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Old 02-18-2022, 07:01 AM   #1
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Breakers and Fuses help

Hello everyone,
I am an amateur and have done a lot of research. Iíve seen many diagram etc and I could just copy the diagram for my own build but I want to understand the system more than just install it. So hereís what Iím working with:

4 100w solar panels in parallel
60A PWM charge controller
2 200AH 12 V lead acid batteries in parallel
DC Breaker box

I will not run any AC loads off this system. If, down there road, I want to install that I will devise a completely separate system.

My question regards the terminal fuse. For the most part, I understand fusing I think. Protect the wire and devices from too much power.

So I will install:
40A breaker between the panels and charge controller
60A breaker between the battery and charge controller
60A breaker between battery and 12V fuse block (likely)

I will also have a switch coming off the battery to be able to stop power down the whole system.

I have seen on many diagrams a terminal fuse directly on the battery, I for the life of me canít find a description of the reasoning behind this. Maybe this is a stupid question but can someone please explain its purpose.

And if youíd like to give any feedback on my proposed plan there I would appreciate it. I havenít bought the wires for this yet, so I understand itís certainly not a complete picture of the system.

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Old 02-18-2022, 09:20 AM   #2
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Hello!

First off, you're right about fusing to protect the wiring. Not so right about protecting devices. Yes, you can use fusing to protect devices, and many devices will include or suggest the use of fuses to do just that. But that's really a seperate concern from the fusing protecting the wiring. The device(s) on any one circuit may change, but the wiring and the environment it's located in will not. So protect the circuit.

To that end, you've listed your anticipated fusing/overcurrent protection, but you never listed the wire gauge, type (pure copper?), insulation temp rating, whether the wires are in conduit / bundled with other wires, or the environment (high temp?). Those are all factors that will determine the max amps the wire can safely carry, which will in turn determine your overcurrent protection.

As far as the terminal fuse (I assume you're talking about the fuses that connect directly to the post of a battery?)... they're one way of fusing in the recommended manner... as near to the source as possible. Basically, you want your fuse to be as near to the source as possible because any short to ground between the source and the fuse won't be protected. With a terminal fuse, there's zero length of wire that could be unprotected, so you don't have to worry about that. If you don't use a terminal fuse, you want the fuse protecting the output from your bank to be within 6" or so of the post, and make sure the cabling in between the post and this fuse is well secured & protected from chafing.
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Old 02-18-2022, 11:51 AM   #3
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Thank you so much. That’s very helpful info.

When I say “terminal fuse” I am referring to a fuse directly connected to the positive post of the battery. What I’m confused by though is how to size that fuse. With the system described above, what would you recommend and why?

Also, I understand what you’re saying about all the other variables impacting the safe amperage a wire can carry. Thanks for including that information as well.
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Old 02-18-2022, 12:21 PM   #4
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The fuse you're talking about is sized just like every other fuse... according to the amperage the cable coming off of it is safely designed to carry. And again, it's just one way of doing what you could also achieve w/ another fuse type w/in 6-8" of the post.

Working backwards, you'd generally first determine your peak electrical needs (a worst-case scenario of everything you expect to be powering at any one time, now & in the future). This usually requires significant time & planning.

Once you've settled on that max-amp figure, you can then choose one or more appropriate distribution box/panels that support at least that much current in sum. So if you figured 50A max of all items that could be running/starting at any one time, including room for expansion / additions for things you may want in the future, you want at least a 50A panel (with fuses / breakers for each individual circuit coming off of it).

Let's say you chose a 100A panel to give you lots of headroom (all these numbers are all just pulled out of the ether for purposes of example. Don't think I'm saying "buy a 100a panel" or "50A is what you need". I am not). It's 50A higher than what you need. Do you wire it to support 50A? You could (as long as you fuse appropriately). But it's better practice to wire it up to support its full capacity, and then fuse that wiring appropriately. So if it were me, I'd wire for 100A, and naturally fuse that wire for 100A. That fuse protects the wire (chosen to support 100A), and the panel (chosen to support 100A).

That fuse... that's what's coming off the battery bank.

One other concern is your batteries max discharge rate. Consult the manufacturer documentation. With two identical batteries in parallel you'd double the continuous discharge current rating of each battery to arrive at the absolute max continuous current (amps) your batteries can deliver. I'd make sure your fusing / cable selection coming off the battery was at or (preferably) below this figure.
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Old 02-19-2022, 07:16 AM   #5
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Ok I understand what you’re saying. So last question (I hope). It seems then redundant to me to have a 60A beaker between the battery and the 12V fuse block. The fuse block I have is rated for 100A. I was planning on putting a 60A breaker between the battery and fuse block. I’m not 100% on this number yet, it’s based on roughly all 12V loads I think I would run and then a safety margin. So does the terminal fuse not do the same thing?

Or have I just exactly described the situation that you explained in your first paragraph of the last post?

So in the diagrams I see where there is both a terminal fuse and a fuse on the positive wire from the battery to the 12V load, that is due to the fact that their systems have other loads being powered by the battery. In my proposed system, as long as the 12V fuse block holds the only loads my batteries are powering, I only need to fuse that line, the terminal fuse and a fuse on that wire are essentially doing the same thing.

Does that check out?
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Old 02-19-2022, 07:47 AM   #6
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(nevermind)
just passing through...
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Old 02-19-2022, 09:29 AM   #7
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Quote:
Originally Posted by Newbusguy View Post
Ok I understand what youíre saying. So last question (I hope). It seems then redundant to me to have a 60A beaker between the battery and the 12V fuse block. The fuse block I have is rated for 100A. I was planning on putting a 60A breaker between the battery and fuse block. Iím not 100% on this number yet, itís based on roughly all 12V loads I think I would run and then a safety margin. So does the terminal fuse not do the same thing?

Or have I just exactly described the situation that you explained in your first paragraph of the last post?

So in the diagrams I see where there is both a terminal fuse and a fuse on the positive wire from the battery to the 12V load, that is due to the fact that their systems have other loads being powered by the battery. In my proposed system, as long as the 12V fuse block holds the only loads my batteries are powering, I only need to fuse that line, the terminal fuse and a fuse on that wire are essentially doing the same thing.

Does that check out?
If I understand correctly - assuming your battery (bank) is only powering the 12V distribution panel - there's no need for a fuse coming off the positive battery terminal, and another means of circuit protection (fuse or breaker) past that point between the battery positive and the 12V panel. You would, however, want some means of shutting down the power from the batteries... a properly-rated battery cutoff switch.

Also note that a breaker is not a substitute for a fast-blow, current-limiting fuse for protecting your battery bank / cabling. In your case - with a terminal fuse - that's a moot point. But if you didn't have that, and were thinking "maybe a breaker would work here for both overcurrent protection and a switch, instead of using any fuse", it wouldn't be an appropriate solution. You want any fault that could lead to excessive currents pulled from the battery stopped immediately (Right F..... Now ) and that's what a fast blow current limiting fuse does.
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