Well, I just spent a half hour typing up a great addition to the 2nd article that included fusing. Since it had too many images, and my text was GONE when I hit the "back" key, I get to type it all over again.
Anyway...
It was brought to my attention that I never mentioned fusing in the first two articles, so I'm adding it now.
Fuses & fuse holders
Whenever you're doing any electrical wiring that involves a power source (basically everything in your car except speaker wires), you should always use a fuse as close to the source as possible. There are a lot of different fuse types out there, but the most common for automotive use are the blade type and the glass type.
Here's a picture of a blade fuse:
And a blade fuse holder:
These are the most common fuses used in an automotive environment, and are easy to find.
For some applications, you may prefer the glass style fuse:
Which uses a somewhat more streamlined fuse holder:
You can typically find both styles in current values up to 30 amps, although sometimes you can find a slightly larger value. When you need more than 30 amps, it's also likely that you need a rather large gauge wire, so using a special car-stereo type fuse is usually the best solution. These are just like the glass fuses, except that they are larger. You can find fuses in values up to 80 amps. These use a holder that looks like this:
For even higher power applications you have circuit breakers and wafer fuses. Wafer fuses are available up to 300 amps:
Wafer fuse holders:
No matter what style you use, it's important to mount it very close to the battery. Typically you want to keep your wire length between the battery and the fuse under 12", but it's also important to mount the fuse in a secure, safe spot. There is almost always a safe spot somewhere within 12" of the battery. It's important to keep the unfused section as short as possible, because any section of wire before the fuse is never protected.
When wiring to the battery, it's also important that the connection at the battery is good and solid. Your battery should be firmly bolted into place so that it doesn't move, and you should make sure that you have good connectors. I personally replace all of my side mount batteries with dual-terminal batteries. This allows me to keep using the factory side mounts, but gives me a set of top mounts to use exclusively for my additional wiring. There are dozens of different top mount terminals that you can buy at auto parts stores and car audio shops. If you prefer to keep just the side terminals, you can buy a new side mount terminal that holds the factory wire in place, and gives you a spot at the end for all of your additional wiring:
What size fuse?
One source of confusion for people is how to determine what size fuse they should use. The best way is to read the owners manual or specification sheet of whatever it is you're installing, and try to find the maximum current consumption number. If it's for a device like fog lights which will operate at or near their maximum consumption all the time, you want to use a fuse that's rated at approximately 5% or 10% above the maximum current rating. If it's something like a car stereo amp that tends to only reach its peak draw for very brief moments, you might want to try using a fuse that's rated slightly below the maximum number. If you find that you're blowing it, you may need to increase it, but NEVER more than 5 or 10% above the rated current draw. If your product is blowing fuses that are 10% above the rated draw, it's got a problem that you need to fix.
If your device comes with a fuse, or comes with a fuse recommendation, ALWAYS use that value. If the product is blowing the suggested fuse size, it's got a problem. My guideline above was just intended for projects that may not have come with a fuse or a fuse recommendation.
In cases where you're running multiple products off of one fuse, you should also have a distribution point, and from that, each product should also be fused. Having 1 big fuse up front won't provide adequate protection to each individual component after the distribution point. A good example of this is a car stereo installation where you have 2 amps, plus a signal processor of some sort. It's foolish to run 3 individually fused wires to the back of the car from the battery. It's easier to run 1 wire to the back, and then feed it into a fused distribution box. If you were to do this without fusing each individual component, you could have problems. In this case, having those 2 amps requires a big fat wire, and a big fuse. But... that signal processor probably only draws 5 or 10 amps, and will use a very small power wire from the distribution box. If you didn't fuse the processor at the distribution point, the potential exists for 100 amps of current to be able to flow through the little tiny wire if it were to short out. 100 amps is no different than a completely unfused system as far as that tiny wire is concerned. You'll burn it up, and possibly your car.
With the fused distribution panel, you can make sure that each individual component is properly fused, in addition to the fuse that's protecting the wiring that's bringing the current to the distribution box.
