PV array wiring upgrade dilemma

[rccapps]

I'm upgrading our off-grid household power system, and I have a choice to make.

Currently, the PV-only input is 8 x 375W panels. Each panel has a max amp rating of 9.43A and a max voltage just a hair under 40V. There are 4 serial strings (of 2 panels each) wired in parallel. The array feeds into a Renogy Rover 60A charge controller, which in turn feeds a 48V battery bank.

We are adding 4 more 375W panels (total of 12) and replacing the 60A charge controller with a Midnite Solar Classic 150 SL. The Classic doesn't have a fixed max output amp rating like the Renogy, because (according to the manual) it varies based on the PV input voltage. Here's the table showing the respective power curves for the Classic:


If I kept and expanded the current 80V configuration - 6 serial strings (of 2 panels each) wired in parallel - the max output amp rating for the Classic would be approximately 85A. Since 85A x 58.4V = 4964W, it could easily handle a maximum of 12 x 375 = 4500W from the panels. However, the max input amps would be 9.43 x 6 = 56.58A. This barely exceeds the NEC max current for 8 AWG wire (55A), which is the largest size MC4 connectors will fit.

On the other hand, if I rewired the panels for 120V - 4 serial strings (of 3 panels each) wired in parallel - the max input amps would go down to 9.43 x 4 = 37.72A, which 8 or 10 AWG wire (with MC4 connectors) could accommodate. However, the max output amp rating of the Classic at that input voltage drops to 76A. This would yield an output wattage of 4438W, which would lose 4500W - 4438W = 62W at the high end. It's not a lot, but I'd rather keep it, if possible.

So, is the 1.58A in excess of the NEC rating enough of a concern to avoid using 8 AWG wire, and rewire the panels for 120V and take the small power loss? Or is 1.58A close enough that I could keep the 80V wiring without worrying about it?

Of course, I realize there might be a third option: most of the length cabling from the panels to the charge controller will be two parallel lines of 10 AWG, for two reasons: 1) I can bring two separate 3 x 2 series-parallel arrays together with a 2:1 branch connector a few feet before the charge controller, and 2) the current pre-upgrade array already uses a 10 AWG cable, so I can just repurpose it and buy one more. If there's some easy off-the-shelf solution for connecting a 6 AWG length to an MC4 branch connector, I'm open to it. Even if it required a little crimping, it would still be preferable to the alternatives.

Any advice would be much appreciated.

 

[rhino]

I would not go over the ampacity of the cable, it may start heating up and degrading the insulation. What is distance from panels to charge controller? Are you going to expand any more after this? What is your battery bank voltage? You have to remember you could already lose a lot of power sending that much amps over 8AWG to begin with so switching to 120V may not result in as much loss if any.

 

[rccapps]

I would not go over the ampacity of the cable, it may start heating up and degrading the insulation. What is distance from panels to charge controller? Are you going to expand any more after this? What is your battery bank voltage? You have to remember you could already lose a lot of power sending that much amps over 8AWG to begin with so switching to 120V may not result in as much loss if any.

Between 30-45 ft from panels to charge controller; the controller is closer to one side of the bank of panels than the other, hence the range.
No plans to expand the system further; if we DID need more power, we'd probably just set up the old system (based around the Renogy charge controller) separately with its own panels, batteries and inverter.
As stated in the original post, the battery bank is (and will be) 48V.

 

[MisterSandals]

Of course, I realize there might be a third option:

How about a 4th option?

Is it possible to have 2 different arrays? If you point some of your panels east-ish to start harvesting earlier in the day with one SCC and point the rest of your panels west-ish, to harvest later into the afternoon, you can harvest for longer and at lower amp ratings.

And, running 2 different, independent systems, you will have redundancy so a single point of failure doesn't stop some solar harvesting.

I'm sure you will know if its better to have equal parts east vs west or if its better (fewer trees for example) to have one system larger.

Just a thought. You're trying to maximize your solar harvesting right? And maybe minimize the number of amps you are pushing around thru your already stressed wiring?

 

[rccapps]

I think I've got a solution to my problem, and I'm posting this in case anyone else faces a similar issue with a similar level of ignorance about matters electrical to my own. To an extent, I went with the vaguely proposed 3rd option, but instead of an MC4 2:1 branch connector, I'm buying a 3-way insulated terminal block connector:

The ends of the MC4 connectors on the long solar cables pop/screw off easily, and the cylindrical metal "nubs" inside slide neatly into the terminal block ports, as does the bare end of a 6 AWG cable. Clamped in place by hex screws, this seems like it would be a secure connection. With two of these at a cost of less than $30, I think my problem is solved.

I strongly suspected a connector like this existed, but my lack of knowledge/experience with residential-scale electrical components meant a bit more internet search fumbling than expected.

That being said, I appreciate the suggestions and musings in response to my post.