A car's wiring has to distribute power from the battery to devices located all over the car. It also has to transmit data on a data bus, as well as a variety of digital and analog signals from switches and sensors.
This means that there are many different types of wires in your car. Some wires that transmit signals from switches or sensors carry almost no current. Those that provide power to large electric motors carry lots of current.
If too much current goes through a wire, it can overheat and melt. The amount of current that a wire can handle depends on its length, composition, size and how it is bundled. Let's take a quick look at how each of these properties affects the wire's current-carrying capacity:
- Length - Each type of wire has a certain amount of resistance per foot -- the longer the wire, the larger the resistance. If the resistance is too high, a lot of the power that flows down the wire will be wasted; the energy lost as heat builds up in the wire. Ultimately, heat build-up limits the current-carrying capacity of the wire, as the temperature must not get hot enough to melt the insulation.
- Composition - Automotive wire is usually composed of fine copper strands. Generally, the finer the strands, the lower the resistance and the more current the wire can carry. The type of copper used has an effect on the resistance of the wire, too.
- Wire gauge - The wire gauge, or size of the wire, also determines how much resistance the wire has. The larger the wire, the less resistance. The smaller the gauge, the larger the wire -- so a 16-gauge wire is bigger than a 24-gauge wire. Wire gauges go all the way down to zero, which is also called 1/0 (one aught). Even bigger than 1/0 is 00 (2/0, or two aught), and so on. The diameter of a 4/0 (four aught) wire is almost half an inch (1.27 cm).
- Bundling - The way a wire is bundled affects how well it can dissipate heat. If the wire is in a bundle with 50 other wires, it can carry a lot less current than if it were the only wire in the bundle.
(See also this Question of the Day for a good explanation of current, voltage and power.)
You can see how important it is to choose the correct wire size. The job is made even more difficult by the number of wires in a car, which continues to grow each year as new features are added to even the most basic automobiles.