question

centaur16 avatar image

Do I need a fuse here?

Hello,

I have my 12v side already setup in a van conversion and I'm about to add the MultiPlus 3kw.


I have 3x 200ah batteries with 300amp MRBF fuses at their terminals, in parallel brought together with 120mm2 cable to battery posts.


From the posts, 120mm2 cable connects the bus bars.


The MultiPlus will connect to the busbars via a 400amp T Fuse and 120mm2 cable again.


Should I also have a 400A T Fuse between the battery posts and busbar?


My original wiring diagram had this, but I then opted for the terminal fuses, and an electrician said the extra 400a in the single cable from the post therefore wasn't necessary. I'm now thinking that single cable is not fused correctly (as the 3x 300A MRBF fuses combined are too high) to the bus bar, except for the fuse between busbar and MultiPlus which is (I believe) the only route for that amount of current to run without blowing another much smaller fuse.


Thanks in advance!

And thank you all for your contributions across this whole community, it has been very useful in my learning.


MultiPlus Quattro Inverter Chargerparallelfuses
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2 Answers
michelg avatar image
michelg answered ·

Hi Centaur16, a 3Kw inverter will use around 250A, so a 300A fuse should be fine. Having 3x300A fuses brings 900A, which is really a huge amount. I am afraid the 3x300A will never blow up if any short-circuit happens, I will recommend you using 3x100A/3x125A fuse and a 250/300A fuse for the line connected to the inverter.

In my setup, I have breakers (24V/125A) and a fuse of 150A at the battery bank, my inverter being a 3KVA (around 2400W). It will use around 100A, but 125A gives a margin for spikes, burt enough secure for short-circuits.

Michel

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Hi Michel, thanks for your comment here. I've been following Victron's schematic linked here to get the above fuse sizes: https://www.victronenergy.com/upload/documents/MultiPlus-3KW-120VAC-12VDC-400Ah-Li-VEBus-BMS-generator-MPPT-BMV-CCGX-Orion-Tr-Smarts.pdf

I note they don't have the additional fuse my question asks about, but they only have copper bar joining the multiplus wiring rather than cable to and from a busbar in between.

michelg avatar image
michelg answered ·

I see. I still think that 900A can be a problem if a short-circuit happens. Reducing the size of the fuses will increase the security of your setup. I have always been considering the "smallest" fuse with a 25% marging for my needs.

It is just my point of view, but I consider that 900A can very easily melt and burn cables ... 400A is also a lot ...

Wait for other members of the forum to give their point of view.

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Thanks Michel, makes total sense and I follow your logic too. Victron diagram is confusing me though, as is the manual which states 400A with two 70mm2 cables to the MP. Yet believe 400A is too much for a single 70mm2, and good practice is for the fuse to be rated for single cable.