As simple as fusing is, it is all the more critical for safety and reliability to get right. There is just 2 aspects to consider.
- Sizing
- Placement
Sizing
Find out what is the absolute maximum current that your load will consume, add a 20-50% margin and choose from the available fuse values accordingly. The fuse value you should use may even be listed in the manual for your load. Avoid installing a fuse that is rated at more than 50% current for your load. Allowing too much current into a device can easily cause overheating, explosion, or even fire, since they are not designed to handle that much current internally. This is why it is crucial to size correctly.
Isn’t the fuse size affected by the cable size? Shouldn’t be the case: Fuses are sized to the load, and cables sized to the fuse.
The battery fuses are also sized for the loads, and not the batteries. Add up all the load currents in your system and include a 20% safety margin—this gives you the required main fuse rating. If multiple batteries are connected in parallel, each battery should be individually fused, and the main fuse rating should be evenly distributed across the parallel batteries.
The most common type of fuses and holders(affiliate links):
- Below 5A: 5×20 glass fuses, fuse holders
- 5-25A: Bladed fuses
There are a lot of different sizes of bladed fuses used by car manufactures. The most common is the standard size, just stick to that. There are really great fused distribution blocks for those, or you can install it using an inline fuse holder.
I don’t recommend using bladed fuses above 25A, because of the spade connector design. They can weaken overtime, loose contact pressure, and melt. - 30-100A: Midi fuses
The screw terminal type design is more reliable at higher currents, since the clamping force is provided by bolts.
Fuse blocks for these type of fuses are available but not that common. Usually they are used with single inline fuse holders. - 100-500A: MEGA fuses
These are the go-to type fuses for the main high current distribution directly from the battery.
Great fused distribution blocks are available for them, as well as seperate inline fuse holders. - Terminal fuses : These are a special kind of form factor designed specifically for installing on battery terminals. They are a great solution for fusing the batteries.
Placement
First, let’s clear up a common misconception that affects fuse placement. Fuses are not only there to protect the loads, but the cables as well. This is why it is important to place the fuse as close as possible to the power source, or distribution solution. This way the power cable runinng to the load is also protected. Anywhere where the conductor cross area is decreased, a properly sized fuse must be installed. Use the tree analogy. Anyhwere the tree branches into a smaller branch, a fuse is needed.
Thats it!
If you stick to these simple rules, your system will be safe and reliable. If anything shorts out, the fuse will simply blow without affecting other parts of the system, and it will prevent the cables from overheating, melting, or catching fire.