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Automotive Wiring Tips – Using Relays
There are a few things to consider.  Number one, make sure you use wire that is rated for the amperage that the accessories is going to pull. Automotive Wiring Tips - Using Relays It is always better to have wire that is OVER rated rather than wire that is not rated high enough.  If wire is used that is not rated to handle the current that your accessory will pull, the result could be overheated wires that could melt the insulation or the electrical plugs found throughout your vehicle (see image below), causing a short or worse yet it could result in a fire.  If you know how much current your accessory will draw you can determine what gauge wire is appropriate for your application.Automotive Wiring Tips - Using Relays

Personally I like to use wire that far exceeds the current draw of my accessory.   It’s overkill but in a few applications I’ve used heavy gauge stranded industrial wire with water and chemical resistant insulation. That way there is no question as to whether the wire is rated high enough or not.  If this approach is taken, it is very wise to place a fuse at the battery end as close to the battery as possible.  Most wire in a vehicle, if shorted out, will burn up before the battery overheats and possibly explodes.  If wire that is over-rated for vehicle use is used and a short occurs, a short will most likely result in damage to the vehicle of some sort unless a fuse is put in line as close to the battery as possible.  With the fuse there, in the case of a dead short, the fuse will burn out first before any damage could occur.Automotive Wiring Tips - Using Relays

The method I use for wiring the lights and other external accessories, for the most part, follows the diagram pictured above. As in the diagram a wire is run from a 12 volt power source to the switch in the cab and out to the relay placing a fuse at the source of the power. (Follow the relay’s wiring schematic when connecting the wires to the relay) One of the relays terminals goes to ground. Then run a heavy gauge wire from the battery to the relay placing a 30 Amp fuse in line very close to the battery. Do not connect the power to the battery until all wiring is done. Then I run a single heavy gauge wire out to the lights or other accessory. If installing lights, split it into two leads at the lights. If you do this be sure the wire is rated to handle BOTH lights since it will carry the current of both. The diagram shows two leads coming from the relay. Automotive Wiring Tips - Using Relays Then I run the second wire of the lights or other accessory to a good ground on the frame of the vehicle. If the wires will not be soldered together and crimped connectors will be used it’s a good idea to put a dielectric paste on the connectors where they come in contact. This will prevent corrosion as time passes ensuring a good connection. Then double-checked all wiring before plugging in the power.