Parallel inputs setting on SMA SunnyBoy.

[bartekw]

Good day!

I am installing SMA Sunnyboy 7.7 (SB7.7-1SP-US-40) with 24 modules (CS3U-380MS KuMax 380W 144 split cell Mono PERC 35mm T4).

The array is quite far from the electrical panel (80 m-inverter by the electrical panel not array) therefore to keep the wire cost down, I wanted to use 2 inputs on the inverter:
A||B (A and B in parallel) and C. Instead of standard A, B, C to avoid running 6 wires.

2 strings of 8 modules combined on A||B and 1 string of 8 on C input. All checks below:

sma 8x2 + 8.jpg



For A||B I will be having combined homerun of 2 wires (pos and neg).

My question is:

How to properly connect my homerun for A||B inputs in parallel?

Are there any settings that needs to be changed in the inverter?

Thank you so much in advance!
Bartek

[Markyrocks69]

Pretty sure that the installation manual shows exactly how you need to do this. I doubt any settings need changed. It looks like parallel strings still require 4 wires. I never understood as what the main benefit of this configuration was. The load still needs to be equally distributed across both channels

For the record I am in a similar situation and I determined that the easiest way to go about it for me was to install the inverter at the array and then youd just need to run 4 wires appropriately sized ac wires to the house, also take into account voltage drop for a long run. But honestly doing pv it's better to spend more money on wire now than have a system that has big losses over 20 0r 30 yrs bc you cheaped out now.

[solarix]

Generally desirable to keep the AC wiring short to minimize voltage drop to the mains voltage. If the mains voltage is high and you add voltage drop to it your inverter can and will drop out. Especially if and when your neighbors add solar and your neighborhood's voltage goes up in the middle of the day. (This happened to me) So you are right in locating the inverter near the service. The idea for the parallel input mode of the Sunnyboy inverter is to allow arrays with 4 strings by putting 3 of them in parallel across the two A & B inputs of the inverter. This is okay for older small PV modules where the three strings would add up to less than 20 amps max, but very few new PV panels are this small. Your 380 watt modules are no doubt around 10 amps max so you can only do two strings in parallel so is no advantage. Maybe four wires instead of six, but they have to be bigger wire. Just bite the bullet and wire them as 3 strings (6 wires plus ground) and get the benefit of three separate inputs with MPPT optimization on all three. PV panels are expensive, don't cripple them by wasting some of their power in 50 years of wire losses. We use 10ga wire for all our DC wiring even though the PV strings are just 10amps max. If you look at the derating required in the Code, because of being a continuous load, more than three conductors running in conduit, on a hot roof, with high ambient temps, your basic THHN rating of 30amps for 10ga wire can go way down...

[oregon_phil]

Agree with Solarix on running your array as three strings.

I have a SMA 7.7 and have parallel inputs on A&B due to SMA Rapid shutdown device on my roof. The SMA 7.7 does not need any special setting for parallel inputs on A&B. Firmware determines configuration when the inverter is turned on and when enough DC power is available. Upon boot up, the inverter assumes independent strings ("No bridge strings determined"), then will switch to parallel mode if requred ("bridge strings determined").

[Markyrocks69]

In my defense what I described is no different than the average microinverter setup and you avoid having to jump through extra hoops bc of high voltage coming into the house. But after revisiting the idea its probably not going to save you any money on wire. If you want to save money on wire b4 the wire goes into the ground use aluminum wire for the long runs. I'm assuming that youd put in a disconnect anyways. Then inside of the inverter in the house put a copper tail on it. But make sure you heavily insulate the splices.

[nwdiver]

you avoid having to jump through extra hoops bc of high voltage coming into the house.

DC runs just need to be metal clad when they enter a building. The other great thing about running DC instead of AC in addition to lower line losses because of high voltage, is that if inverter output is saturated your line losses are 'free'. If you have 9.1kW on a 7.7kW inverter some of that 1.4kW you can't use anyway is your line loss.

[Markyrocks69]

Ya not to mention if you need to go through boxes that have knockout rings remaining you need bonding bushings and a jumper on those connections, high voltage labeling on everything. Not to mention no ocpd on the DC side so if that conduit is energized youd get a nice shock out of the deal if not worse.

[bartekw]

Thanks to everyone for their input, much appreciated! Most definitely inverters will stay by the electrical panel.

@oregon_phil to make sure I got you right, so you have your two strings combined to one and then you have that connected just to input A?

Thank you!

[Markyrocks69]

Thanks to everyone for their input, much appreciated! Most definitely inverters will stay by the electrical panel.

@oregon_phil to make sure I got you right, so you have your two strings combined to one and then you have that connected just to input A?

Thank you!

The amp limit per input is 10 amps. You absolutely cannot have 2 stings connected to one input. If you absolutely must combine them into 2 main wires you're going to have to use a mult tap to uncombine them. The problem I have with this is theres no ocpd for the wires that come out of the split but it may be fine bc the inverter is basically limited to what it can draw.

[oregon_phil]

I have three strings: 1 x 9, 1 x 9 and 1 x 10, all on the same roof surface on my outbuilding, installed in 2018 under NEC 2014. They used an SMA rooftop rapid shutdown that has four inputs and two outputs. My two 1x9 strings are paralleled at the rapid shutdown box to Channel A, the third string is passed through the rapid shutdown unchanged to Channel B. In addition to the rapid shutdown control wires, I have 5 wires coming into the inverter: Channel A +/-, Channel B +/- and ground.

Channel A has the two paralleled strings. Since the current on Channel A exceeds 10amps, they used a 3 port Polaris block (IPL4-3 or similar) on Channel A + to split into two wires for Input A + and Input B +. The same 3 port polaris block for Channel A- for Input A - and Input B-.

I have commented on my particular SMA in the past. As others have said, paralleled strings on a string inverter can make troubleshooting more difficult.

[bartekw]

Will be using 3 inputs. Keep it simple

Thank you and have a great day everybody.