Sunny Boy string Inputs

[voodoocr]

Installing a Sunny Boy SB7.7 SP1-40 and noticed it has no combined Input on the DC side.

I have also already installed a combiner box on the roof and running 2 wires down to the inverter from the combiner box which has its own fuses for 3 strings so about 25A @ 370v DC.

So my question is can I splice each of those wire just prior to connection to feed each fused input A- B- C- and A+ B+ C+ on the inverter thus feeding 8A to each string input which are rated at 10A.

I don't see why not but just wanted to check with someone with experience !!

Thank you

John

[inetdog]

Is this the US model? If so the manual seems to indicate that only the A and B inputs can be paralleled.

Three inputs, 245-480V, 10A each, max of one string per input.

[solarix]

I'm not clear on what you are doing, but the only time you can have more than one string per input on the SunnyBoy TL's is if you configure it with the A and B inputs paralleled - then you can tie three equal strings into these two inputs. With another string on the C input, it gives you a way to wire four strings to this three input inverter. The three strings need to be tied together as well as the A and B inputs tied together - which means you ought to use a DC combiner box (with fuses) to protect the strings.
BTW, on the smaller inverters in this series (3.0, 3.8), there are seemingly three inputs as they all use the same connector hardware, but input C is not active.

[sensij]

OK, I stand corrected. On further review, and based on the responses above, combining three strings on the roof looks incompatible with this inverter.

You will need to replace the fused combiner with a string pass-through, and replace the two wires you ran for the home run with 6 wires, so that each string has its own circuit. If you'd like, you can combine two of the strings, no fuses necessary, so the home run is just four wires.

I don't really get why the combined approach doesn't work. If the A and B inputs can be paralleled without screwing up their independent mppt control (and I don't see a software setting to indicate whether they are combined or not), I don't know why all three inputs couldn't be paralleled.

Is it possible that all three really could be put in parallel on the roof into one home run (and then split equally into the three inputs in the inverter), but the SMA manual doesn't want to get into fusing requirements for individual strings, so they don't illustrate that situation?

[inetdog]

OK, I stand corrected. On further review, and based on the responses above, combining three strings on the roof looks incompatible with this inverter.

You will need to replace the fused combiner with a string pass-through, and replace the two wires you ran for the home run with 6 wires, so that each string has its own circuit. If you'd like, you can combine two of the strings, no fuses necessary, so the home run is just four wires.

I don't really get why the combined approach doesn't work. If the A and B inputs can be paralleled without screwing up their independent mppt control (and I don't see a software setting to indicate whether they are combined or not), I don't know why all three inputs couldn't be paralleled.

Is it possible that all three really could be put in parallel on the roof into one home run (and then split equally into the three inputs in the inverter), but the SMA manual doesn't want to get into fusing requirements for individual strings, so they don't illustrate that situation?

The problem, it seems to me, is that there are three independent control loops in the firmware of the inverter, each one controlling the voltage/current at one of the three MPPT inputs. If the setting of one loop affects the voltage seen by another loop, you have the same problem you get with two totally independent MPPT devices connected to the same panel array.

it is easy enough to detect that A and B have been paralleled: You just change the A setting and see whether the B voltage changes in response.
But trying to do that for any combination of two out of three or all three inputs is more complex than their programmers were confident in implementing.

[Tissouk]

Hi Everybody. I am new to this forum . I have a straight question and would be very grateful for your guidance. I have a small PV system of 3.0 kW capacity. The array is made up of only one string of 12 panels of 250Wp each . The Inverter has been removed away by the installer for repairs and never came back with it. I am looking forward to replace it by a SMA Sunnyboy Inverter to get the system operational again. But the Sunnyboy has 2 MPPT. My question is: Will the Sunnyboy work fine with only one string ?

[SunEagle]

Hi Everybody. I am new to this forum . I have a straight question and would be very grateful for your guidance. I have a small PV system of 3.0 kW capacity. The array is made up of only one string of 12 panels of 250Wp each . The Inverter has been removed away by the installer for repairs and never came back with it. I am looking forward to replace it by a SMA Sunnyboy Inverter to get the system operational again. But the Sunnyboy has 2 MPPT. My question is: Will the Sunnyboy work fine with only one string ?

I believe the Sunnyboy will work with on a single string provided you input enough wattage and voltage per it's specifications.

From what I have heard the second input allows for multiple panel orientation but others should be able to provide more hands on experience.

[oregon_phil]

To use one input, manual says OK. As long as the string voltage is under 480 Vdc and amps is 10 or lower.

[Tissouk]

Thanks a lot for your help. Its good to know it can work with only one string. I shall check the voltage and amps conditions and do needful.