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Wiring 2 Fronius Primo inverters on the same system

Saikele

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Dec 27, 2021
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I purchased 2 Fronius Primo 10.0-1 grid-tie inverters for my residential solar project. I plan to deliver 3 strings of 7 panels each (Sunpower 435W, 5.97A, 72.9Vmp, 85.6Voc) to one inverter, and 4 strings of 7 panels to the other inverter. Each inverter will then receive maximum 673V (calculation based on Voc + temperature correction for the coldest date on record in my area) with one inverter receiving roughly 18A and the other roughly 24A of DC current. The Primo inverters are rated up to 1000V so that shouldn't be a problem... The Primo inverter installation manuals state that they can be wired in parallel, but it doesn't go into details on how one would actually make the connections. From my inverters I will run the 240V output wires to an AC disconnect box, and from there to the meter (the local utility company will be replacing my current meter with one that is capable of metering both directions). If I were going with a smaller system and just one inverter, my understanding is that I would just wire directly from the inverter to the AC disconnect. Pretty simple. But with 2 inverters, I have been told that I need to run each set of inverter outputs to a subpanel/load center with 2 60A breakers first, and then on to the AC disconnect. Does this sound correct? And if so, won't the distribution be wired "backward" from the normal flow pattern? meaning, I would have to wire each inverter into the "load" side of the breakers, then the single "output" from that load center would be going "out" the top 2 hot L1/L2 lines and from there over to the AC disconnect? Is that a problem with electricity "flow" going backwards thru a breaker? I am planning to have a licensed electrician do this work for me just for safety, but I enjoy learning about this stuff so I am trying to understand what all is involved beforehand.... I know the Fronius manual says that the inverters need to be "linked" to each other with a shielded Cat5 cable - it sounds like this is how they communicate with each other, and it looks pretty straightforward to make that connection.
 
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Most breakers can allow current to pass both ways, unless it specifies "line" or load only. So, 2 inverters feed into a load center with each on a protected circuit of 60A. Depending on the AHJ, you may require another disconnecting means outside and near the meter. Again, your AHJ probably counts the inverter output as a load, so the sum of the overcurrent can only be 1.2 (or 1.25 residential) times the rating of the busbar that it is connected to, or the rating of the service feed. If your service is 200A and you have a busbar rated at 225A, then your maximum input could be limited to 81.25A (225x1.25-200=81.25). Your AHJ may allow higher current if the connection is on the meter or a splitter between the meter and the main breaker panel. In this scenario the limiting factor could be the capacity of the wiring or splitter.

I believe Fronius requires the units to be connected Master/Slave for data transfer only, not required for simple operation. The "Full" or "Advanced" has the data manager included, the "Lite" doesn't (less expensive).

As far as wiring goes, you have a number of options. MPPT1 has 3 inputs and MPPT2 has 1 therefore, you can just have each string go to its respective terminal. Again, your local AHJ may require an external DC disconnect, and this can get expensive for multi-strings to the inverter, especially if the Voc is above 600v dc. A fused combiner with a 2P 36A disconnect switch (using your data) and a 1-4 finger bar is often used for this purpose.

FWIW, if you can not legally run 2 10kW inverters, you could get close to the same output with one 15kW inverter and close to the same number of panels! The further north you go and the cloudier the location, it is probably best to run the DC side to the maximum allowed by the inverter. For your set up, You also may be faced with limitations due to the higher voltages of the selected panels.
 
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