Calculating the Fuse Size

A common car audio question I get is, "What size should my fuse be?". This is almost always in regards to the main power fuse. The most important component you'll install in a car audio system. This is the fuse that protects the system and your vehicle and is located within inches of the battery. Choosing a fuse size that is too large will not provide adequate protection and choosing one that is too small will cause a lot of blown fuses. Here's a few guidelines you need to know when locating and sizing the main power fuse for your car audio system.

1. Location - The only function of the main power fuse is to protect the main power wire. This in turn protects the vehicle. Without the fuse a short in the main power wire could cause excessive current to ignite the wire and eventually the vehicle (it's happened). This is why you want the distance between the battery clamp and the main power fuse as short as possible. Any length of cable between the fuse holder and the battery clamp will be unprotected. The general sound off rule is eighteen inches but the shorter the better.
2. Current Handling - The fuse needs to handle the power that will flow through the cable under maximum operating conditions (full power) and no more (ideally). To determine the maximum amount of current that will be needed you will need to know the current draw of each part of the system that draws power from the main wire. For example, if you have two amplifiers that draw 30 amps and 50 amps respectively you'd want to use an 80 amp fuse.
3. Fuse Type - There are three main types of fuses used in car audio. The most common is the barrel (cylindrical) fuse which may be an AGC or AGU type. The second most common is the blade fuse which is the type most often found in your vehicle's fuse box. Large blade fuses are used in some fuse holders and may be referred to as a MAXI fuse. The least common type of fuse is the ANL or "wafer" fuse. These are used in the larger fuse holders when high current handling is required. Typically 100 amps and higher. The type you use will depend on the current capacity needed and your own preference. For draws of less than 100 amps expect to use a barrel fuse. For currents larger than that you'll probably be looking at the more expensive wafer fuse.
4. Fuse Size - Using the method in item two you'll be able to choose the size of the fuse you need. But what if you have a current draw that is greater than one value of fuse but less than another (i.e. 125 amp). In this case you have the option of either choosing a 100 amp fuse or a 150 amp fuse. I always go for the larger fuse to reduce the chance of accidental fuse blows. However you may choose to go with a smaller fuse for greater short circuit protection. Just keep in mind that if you keep blowing fuses you'll probably need to move up to the next size. But if you're blowing fuses and you've sized it correctly for your current draw you may have another problem. Time to break out the multi-meter and check for problems.