12V distribution box - what fuses?

[Fishd]

Hi all,

I'm having a go at re-wiring some of my electrics as I'm fitting a BMV-712 to monitor my leisure battery.

To simplify and improve some of the electrics, I was looking to fit a 12V distribution box like this one but I'm a little unsure the best way to wire this.

To the folks that have used these, how have you wired them?

Given my equipment (dc to dc charger, mppt, mains charger and a couple of downstream fuseboxes) I can't see any requirement for something the size of the Mega fuse and even the Midi fuses seem overkill compared to what I have currently... I think the largest fuse I have right now in my system is a 30A Maxi for my DC to DC charger.

I was originally thinking of having the DC to DC charger (25A) on the Mega fuse (but I think the smallest Mega fuse rating is 100A?), then the four Midis connecting the mains charger (15A), MPPT (20A max) and the two downstream fuse boxes (using 30A Midis I guess) ...

One other thing that threw me, was the continuous vs blow rating of Mega and Midi fuses... are folks just upping the capacity of their cabling to deal with this?

Any help would be appreciated.

 

[saintclarkey1]

How did you get on with this? I'm about to do the same. The way I understand it you have one main (larger amp rated cable) going from the battery to the Mega fuse point and then everything else (fuse boxes, DC charger, battery charger, battery sensors, solar etc etc connected to the midi fuse. The size of this cable is important as is the Mega fusenfor this cable from the battery. It's not nessesarily about load its about the total fuse capacity of the midi fuses that sit in the box with it. E.g say you have a DC charger coming into the box on a 70amp 10mm2 cable and a 50 amp midi fuse, the same for your mains battery charger, and the same for two additional downstream 12v distribution fuse boxes. With that setup right there you require a mega fuse that's greater than 200 amps and a cable from the battery to the Mega fuse that's greater than that. I'd probably go 225 amp mega fuse and then 35mm2 cable to the battery (240amp rated). I think you then also need to ensure you have a 35mm2 earth cable from the negative not the battery to the chassis eartring point

 

[Grim Reaper]

I think you're over thinking what your requirements are for fusing.
The smallest Midi fuse you can get (that I know of) is 30A, so overkill for most things inside a van.
As an example, in my van I have maybe 3 midi fuses, 60A on the battery end of the feed into the 40A DC-DC charger on the leisure side of the van, 100A on the leisure battery output cable which goes to a busbar point, and a 30A on the feed from that busbar via a small battery cut off switch (so I can isolate to a 12 way fuse box which then has individual fuses feeding out to lights/diesel heater/USB chargers/12V outlets etc.
My inverter has no other fuse on its link (relying on the 100A fuse on the battery feed for any faults there)
My solar input is unfused though I do have another small battery cut off switch on that feed so I can isolate the panels from the DC-DC charger during the winter when I don't want the lithium leisure battery being charged.
I used individual midi fuse holders for these and they fit nicely in the runs of the cables as they go from one point to another.

 

[saintclarkey1]

I totally agree, my numbers were hypothetical for explaining to the Original Poster how I thought it worked (and to prompt a response\confirmation from someone like yourself who will probably know more than me ). My real life situation will be - 15 amp victron mains battery charger (30 amp midi fuse), 40 amp DC Charger (50 amp midi fuse), 12v Distribution block 1 (30 amp midi fuse), 12v Distribution block 2 (30 amp midi fuse) all into the Mega\Midi Dist block. Based on this I think I need 150 amp mega fuse and 25mm2 cable from the leisure battery to it. Agreed? I'd usually use individual inline midi fuses but on this conversion I thought I'd try something different (why not, live and learn right) and with the set up under the seat I'm hopeful it will give me a tidier finish with a lot less cables hanging off the battery terminal