Actually, just to step in here. The lowest possible fuse rating is not ALWAYS the best bet.
If you are close to it's limits..eg.. 25 amp fuse and you are pulling 23 amps through the fuse then it starts acting like a resistor and can actually reduce performance.
The main idea behind a fuse fir lights is purely short circuit protection. A big ole 100A fuse will blew just milliseconds behind a 25A fuse in the event of a short circuit. Since there is no risk of overloading the circuit here, using the 25A fuse would not have had any issues what so ever.
The LOWEST possible fuse is always best. A fuse will not act like a resistor unless you have really stuffed something up. The connections may bring a little resistance, which is why there are stories about melting sockets etc, but the fuse itself is completely different. It is a short piece of wire, very little resistance, very little voltage drop, it will never affect performance (in this case anyway) unless it blows. If you have a 50A fuse in, and you are only ever needing 5A from the circuit, you have a 45A leeway. That is 621watts of power. Enough to easily start a fire. You can be sitting there driving along, car catches on fire, burns everything to the ground and still not draw enough current to blow the fuse. At that sort of current, the wiring can become a fuse, and just start melting. That is why there are always (should be anyway) warnings that say, only replace fuses with same type and rating to prevent fire, something along those lines.
I dont know who told you that mate, but forget it and forget it quickly. A fuse is there simply for safety. Thinking that you are being clever by putting a larger fuse in is asking for trouble.
When choosing a fuse, work out what the normal current will be, and add 25%. This is the fuse that you should use that will balance between safety - not allowing too much current - and reliability - not having the fuse blow every time the circuit is switched on.
If you are running 2x 100watt spotlights, you are using 200watts. At 13.8volts, the typical operating voltage, you are using 14.5amps. If you car was turned off, and the voltage was only 12v, you would be drawing 16.6 amps. Add 25% onto that, and you get about 20A. So start with the 20A and you should be fine. If it keeps blowing, then, and only if you are confident that there aren't any mistakes anywhere, go up to the next value, 25A.
You are absolutely right about the 100A fuse blowing in the same time as a 25A fuse in a short circuit, no problem there EXCEPT not all electrical faults are short circuits. If you have a low resistance to earth through a coil or something, say wasnt a nice short, it was 0.26ohms. Now at 13.8V, thats going to pass about 50A. Thats still not enough to cause your monster fuse to blow. Thats 600watts of heating power, possibly a few hundred degrees celcius, enough to cook everything around it, maybe fuel lines, material, anything flammable, and your fuse wont even worry about it. The 25A would have blown and saved you. The overrated fuse has done nothing. Thats what all the warnings are about.
