From my experience (25+ years in the amp repair business), it seems that most people don't fully understand what a fuse does and why it's necessary. This page will help you to understand fuses and will make you fully realize the consequences of using a fuse that's rated higher than what is recommended by the manufacturer (for electrical devices) or for the current carrying capacity of the wire in the circuit. Virtually every type fuse and fuse holder used in car audio will be shown below and most will have descriptions telling where it's good to use and when a different fuse may be a better choice.
Function:
A fuse is generally inserted into an electrical circuit for 1 of 2 reasons, either to protect the power source which includes the wire that connects the power supply to the electrical device, or to protect the electronic equipment. The electronic equipment manufacturers specify a fuse rated to open the electrical circuit before damage can be done to the device or open the circuit if the electronic device fails in some way (electronic devices may pull excessive current when they fail). If a fuse larger than the specified fuse is used, a small mistake when installing the equipment may cause catastrophic failure of the equipment. WHEN, not if, WHEN you're thinking of replacing a blown fuse with a higher rated fuse ask yourself if you know more than the engineer who designed the equipment.
It basically comes down to this... If you use the correct fuse and there is a problem that's causing excessive current flow, you're sacrificing a $0.50 fuse to protect an expensive piece of electrical equipment and the vehicle it's installed in. If you use a fuse that's rated for too much current, you're sacrificing an expensive piece of equipment or the vehicle it's installed in to protect a $0.50 fuse. Don't get in a hurry when installing electronic equipment. Take the time to go get the right fuse. 50 cents for a fuse is better than $60-$100 labor plus the cost of the replacement parts to have the equipment repaired (if the damage isn't too extensive) or in the worst case scenario, the cost of the stereo equipment AND the vehicle.
There are two important specifications for fuses. The first is simply the current rating. Fuses are rated for a given number of amps (amperes). Fuses are not rated in watts. If a fuse has 30 printed on it, it's a 30 amp fuse, not a 30 watt fuse. The second specification is the voltage. This isn't really important if you're simply replacing a fuse that has blown with the same type. Fuses typically used in automobiles are generally rated well below 100 volts. If you were to attempt to use a low voltage fuse in a circuit with mains voltage (110v+), it would present a real danger. When a fuse blows, it opens a gap large enough so that an arc (which burns at a very high temperature) can't form between the terminals inside the fuse. A low voltage fuse like an AGU fuse may not be able to open a large enough gap to quench the arc. That would allow current to continue flowing (current would jump the gap in the fusible element in the form of an arc). This could result in an electrical fire. High voltage fuses may also be filled with silica (sand) to fill the gap and quench the arc. Low voltage fuses are not. The following is one example where a low voltage fuse very closely matches a high voltage fuse in size and current rating but is absolutely not a good substitute.
In this first diagram, you have a simple system. You simply need to safely get the B+ (12v) line from the battery to the amplifier. One of the most important things to understand here is that wire segment A is not protected in any way. If it were to be shorted to ground, either the wire or the battery would be damaged and in either case there would likely be an electrical fire. This is why the wire must be short. 12-18" is the maximum recommended. The fuse protects the remaining length of wire. This generally extends from the front of the vehicle to the back of the vehicle. The fuse must be rated to protect the wire that's being used. The wire 'gauge' is generally the deciding factor when it comes to selecting a fuse. More information on the selection of the proper fuse will be covered on the 'wire' page of the site. If the amp has no fuse on-board, you will need an external fuse. If you have only one amp, the fuse at the battery can be used to protect the wire and the amplifier. The fuse would need to be whatever was recommended by the manufacturer for the amplifier and would also need to be rated at or less than the suggested maximum current for the wire that you're using.
In
the next diagram, things get a little more complicated. As you can see, wire
segment 'A' is used to deliver power to the fuse within 18" of the
battery. Wire 'A' is of a large enough gauge to power both amplifiers. 4 gauge
wire is commonly used as a main power wire so it will be used as an example
here. Fuse 'A' must be rated to protect wire 'B'. Any fuse rated at less than
~150 amps is sufficient to protect a 4g wire and the vehicle but I generally
recommend a 125 amp fuse with 4g wire. Fuse B protects wire segment C. Fuse C
protects wire segment D.
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