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Safety Check for Off-grid system: Solar, battery and inverter

wrightmi01

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Oct 12, 2020
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I've been studying the posts and videos in an effort to design an off-grid system for a rural property I have. There is a barn with a small cabin-like area above and tools and equipment below. I'll be occasionally running power tools (e.g., table saw, bench grinder) in the working part of the barn. Along with lights, the cabin will have a few amenities like a small refrigerator, TV, etc and used for occasional weekend stays. I've done and energy audit and arrived at 3200 watt hours/day. I assumed running all the tools, lights, refrigeration, TV, and more for very a generous amount to arrive at that value. The panels will not have any shading to contend with and lowest temperatures are in the low 20's * F for a handful of days.

Using the resources available on this site, videos and posts I've arrived at *what I think* are appropriately sized fuses and wire sizes for the equipment and load. I would very much appreciate your thoughts and feedback to ensure I've designed a safe system. All of the components will be tightly grouped so wires runs will be very short. The solar wire run will be more more than 25'. I intend to add more panels in the future

Specific areas of concern:
- Ground: Is an earth ground from the AC breaker panel all I need?
- 300A fuse on the battery. It is a 280Ah battery, but with the 3000W inverter I will never pull more than 125A continuous. Have I got this right? Too big?
- Following the logic in the previous bullet, I sized the fuse off the inverter at 150A. Is that sufficient/safe? Fused for continuous current not peak, correct?
- I'd like to add a breaker/switch to isolate the panels, but have not identified one yet.



Blackburn Solar Diagram.png
 

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Your 100/50 is likely not adequate. At 20°F, you'll see a Voc over 100V. Victron states that a 1.22 margin should be allowed for cold weather operations. I made the mistake and went 3S on my 47Voc panels on my 150/100. Upgraded to a 250/100.

The panels will need their own ground - panel frames directly to earth.
Fuses are sized for the WIRE, not the components. Your fuses/breakers should be wire current capacity * 1.25.
Midnite Solar sells a line of breakers designed to handle 150VDC for PV panels.
 
Yeah, every time I look up the datasheet for that particular panel the numbers change. At 88+ Voc * 1.14 (for 22-14* F) it's right at 100V. If they suggestion from Victron is 1.22, then I'm over. I haven't purchased the panels so I can adjust there or the SCC. 250/100 would allow for adding more panels without another SCC...that might be the way to go.

I've seen quite a few different ampacity numbers for a given wire size, which was confusing, but believe I have arrived at wires that are well under the allowed current. So, I think I'm alright there.

Good to know on grounding the panels independently. I'll work on figuring that out. The Midnite solar breaker sounds like what I need. Thank you!
 
A 250/100 is about $900. I was in a situation where I needed to rewire to 2S AND get a second SCC. I opted for the lazy way and went for the 250/100

60 cell panels might get you more margin. If you're willing to go with the NEC2017 guidelines, then you're right at that limit. 72 cell panels seem to be more pain then they're worth when it comes to 100 or 150V limits.

A 150/100 is about $200 cheaper than a 250/100. That would get you plenty of margin and can work with up to 2900W of panels on a 24V battery.
 
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