Semi-offgrid System

timknock28

New Member
Hey all, total solar newb here, I'm looking into building an off grid system with grid tie backup. For now I have 8 x SOK 100AH 12v batteries and a MPP Solar Hybrid LV6048. I'm looking at purchasing 20 or so 370w panels from Signature Solar (I live nearby so no shipping). I'm planning on having the panels in a pasture about 300 feet away from the inverter and battery location. How do I determine the correct size wire to use? What else should I be considering for the design process?

Thanks,

Tim
 

FilterGuy

What, me worry?
Welcome to the forum.

This is from the manual:

1628555746381.png


The total PV wattage the inverter will handle is 2 x 80A x 48V = 7689W (If the batteries are low)
20 370W panels will provide 20 x 370W = 7400W. So 20 panels are not too much

Most 370W panels run an Voc of 35-40V. Since the Max Voc is 145V on the MPP input, you will only be able to do 2 or 3 panels in series. If you are in an always warm climate, you might get away with 4 in series, but I doubt it. (To narrow the string count down we would need to know the detailed specs of the panels and the coldest it ever gets where you are.

Since you are talking about 20 panels, strings of 2 kinda works best. (If you want to do strings of three, a total of 18 or 21 panels works better)

If you go 2 in series you have 10 strings of 2. You will split that between the 2 MMPT inputs to get 5 strings on each MPPT. You will need two combiner boxes with appropriate breakers for 5 strings of panels.

I could speculate further on things like wire size and breaker size based on assumptions about the solar panels.... but that would be a waste of time.... However, if you review this resource, you should be able to calculate it yourself:

 

FilterGuy

What, me worry?
I'm planning on having the panels in a pasture about 300 feet away from the inverter and battery location.
The resource I pointed to in my previous post does not try to address voltage drop over long distances.
Because of the 300Ft (600Ft round trip) you want to keep the voltage high, so if you can get strings of 3 in series it would be better. The limiting factor is going to be the total Voc after you take cold temps into consideration. If the VOC works out, something like 3 strings of 3 in series on each MPPT would work reasonably well. You could even do 3 strings of 3 on one MPPT and 4 strings of 3 on the otther MPPT. (4 strings of 4 will theoreticly produce more power than the MPPT will take in but 1) Panels rarely produce as much as they are advertised for and 2) You will get more power in the morning, evening and cloudy days)

With 3 in series, each string will produce ~10A. For 3 strings of 3, you will get ~30A, but to do it correctly the wire needs to be sized for 30 x 1.56 = 47A. That means the 8AWG mentioned in the manual is the minimum size for the run from the panels to the inverter. However with 600ft round trip at 8AWG you might have a healthy line loss so I would go with something larger. (Too bad copper has gotten so dang expensive).

Warning!!!! All of the above has a lot of assumptions about the panels. You really need to go through the calculations with the actual panel specs.
 

timknock28

New Member
Thank you @FilterGuy ! I'm not opposed to 18- 21 panels, still in the design phase. And I haven't quite decided which panels exactly to go with. I was going to wait a bit and see if they were going to get any more polycrystalline panels in.

I live in NE Texas so the winters are typically mild but last winter got very cold for a couple weeks (below 0F).

And as for the 300' run, I could put the batteries/inverter closer to the panels and do the long run as AC instead if that's better.
 

FilterGuy

What, me worry?
I just ran some numbers:

Here are the specs of a silfab 370W Panel:
1628825254064.png

If you do 3 in series, it will operate at a typical VMP of around 35.91 X 3 = 108 V. That is getting close to the 120V so the gain of converting to AC will be limited.

HOWEVER: If we assume a worst case temp of -10F (-23.3C), the Voc from 3 in series will raise (25 - (-23.3)) x .279 = 13.148%
So the max Voc is 3 * 45 * 1.13148 = 152.74V..... and that is too high for your MPP input. Even at 0deg F it comes out to 151 volts so you should not run 3 of these panels in series.

EDIT: I used round trip distance but the calculator needed one-way distance so my numbers were way off.
I corrected everything below in purple
I ran a voltage drop calculator and with a 600ft round trip and 5 sets of 2 panels in series would need 4AWG and would still see a 9.2% drop.

However if you could run 3 sets of 3 in series, you would need 8AWG and get about a 8.9% drop.


Keep in mind that this is just for ONE of the two MPPs. We are talking about a total of 1200ft of copper and with the copper prices where they are this is going to get painful.

Even running a single run for 120V AC will get expensive. You will need 4 AWG and still see a voltage drop of 6.62% ... that means you would be at around 112V at the house, and some appliances may not like that very much....so the run should probably be 1/0 AWG!!!.

Have you already purchased the LV6048? Can you return it? If you could get something with a higher max PV voltage you would be better off. (Some all-in-ones with high voltages have recently come on the market)
 
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FilterGuy

What, me worry?
Bad news & good news & bad news:
Bad news: I made a rookie mistake and did not check to see if the voltage calculator wanted round trip or one-way.... it looks like it uses one way and I put in round trip...... That changes things by a lot
Good News: The wire sizes needed are much smaller. I corrected everything in my previous post. (Corrections in Purple).
Bad news: The wires sizes are still rather large, so it would still be better to find an all in one with a higher input voltage.

In this video Will released today, he used an LV6548 that has a 250V PV input. With this, you could run 4 sets of 5 panels in series to a single MPPT. With that set up you would only have to run a pair of 8AWG and have a 7.2% voltage drop.


 

FilterGuy

What, me worry?
In this video Will released today, he used an LV6548 that has a 250V PV input. With this, you could run 4 sets of 5 panels in series to a single MPPT. With that set up you would only have to run a pair of 8AWG and have a 7.2% voltage drop.
Note: I used the NOTC values to check for voltage at -10F and it came out to only 203V..... well under the 250V max for the input.
If you use the much more optimistic Standard Test Conditions, you get 255V at -10F. That is a bit over the 250V max. You could go 5 strings of 4 in series and run a single pair of 6AWG if you want to be extra safe.
 
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