Prevention of wire conductor overheating and resulting burning of the wire insulation is the primary reason to install a fuse or circuit breaker. In some cases they are also installed to protect electrical or electronic equipment from damage. Fire results when too much amperage travels through a wire.
Amperage is electron flow through a conductor.
If too much amperage flows through a wire, enough heat can be generated to melt and burn the wire insulation or surrounding materials. Calculate the maximum sustained amperage of the circuit. Measure the length of the circuit from the power source to the load and back. Decide whether the circuit runs in an engine space or non-engine space. Starting in the column which has the right voltage and voltage drop shown at the top, run down the list of numbers until arriving at a value which is greater than the calculated Famps.