Will being slightly over voltage cause issues

[JayeTH]

I am in the planning stages for my solar system form my Tiny house. i have already purchased my Growatt 3000w 24V all in one units.

Due to the design of the house roof the plan was to put 3 panels on each side. I had looked at Trina 400 W panels but if i did the math correctly they would be to much for the MPPT charge controller. I also found some REC 350 watt panels that I think will work.

Growatt
Max PV array = 2000W
MPPT Range = 30VDC - 115CVDC
Max PV OCV = 145 VDC
Maxium Solar Charge Current = 80 A


Trina panels
Rated power = 350W
Open circuit VOC = 45.4
Max Voltage (VPM) = 41.1
Short Circuit Current = 10.18 A
Max Power Current = 9.74
Max System voltage = 1500 VDC
Fuse Rating = 20 A

REC Panels
Rated power = 350W
Open circuit VOC = 46.7
Max Voltage (VPM) = 38.9
Short Circuit Current = 9.27 A
Max Power Current = 9.0
Max System voltage = 1500 VDC
Fuse Rating = 20 A

With both I think i would have to wire in series. Going in parallel would be over the Fuse rating if i understand where that comes in correctly. The short circuit current would be almost 30A.

With the REC panels in series i would have VOC=140.1 and VPM=116.7 Current would remain at 9A. This is slightly over the 115. Is that ok?

With the Trina panels I would be at VPM=121.3

I am thinking i would be better going with the REC panels. I have attached how I plan to wire everything. Any thoughts or suggestions are welcome.

 

[snoobler]

Welcome to the forum.

It's a massive no-no.

Voc is always used to establish max PV voltage, not Vmp.

Unless you live in an area that never drops below 25°C, you can't put 72 cell (~47Voc) panels 3 in series on a 145Voc limit controller.

80A *24V = 1920W - what the 80A charge controller can use.

You're proposing about half that much.

You could do 2SXP. The 20A has no relation to the Isc from a planning perspective. If you go more than 2 strings in series, each string will need a 20A max fuse/breaker. That's all that means.

For each X, you would add 10A. Since there's no published limit form PV in, typically assume the same as out, 80A.

You could have 2S3P on each SCC and be about 200W over-paneled. Your total input to the SCC is 30A, no big deal.

I prefer the REC due to their slightly lower current.

 

[JayeTH]

Thank you so much for your help

If i follow you are saying i should wire the 3 panels on each side in parallel. I could then either wire each string of parallel panels to a charge controls or combine the two strings in series to one charge controller.

With the higher amperage I would have to change wire size. I believe. Run is about 15-20 feet. From running the online calculators. At 20 feet would need 9 gauge but at 15 feet can do 10. The length would only from where they are combined to where they enter the charge controller, correct? If so I think I can get it down to 15 feet. I had planned to use 10 gauge wire with MC4 connectors

 

[snoobler]

Not sure what wiring calculators you're using, but 10awg (typical panel wire) @ 39V, 30A, 20' has a voltage drop of 3.1%. That's not optimal, but it's not bad.

If you only have the six panels to work with, I would go 2S3P on only one controller vs. 3P on each controller. 1.5% voltage drop, and you'd likely have a small performance increase from the higher PV voltage.

 

[John Frum]

@JayeTH is there a reason that you put the negative switches on the negative leg?
Not saying its wrong, just curious.

 

[JayeTH]

i was thinking you had to put them on the negative side. For some reason i recall reading that someplace that is it better to put your cutoffs on the negative side.

 

[JayeTH]

The reason for not running the two parallel strings in series is that they will be on opposites of a gable roof. When one side is in sun (3 panels) the other side would be in shade. I have looked at do a lift/tilt bracket so i cold put the "down side" into the sun, but not sure it would always be worth the effort.

 

[John Frum]

i was thinking you had to put them on the negative side. For some reason i recall reading that someplace that is it better to put your cutoffs on the negative side.

Mayvbe from chassis return in cars.

 

[JayeTH]

Mayvbe from chassis return in cars.

that is probably where. My brother is a mechanic and we added one to my semi truck which will be pulling the tiny house to keep it from draining the starting batteries.

 

[JayeTH]

@snoobler you siad you use VOC for determing MAXPV...so then on the SCC what is the MAX VOC listed separately? I am really curious

 

[snoobler]

Voc is a solar panel property. The sum of the series panel Voc MUST NOT exceed the listed 145V. Additionally, you need buffer to allow for cold weather voltage increase. Voc increases roughly by 1% per 3-4°C.

The MPPT range is where the sum of your panel Vmp should fall.