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diy solar

Two strings with different amperage on one Victron MPPT 250/100

TouchHarder

New Member
Joined
Apr 29, 2021
Messages
28
Hey
My setup is as followed:

27x Trina Solar Vertex s TSM-400 DE09.08
2x Victron Smart solar Mppts 250/100 MC4
3x Victron MultiPlus 48V 5000 70/50
8x Pyxlontech US3000C



The caption is a little misleading, but i don't know how to describe the question any better.
I want to combine the panels 5 in series 3 in parallel for one MPPT and 4 in series 3 in parallel for the other.
The panels have a short-circuit voltage of 41,2V and a short-circuit current of 12,28A.
The temperature coefficient is
Pmax -0,34%K
Voc -0,25%K
ISC 0,04%K



Question:
Is there a problem combining a 24,56A and a 12,28A string at the MPPT? The MPPT puts them in parallel anyway...








Why do i want to do that?

The plan was to get a cable 6mm2 from each 5 (or4) in series to the MPPT and let the MPPT combine the 3 strings in parallel (the victron Mc4 inputs are internally paralleled)

I don't want to combine them all on the roof because each MC4 input of the Victron MPPT has a maximum of 30A (12,28Ax3=36,84A) The Victron has 3x MC4 inputs.

There is a law in my country which needs me to put an overvoltage surge protection each 10m of cable length from the power source (the panel) which is why I'm thinking of paralleling two of the three strings together on the roof. 24,56A is doable for the MPPT input and I can still use the 6mm2 cable. So that I can cheap out a little on buying overvoltage surge protection. Those cost 100 bucks per string (I'm not going to buy the cheap china stuff)


Would be happy for constructive input from you guys.
 
I'll confirm that your configuration has no electrical issue at the MPPT. Each of the two "down-cable" pairs has the same Voltage differential, and the total current on the two parallel input cables is additive, as you correctly described.

My only issue would be the power loss within the already-paralleled-once "doubled" cable, versus buying more cable and spending another $100 over overvoltage protection. I would express that question in this way:

If you can make use of the additional solar power arriving at the MPPT, what would be the number of months/years need to generate 100% return on investment for the 3rd cable pair and associated over-voltage protection cost?

It's a hard question to answer, because the "usefulness" of that additional power is possibly a lot more in winter months than summer months.
 
For me, it's actually just as useful in the summer as in the winter, because it's going to power an electric car :)
The wire gauge calculator says even with 4mm2 the loss is under 1% so with 6mm2 it should be way less.
I was just asking myself if the Victrons PowerPoint tracking software will be happy with that setup.
 
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