Fuses protect the wires in your camper’s electrical system from overheating, melting, and then starting a fire. They are not intended to protect the devices that you install (devices should have internal protection).
Fuses must have a lower amperage rating than the wires they are protecting, however, their rating must still be higher than the devices you’re trying to power (otherwise the fuse may trip prematurely).
Before trying to calculate fuse sizes yourself
More often than not, component manufacturers list the required fuse size, wire size, and maximum wire length in the instruction manual.
Above is taken from the instruction manual of a DC-DC charger. Fuse and cable sizes are clearly listed. In this case, follow their guidelines and you will have no doubts.
How to calulate fuse size manually
If there are no guidelines to follow from the manufacturer, you can use this simple formula to calculate the required fuse size.
Total Circuit Amperage x 1.25 = Approximate Fuse Size
Figure out the maximum amperage that will flow through the circuit you’re trying to protect. This is the amperage of all components on that circuit, combined.
For example, if there are 2 waterpumps sharing one circuit, and each water pump draws 4 amps, the Total Circuit Amperagel is 8 amps.
Once you have this number, you’ll need to round to the nearest available fuse size. If you round down, make sure the fuse is still large enough to power the device. If you round up, be sure the fuse rating is still lower than the cable’s rating.
Here is an example in practice
In this example we have a component that has a nominal amperage* or 13A.
If we multiply 13 * 1.25 we get 16.25, the approximate fuse size for this system.
Since they don’t manufacture 16.25A fuses we’ll have to round to the nearest available fuse size.
In this case, a 15A fuse is still large enough to power our 13A component.
Where to place fuses in your electrical system
For a typical camper van electrical system, fuses should be installed on the positive cable closest to the source. Each individual circuit requires protection.
The following diagram shows a typical circuit design that you will find in a camper electrical system.
In this circuit, the main positive cable is fused closest to the battery. If there is an over current after the fuse, the fuse will blow, disconnecting all of the wiring and components.
The section of cable between the power source and the fuse will still have power, for this reason, that section should be as short as possible.