A fuse is an intentional weak point in a circuit. Fuses are designed to fail! When too much current flows through a circuit, the filament in the fuse melts. This stops the flow of electricity and protects the wire and equipment.
Fuses and breakers are essential in any electrical system. It will protect the circuit, wires and the components against over current and possibly fire.
There are two types of fuses used in the van build, spade fuses and ANL fuses.
Spade fuses are those colorful plastic fuses used in your car. Common Spade fuse sizes are: 2A 3A 5A 7.5A 10A 15A 20A 25A 30A 35A
These are higher current fuses used at your main electrical panel for the batteries and inverter.
ANL are fuses used for circuits of more than 20 amps. Select fuses that are above the 50% above max current of your circuit load, but below the amperage rating of your wiring.
DC Circuit Breakers
Circuit breakers are like resettable fuses. When there is too much current the circuit breaker “throws” and switches off. After fixing the problem that caused the overcurrent, the breaker can be turned back on. Breakers also allow you to easily disconnect a circuit from service.
I use circuit breakers in the electrical system in places I need to disconnect a portion of the circuit for maintenance. A good example is the solar panels. Solar panels should be disconnected when servicing the main electrical panel or batteries.
Fuses are more reliable than circuit breakers. I like fuses because they are simple and I have confidence they will work when needed. But fuses must be replaced when there is an overcurrent.
What size fuses/Circuit Breakers?
The fuse must protect the wire, and electrical components. I will use a 12 gauge wire for all 12 volt connections in the van. 12 AWG wire will carry a 15 Amp load 15 feet with only a 3% voltage drop. Use this calculator to determine your wire size – http://circuitwizard.bluesea.com/
So, the wire is safe for 15 Amperes or less. Next, I need to protect the individual components.
Take a look at your ‘Electrical Load List”. This is the list you created when sizing the electrical system. On that list you should have the normal operating amperage of each device. Take that number and multiply by 1.5.
For example SEAFLO 42-Series Water Pump is 7.5 Amps. 7.5 x 1.5 = 11.25 Amps. There are no readily available fuses at 11.25 amps, so round up. In this case the closest fuse is 15 amps.
The 12 volt fuse block can handle a total of 100 Amps
If you only have a wattage rating for the device divide by the system voltage. For example a 12 volt fan that is 9 watts.
Current = Watts / Volts Current = 9/12 = .75 amps.
If this fan was on its own circuit a 2 Amp fuse would be used.