This afternoon I had a message from David at @comparethecampervan after talking a little bit about various systems he asked me if I would mind contributing with some knowledge snippets that could help the van building community to raise it’s standards therefore improving safety.
Why are fuses so important and why can’t we just wire the batteries straight to the source, after all a 12v fuse box is a fuse box? The issue is if the fusebox itself has a melt down or the cable running to it shorts out there would be no fuse between the battery and that fusebox to prevent the battery shorting, this goes for all appliances.
If you look at most modern batteries in vehicles there will be a master fuse box connected directly to the terminal, this ensures the fuses are as close to the positive terminal as they can be therefore protecting the battery from a wire that could short out on the body.
Think of the fuse as the gate keeper to the appliance where the power will end up or come from, this ensures that a short can’t return to the battery and that the appliance cannot draw too much current, the fuse should always have a lower rating than the cable it’s in front of to ensure that the cable doesn’t act as the fuse and start a fire.
I wanted to share a little piece of kit that I’ve been using recently called the Mega/Midi fuse holder, Victron sell this piece of kit and it’s branded as the “lynx distributor”
This makes a very neat little hub where all of the loads and inputs can be connected in one place, a fused positive bus bar, it would be best practice to install a terminal fuse but if the cable between the battery terminal and the midi fuse box or Lynx but if the cable is away from earth cables and body work you can use the midi/lynx as the primary fuse just try to ensure this is as close to the battery as possible.
Fuses prevent fires, make sure they’re used and use the correct sized fuse, make sure the fuse has a lower rating than the cable it looks after, if you’re using 10mm2 cable that’s rated for 70 amps I personally would not go above a 60amp fuse, also remember with 12v dc systems you need to account for voltage drop, you can use this calculator.
When trying to size your fuses don’t go above 1.5x the amp rating of the appliance you’re using, sometimes on start up there is a higher demand for power for a split second so if the appliance is rated for 5 amps it may well surge to 6 or 7 before settling down to 5, make sure your cable and fuses allow for this.
If you don’t have the appliances amp rating you can calculate this by dividing the watts by the voltage e.g 70w/12v = 5amps this can be applied to any voltage.
I hope you’ve found this helpful, if you ever need any help with your electrical systems you can reach me on Instagram @aboutavan
Below you can find links to some of the products I’ve mentioned, if you buy the items I will receive a small commission.
About A Van 
Nice Blog. Thanks for giving this information.
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