These are the recommeded minimum fuse ratings for a the corresponding wire size. Using a smaller fuse than what's recommended here will be perfectly safe. Using a larger fuse than recommended could cuase an unsafe condition, and should NOT be attempted!

Wire:

As you already know, wire comes in many different styles and sizes. I will touch on a few design parameters that you must consider when choosing wire. The most important consideration is the amount of current that will be carried by the wire. The wire's size is indicated by gauge. The most common wire sizes used range between 4awg and 22awg. The larger the awg (American Wire Gauge) number, the smaller the wire size.

Resistance:

Now you need to realize that all wire has resistance. This is the reason that wire has current limitations. If you remember the formulas from P=I^2*R. The power dissipated in wire will be in the form of heat.

For Those Who Refuse to Fuse:

Now let's see what will happen if excess current is passed through a small conductor. We will assume that some imaginary piece of wire (we don't want to destroy a real piece of wire) has .01 ohms of resistance (e.g. a 15 foot long piece of 8 gauge wire) and that wire is connected directly to the positive terminal of the battery (without a fuse... that should scare you). Now let's say that the other end of the wire is allowed to touch to the chassis of the vehicle (which, in most vehicles, is connected to the negative terminal of the battery). The two battery terminals are basically shorted together by the wire (through the chassis). In this situation, a very large amount of current will flow through the piece of wire.

If we wanted to calculate the current flow through the wire, we would use the Ohm's law formula I=E/R. If we use the ideal automotive battery, which is rated at 12 volts, and divide it by the resistance of the wire which is approximately .01 ohms, we get a current of 1200 amps.

I = E/R
I = 12/0.01
I = 1200 amps
Then plug the current into the formula P=I^2*R. We get:

P = I²*R
P = (1200*1200)*0.01
P = 14,400 Watts

This shows that the wire would dissipate 14,400 watts of heat which would melt the wire's insulation and more than likely ignite everything that comes in contact with the wire (fuel lines, other wires, carpet, plastic, insulation). In comparison, the largest burner on your electric stove will not put out that much heat on high!

Note:

We could have simply used the formula P = E²/R but many of you are new to this stuff so I went the 'scenic route'. To prove that it comes out to the same value:

P = E²/R
P = 12²/.01
P = 14,400 Watts

Safety:

Any time that a tap is made off of a power source (battery, fuse block, distribution block...), you MUST put a fuse inline as close to the source as possible. Another thing to keep in mind is that you must insert a fuse inline anytime that the wire size is reduced, such as a tap off of the main power wire for an amplifier, CB, fog light... The fuse must be rated to open (blow) well before the wire starts to overheat. A secondary but very important consideration is environment. Is the temperature going to be extreme, hot or cold? Is there anything like oil, grease or solvents that will come in contact with the wire's insulation? All of these things have to be considered to keep you, your vehicle, and anyone else around you safe.