Wire size should be chosen based on the current draw of the device in question. The table on page 66 shows the American wire gauge (AWG) sizes recommended for various current load requirements. These sizes are valid for runs up to about 15 feet. For longer runs, it is best to upgrade to a larger (lower gauge number) wire. If in doubt, always go with the larger wire to ensure you have plenty of current capacity and to minimize voltage drop over the length of the wire.
Relays are used when you need to supply high current to accessories and want to switch the battery voltage remotely. This is often the case with high-wattage driving lights. Pick one that meets or exceeds expected current requirements.
Relays
Whenever high-power electrical devices are added, they need large wires to carry the high-current loads to the device. In such a case, it's best to minimize the length of wire run if possible. This is where a relay comes into play. A relay might be more correctly called a "relay switch" because a relay is a switch that is triggered by another switch. The relay can be placed in line between the battery and the device, connecting the two via heavy-gauge wire and minimizing the length of the wire run. The relay is triggered remotely by another switch using light-gauge wire between the remote switch and the relay.
Two good examples of the use of relays (or solenoids) are the solenoid on the starter motor and the solenoids used to actuate a winch motor. In both cases, the relay or solenoid is used to switch large current loads remotely. This setup allows you to engage the starter motor from the ignition switch or command the winch motor remotely without having to route the heavy power cables to the switch location.
Once you understand the basic function of a relay, they are fairly simple to use and wire when adding high-power accessories. As shown in the above diagram, there are two terminals connected to the relay coil: one to the control switch (85) and one to ground (86). The other two relay terminals are simply wired inline to switch the current flow to your device. When 12 volts (battery and ground) is applied to the relay coil, an electromagnetic switch connects the two high-power contacts and allows current to flow to the device under control.
This figure illustrates a typical wiring diagram for a common Bosch-style relay. The switched 12V supply at terminal #85 energizes the relay coil and connects terminal #30 to terminal #87.
A fuse should always be used on a positive supply wire as close to the power source as possible. Fuse rating is chosen based on the size of the "downstream" supply wire. If a problem occurs, the fuse should blow and interrupt power as far up the power supply line as possible.
Fusing
Whenever an electrical device is added to your vehicle it's good practice to add a protective fuse to the positive power line. Many accessories often come with the proper inline fuse included. If this is the case, you can simply wire the device to your battery or other source of 12V power. Other electrical components, such as lighting, may require you to add your own fusing.
Fuses are chosen such that their rating exceeds the expected current draw of the device (by at least 20 to 30 percent), but with a rating that is below the maximum current rating for the wire size you are using. If the manufacturer does not provide the current draw for the device, we can calculate it from the specified power consumption using this formula:
Current (amps) = Power (watts) 12 volts
For instance, a 100-watt light would draw 8.3 amps, according to this formula. If we add two lights on one circuit, our total is 16.6 amps. At minimum, we should choose a wire size to support a 20-amp current flow and fuse our power line with a 20-amp fuse.