system

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