Off grid cabin solar design check

[shadyshabin]

Hey all,
I'm after some fresh and more experienced eyes to double check for any design flaws before I spend some money! This is my first solar design for an off grid cabin.
Appreciate any feedback!

 

[FilterGuy]

Hey all,
I'm after some fresh and more experienced eyes to double check for any design flaws before I spend some money! This is my first solar design for an off grid cabin.
Appreciate any feedback!

Nothing jumps out as terribly wrong. But here are some observations

Edit: Corrected wire gauge on the diagram.



* The wiring going to the 24-12V converter seems larger than it needs to be. With a 40A output and a 90% efficiency, it should only draw (40/2)/.9=22.2A. For a fuse I would take it down to a 30A fuse and 10AWG (6mm^2) wire.

* Every time you turn the batter disconnect switch on, there is going to be a very large spark internal to the switch because of the big capacitors on the input to the inverter. (This will happen even if the inverter is turned off). You may want to consider putting a disconnect switch with a precharge circuit on the inverter circuit. Then you should always make sure the inverter is disconnected before you turn the battery switch on.

* There is not enough detail on the battery and shunt wiring to give any comment.

* It kinda looks like you have 2 strings of 4 panels. Depending on the panels, that might (or might not) create too high of a voltage in cold weather for a 150V MPPT.

Adjusting Solar Panel Voc for temperature

When designing you solar panel system, it is important to adjust you solar panel Voc for temperature in order to ensure you do not over-voltage the PV inputs of your solar charge controller. This paper shows how to calculate the Temperature...

diysolarforum.com

 

[FilterGuy]

One more thing: The 300A fuse on the battery and inverter seems too large.

A 3000W inverter at 90% efficiency will draw ~138A at full load. Since you don't want to run a fuse at its blow point you should have a 188 x 1.25 = 173A fuse. I would look at using a 175A fuse on the inverter and a 200A fuse on the battery. If you are using 105C rated wire, the 1AWG will be OK. If you are using 90C rated wire you may want to go to 1/0AWG (55mm^2)

 

[shadyshabin]

Nothing jumps out as terribly wrong. But here are some observations

Edit: Corrected wire gauge on the diagram.

View attachment 97619

* The wiring going to the 24-12V converter seems larger than it needs to be. With a 40A output and a 90% efficiency, it should only draw (40/2)/.9=22.2A. For a fuse I would take it down to a 30A fuse and 10AWG (6mm^2) wire.

* Every time you turn the batter disconnect switch on, there is going to be a very large spark internal to the switch because of the big capacitors on the input to the inverter. (This will happen even if the inverter is turned off). You may want to consider putting a disconnect switch with a precharge circuit on the inverter circuit. Then you should always make sure the inverter is disconnected before you turn the battery switch on.

* There is not enough detail on the battery and shunt wiring to give any comment.

* It kinda looks like you have 2 strings of 4 panels. Depending on the panels, that might (or might not) create too high of a voltage in cold weather for a 150V MPPT.

Adjusting Solar Panel Voc for temperature

When designing you solar panel system, it is important to adjust you solar panel Voc for temperature in order to ensure you do not over-voltage the PV inputs of your solar charge controller. This paper shows how to calculate the Temperature...

diysolarforum.com

Ahh thank you for sharing what you picked up!!

Re the 24v-12v converter, yes! I was thinking in 12v..

Thank you for the advice on having an inverter disconnect switch, will have to incorporate that.

For the battery shunt, I was thinking 50mm sq. cable?

Ahhh yes, the panels have Voc = 37 (each). Based on the NEC table, the MPPT would be undersized for anything temperature below 25 degrees Celsius... These are second hand panels we're using, I wonder if that would be better or worse? I wasn't able to find the Temperature Coefficient, and the company's website is in Chinese. The MPPT is the one device we already have! We could potentially drop down to two strings of 3 panels though..

I went with a 300A fuse for the battery OCPD as the Multiplus-ii manual recommended this size, I did think this sounded too high, though? Or maybe I misinterpreted. I have attached table.

 

[FilterGuy]

I went with a 300A fuse for the battery OCPD as the Multiplus-ii manual recommended this size,

If that is what they say, then I guess you should use it...... but then the wire seems too small.

A 1AWG (50mm) 105C cable is only rated for 245A (am I doing my conversion to AWG wrong?). Some engineers are willing to spec the wire for a bit under what the fuse is because if there is a short circuit the fuse will blow before the wire gets too hot......but that makes me a bit uncomfortable.. I would at least use a 1/0 (55MM) cable. Even that is only rated for 275A.

 

[FilterGuy]

These are second hand panels we're using, I wonder if that would be better or worse?

I would not make any assumptions.... particularly if you only have 2V of leeway.

 

[FilterGuy]

Yet another thing:
I would not normally put a fuse next to the Solar charge controller. It is internally current limited and it is highly unlikely it would ever cause a problem. (The fuse near the busbar is a must)

 

[shadyshabin]

If that is what they say, then I guess you should use it...... but then the wire seems too small.

Yes... I'll have to do a bit more digging. I might just go with a 2/0 AWG (70mm sq), if the only downside is that it costs a bit more?
Or perhaps I can do as you suggested and have a 175A fuse for the inverter and see how that goes, and keep a 300A fuse for the battery protection.

I would not normally put a fuse next to the Solar charge controller.

Oops, thanks for picking that up. After staring at the design for a little too long I decided to add one in, thinking I remembered the MPPT wasn't internally protected.

 

[FilterGuy]

if the only downside is that it costs a bit more?

The other downside is that it is a PITA to work with.