Series Parallel wiring question

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Solar Expert Posts: 27
Hello,

I'm wiring 3 strings of PV Panels (Unisolar) on a 5th wheel.

Each string contains a 144w and a 68w panel wired in series.
The 3 strings will then be wired in parallel.
The 68w panel is about 1/2 the voltage of the 144 (33v vs 16.5v) and a slight difference in amperage: 4.4a for the 144w vs 4.1amp for the 68.

Due to the placement of the panels on the roof it would be easier to wire two of the strings 144w panel positive to 68w panel negative and the last string 144w panel negative to 68w panel positive.

The end result would be that when I parallel wire the 3 strings, 2 of the positive leads would be coming from the 144w panel end of the string and the 3rd would be coming from the 68w panel end of the string.

I don't think there is any reason to not do it this way but didn't know if the amperage difference between the panels would make an additional difference. I realize that the slight difference in amp output of the 68's will reduce the output on the 144W panels slightly.

Thanks

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Re: Series Parallel wiring question

Assuming that the numbers are correct (33 Vmp is a bit odd for any panel, especially 144 Watts, but Unisolar makes odd panels) here's what you'll get:

Vmp of the 68W + Vmp of the 144W = Vmp of panel strings. 16.5 + 33 = 49.5
Imp of the strings will be limited to 4.1 Amps. The current of the two is close enough to cause no worries.
It will not matter if two strings are "144W + 68W" and one is "68W + 144W".
Your array will be Vmp 49.5 @ Imp 12.3.

It's all good. Of course this is on an MPPT controller.
• Solar Expert Posts: 27
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Re: Series Parallel wiring question

Gotcha... I knew i'd be limited due to the slightly less imp of the 68w panels.

Yes on the MPPT. I'm using a Tristar 60. All the strings are series fused as well.