I'm installing a system at my house, new to solar, reading about reverse currents and noting what my module manufacturer says about this in the installation manual. QCells says I need string fuses if I have more than 2 strings in parallel. I have a Midnight Solar combiner box with their 20A breakers. Is this insufficient for preventing issues with reverse currents? My plan is 3 strings in parallel (a total of 6 strings), with 37A/120V per string.
I called QCells but haven't heard back. Here's what they say in the install manual
Any Idea , Suggestions would be appreciated,
I didn't find the right solution from the internet.
For me to understand the situation I have to dig into what reverse current even is in a solar array. Essentially what happens if one solar module has a fault the other remaining modules will force their currents through the faulted string. This can even happen if half your solar panels get covered in shade and the other half still see sun, current will increase because your shaded modules will act like a load. The video shows this happening very well.
Now to fuse or not to fuse. The primary reason we have fuses/breakers is to protect the wires, then protect equipment (in that order). Some inverters which Im assuming your midnight solar combiner is (DC to AC) Some come with empty placeholders others come with protection devices already installed. Why the difference? Mainly i believe its so you have more control over how you set up your array and can choose the protection required for your system.
Module manufacturers by code are required to to have integrated over current protection device (OCPD) of 1.56 times the short circuit current (ISC).
In a single string - no extra protection required since it should be protected by the OCPD. This goes for 2 string as well.
In a 3 string, current can reach 1.25 times that of the ISC, if a fault were to occur in one string the other two would force there way through the faulted module, this means you would have 2.5 times the ISC rating which exceeds the required 1.56 OCPD and now you need to protect your output with fuses.
So here is the formula: ISC x 2 strings x 1.25 so.... 8.87A x 2 x 1.25 = 22.18A > 15A
need fuses on output.
IF you have the data for your system you should be able to calculate this way.
At the end of the day follow the owners manual specs and installation instructions, I hope this provides some kind of lead to get things moving.