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diy solar

Prelim off grid, SOK + Growatt

aurfalien

Solar Barney
Joined
Jul 20, 2021
Messages
78
Dear community,

This is my very first thread post so it's a very exciting time for me (minor sarcasm for a cheap laugh).

At any rate I have purchased a Growatt 6kW 48V MPV hybrid and the unit looks very nice, albeit it's still in the box.

I'm still deciding on batteries and am very seriously looking at SOK amongst others.

I'd love to over size the system in terms of storage capacity and thought that a 4S2P 12V/206Ah SOK battery might be very nice.

This would yield a 48V/412Ah potential (19,776 Wh) however my inverter at max would/should not pull more then 125A based on P/V = I; 6,000/48 = 125.

So would a fuse between the batteries and inverter be wise to size at 125A?

I feel as though I am missing something very basic and sorta yooooge!

I'm confident in my math but not so much in my understanding.
 
6000W / 48V is 125A. So you should size your wire to handle at least 125A. For that I would suggest 1AWG high quality wire. Then you choose a fuse to protect the wire. It's common to multiple by 1.25 so 125A * 1.25 = 156A. For a single wire you can fuse up to 250A on a 1AWG wire. So I would choose a 150 - 175A fuse between the battery and inverter with the 1AWG wire.

Choosing a 125A fuse would work but if you ever actually tried to use the full 6kW of the inverter then the fuse might blow needlessly.
 
Ah thank you, makes total sense. Also accounting for surge would indicate the need for a larger fuse as well which probably results in the 125% of 125A fuse that you mentioned.

However I've read many times here that one should fuse for protecting the conductor which I find odd. The fuse should protect the load and you size components accordingly yes?

It might be splitting hairs but the Devil is always in the details.

Thanks again and I'll go on the higher end and fuse it at 175A as the battery bank of 4S2P should be able to handle it.
 
Also accounting for surge would indicate the need for a larger fuse as well which probably results in the 125% of 125A fuse that you mentioned.
No. The 125% factor has nothing to do with a surge. It's just a typical way to find the best fuse size to protect the wire. More on that below.

However I've read many times here that one should fuse for protecting the conductor which I find odd. The fuse should protect the load and you size components accordingly yes?
The only job of the fuse is to protect the wire. The fuse's job is to blow before excess current in the wire (possibly caused by a short or some other equipment problem) would turn the wire into a flaming torch. The fuse is not meant to protect the load in any way.

This is where the 125% calculation generally comes into play. You have a known maximum load (125A in this case). You pick a wire that can safely handle that load without getting too hot or losing too much voltage over distance. Now you choose a fuse to protect the wire. The fuse needs to at least the size of the maximum load and it needs to be less than the maximum current that the wire can handle before it becomes dangerous. So you pick a fuse between those two. Too close to the load current and the fuse might blow early (often called a nuisance trip). Too close to the max current of the wire and you risk the fuse not failing before the wire. The 125% factor typically gets you a value in that range.

It might be splitting hairs but the Devil is always in the details.
It's not splitting hairs at all. It's important to understand the choices. Pick a wire based on the load and allowable voltage drop. Pick a fuse/breaker to protect the wire.

I'll go on the higher end and fuse it at 175A as the battery bank of 4S2P should be able to handle it.
Your choice of fuse is not based on the battery configuration being able to handle it. Two SOK batteries in parallel can actually provide a max continuous discharge of 200A. It's fine to have a lower fuse because you are fusing the wire based on the max current of the inverter. What's important with your choice of inverter and battery configuration is that even with the inverter running at the max of 6kW you will only be discharging the batteries at 125A which is less than the 200A max that the batteries can provide.
 
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