The general rule in car wiring is that the thicker the wire is, the better. However, thick wire is expensive, and financially it's worth learning how thick a wire you really need to bring power to all the devices you add to your car.
The width of wires is usually given in a unit called gauge. As with shotguns, a smaller number represents a bigger diameter. Continuing our electricity-as-water analogy, the wider your wire is, the wider a "pipe" the electricity has to go through. Thus, if you are trying to carry a lot of power to your trunk to power a small studio of TV screens, computers, and audio amplifiers, you will want to use a thick wire. But how thick?
Using a wire that is too thin can be a fire hazard: too much amperage will overheat the wire, potentially melting the insulation and lighting the carpet on fire. However, using a wire that is too thick can be a financial hazard: you will spend hundreds of dollars on wires alone and have no money left for gadgets.
Wire comes in several types, including solid core, which is just thick, bendable copper, and stranded, which consists of many smaller wires braided together. For automotive applications, stranded wire is more desirable than solid because it's more flexible, especially at colder temperatures.
The maximum amps a wire can carry safely depends on the gauge and length of the wire. You probably won't be running wire longer than about 6 meters (20 feet) in a vehicle, but it's good to know that the longer you run the wire, the thicker it should be, because some of the electricity gets lost along the way as heat.
Wire gauge differs in the U.S. and Europe. In the U.S. you will see the term AWG, for American Wire Gauge, which I will be using here. As mentioned before, gauge is a bit odd in that the larger the wire is, the lower its number is. (Europe uses a more sensible metric-system-based wire gauge scheme. I won't cover it here, but I will give you the metric equivalents of the AWG numbers so you can visualize how thick the wires are.)
If you are comfortable inside a computer, you are probably already familiar with several gauges of wire. The wire used to connect the reset/power buttons and front panel LEDs to the motherboard in most computers is between 24 and 22 gauge (about 0.5 mm in diameter). The wires that come out of a power supply to the ATX power connector on a motherboard are 18 gauge (about 1 mm in diameter), as are the yellow/black/red wires that go to peripherals.
To carry 12V around a vehicle, you will normally use 18-gauge and larger (i.e., smaller number) wires. The red or yellow 12V power cord in a dashboard going to the back of the radio might be 18 or 14 gauge (1–1.5 mm in diameter).
Wires get hot, and when you bundle them in groups, they get hotter. Thus, while 18-gauge wires are rated to carry 16A (according to the AWG Handbook of Electronic Tables and Formulas), this rating decreases tremendously when the wires are bundled and wrapped and run together in a plastic sheath under the carpeting.
There are many tables online (Google "wire gauge table"), most with somewhat conflicting data. Generally, you will use the gauges 12 (20 mm), 10 (25 mm), 8 (33 mm), and 4 (about 50 mm, or 0.5 cm). If you are powering a single device under 5A, 12-or 10-gauge wire can handle it. If you are going to power a full-sized computer, or several devices, use an 8-or 4-gauge wire. These are thick, heavy-duty cables that look like they should be powering a big motor. In fact, 4-gauge wires can be used to bring power to a winch, so they should certainly handle your in-car PC. 2-gauge wires (64 mm in diameter) are overkill, and if you go any thicker than 2 gauge, you're probably using the same size wire as the battery to the engine. If you need that many amps, you might consider adding another battery in the trunk [Hack #10] .
You should use the same gauge for the ground wire as you use for the 12V wire you run to your device, as the current path is only as thick as your thinnest link. Also, run the ground and 12V wires straight to the battery, with a fuse near the battery between the 12V wire and the positive battery terminal. Try not to tap off other connections if possible, unless you're powering a low-power device (not a computer or screen). This includes the chassis (car body)—even though it is connected to the negative battery terminal, this can add noise to your audio system or device.
A quick way to determine your power needs is to double the amperage you need to power, because when devices first turn on, they draw a lot more current. This "spike" in current needs to be accommodated. Thus, if you know your computer will run at 200 watts (200W / 12V = 16A), you should double that to 32A and add that to the amperage required by any other devices you are running (amplifiers, subwoofers, etc.) before you decide how thick a power line to run and what fuse to install.
8-gauge wire (about 33 mm in diameter) can easily carry 50A (600W) to the trunk, and 4-gauge wire (about 0.5 cm in diameter) can carry over 100A (1200W) to the trunk. A 2-gauge wire (64 mm) can carry upwards of 150A (1800W).
To tap directly into the battery, you need an additional wire tap for the battery terminals. Whether your battery has posts or screw-down connectors, it's as simple as purchasing an inexpensive extendable connector (Figure 1-9) that enables you to stack on a few connectors. A small power distribution block mounted next to the battery is even better if you have more than a few additional power lines to run. A power distribution block is a hub that receives power from the battery and splits it neatly into several fuse-protected terminals (places to connect a wire). There are many "show-quality" devices that are translucent and have big, pretty fuses. Your budget and appearance concerns will dictate how elaborate a power block you purchase. You can use one large fuse between the power and this block, and it can be used to turn off all power in an emergency or just act as a backup fuse for your system. Make sure the distribution block has a protective cover to prevent any short circuits.
If most of your devices are going to be in the rear of your vehicle, Figure 1-10 shows an excellent way to provide ample power for all your devices: running one very thick wire to a power distribution block in the trunk.