MY ANSWER-- Different devices require different amounts of current, so different fuses are necessary e.g. the motor requires 40amps whereas the headlights require 4.5amps, if there was only one fuse, then either the headlights’ circuit will be damaged or the fuse would blow as soon as the motor is switched on.
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I believe you are on the correct track. But, remember, the fuses prevent over-current on the circuit it is protecting. In fact, the actual rating that determines the fuse is the wire size feeding the circuit. For example, the heater motor is on a single circuit (in my car) it draws 20 amps. the wire used for that is 12 Gauge, which works fine delivering 20 amps of current. So, you must use a 20 amp fuse. If the motor were to fail due to a short or because of a frozen bearing, it would sudden begin drawing much more current from the battery bus- and the fuse will then blow, cutting off all current. If I were foolish and installed a 30 amp fuse to keep the motor going, the motor then would draw more current than the wire could stand and I would probably have a car fire. Imagine how heavy our cars would be if the mfg used super duty cables to feed every thing- even then, a failed device would simply draw maximum current and still set the car on fire. So, thinner wires, are protected by smaller rated fuses while thicker wires, are protected by larger rated fuses. So, the correct answer is, the fuses are rated to match the rating of the wires feeding the circuits which are rated to meet the minimum electrical demand of the individual circuit..
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Makes sense
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Good Job