But you can ALWAYS go overkill on the wire, you can ALWAYS use larger than what you need or rated. This allows you to correctly fuse and make sure that your under the max amps of the wire. Always choose a wire rated at or ABOVE what your accessory requires. That chart and most companies list the max amps at the continuous draw with the wire by itself cooling in the open air. Wires get heated under normal use by the electricity running through it, but with too much amperage the heat melts insulation, this causes a short and hopefully not a fire. So when you have a bunch of wires tight together or in a loom you may wish to up the gauge just for that reason alone. Then you add the heat from under your hood and you are cutting it kinda close with the point of failure of the wire. Take these into consideration when choosing wire size.
Always fuse the wire, not the accessory !
! This means to make sure your fuse is less amps than what the max amps of your wire. Yes, your accessory may require a 20a fuse, but if your wire will catch fire at 18a you need to fuse it before that point. The fuse needs to be your weak point of failure at all times.
Choosing your fuse
Now that you know your amp draw and the correct gauge wire you will need to fuse it correctly. Luckily, that is the easy part. You just make sure the fuse will pop before the wires failure point. You also want the fuse as close to the battery as possible so there is as little unprotected wire as possible. You do not want the fuse far away from the battery because you do not want the chance of a short before the fuse, defeating the whole reason a fuse is there. Again, your fuse should ALWAYS be your weak point.
Thatís it, almost.
You will need to figure the performance needs for your accessory. A common automotive fuse is set to fail at a set amp, this works fine, unless your item requires something different. If your accessory is a high amp item, say above 30a, a common automotive fuse may not be high enough. Or if your accessory has a high amp draw only at start up you may require a slow acting fuse that still protect the wire at its continuous rating but allow the high start up amp. An example would be an HID light or a compressor that has a peak draw when it first starts up.
Or what if you donít want to keep buying fuses or have an item above a common rating? In that case you would run a circuit breaker. These can be automatically resetting types (resets when the power is turned off) or manual reset (throw a lever or push a button) and can vary in size from a 2.5a to 300a and above.
I myself like to run manual reset circuit breakers because I want to know WHY my circuit breaker tripped and solve that problem before I reset it.
Now as mentioned before, what the type and brand of fuse will dictate when it blows. The amperage at which fuses actually blow, and circuit breakers actually trip, are sometimes considerably higher than their nominal ratings. SEA, Maxi, ATO and AGC fuses, and most circuit breakers, blow or trip at about 130% of their rating. ANL fuses blow from 140% to as high as 266% of their rating.
You NEED to take this into account when fusing a high amp system, but luckily, the information is easy to find from the company website for the manufacturer of the fusing system.
You will also need to choose a holder or fuse block. Fuse holders work great for a single item or when the total amperage of all your individual items will exceed the capacity of the fuse block. They are also very easy to waterproof or come waterproof out of the box.
Here are a pair, now they are rated the same amperage but you notice the different size. one is an ATO-ATC fuse and the little one is an ATM. Your JK uses ATM. Its your choice as to what one you want to run, but the smaller one will make your jeep fused alike.
Choosing your switching
You got your accessory, you got the correct wire, you got the correct fuse, so how are you going to turn it all on and off?
Just a simple switch is easy enough. Make sure the switch can handle the max amp of the accessory and you are good to go.
Even a not so simple switch is easy, once again you just make sure the switch can handle the amps, follow the wire diagram on the back and you are ready to rock.
You can switch either the positive or the ground. Most of the time if not all you switch the positive because it requires less wiring. The positive wire goes to the switch, then from the switch to the accessory. The ground from the accessory go goes right to the frame or body right next to it. Switching the ground requires a wire from the battery to the accessory, and then from the accessory all the way back to the switch and THEN to a ground. But either way works depending on what your requirements are because there are at times, benefits to a switched ground are like when you need an item to turn on if the switch fails, for example a cooling fan.
What limits us in running "just" a switch is the same problem that automotive manufacturers figured out years ago.
When you have 5 or 6 high amp items that need to be turned on/off where do you run all that thick wire? What if you have a accessory with a amp draw higher than your switch is rated? how do you use it?
You run a relay.
A relay is, in its basic terms, a electromagnetic switch that is turned on and off by low power. You apply power to the relay, the electromagnet in the relay causes the contacts to complete a circuit and the current flows to your accessory. This allows you to turn on a high amp item using a very small or low amp switch using small gauge wire ran inside your jeep.
Could you imagine running your starter wire all the way to a huge switch on your steering column and then all the way back? Along with controls for wipers, blinkers and lights? You would have a steering column that would be huge. Thatís why car makers use relays.
The great thing about running relays are:
Safer wire routing as only low amp wire is ran though the firewall into the cab.
Safer wiring as there is less wire required for the entire run of the circuit.
Smaller gauge wire required for the accessory as the run of the circuit is shorter.
Can handle higher amps than just a simple switch.
Can be switched in multiple ways (more on this in a bit).
The bad things are:
Can be confusing
Not as simple
Although there is nothing wrong with just running a fused wire through a switch to your accessory, I prefer to have mine run with a relay to keep the wire diameter in the passenger compartment to a minimum. I also like to run switches that are smaller but still over rated for the requirements of the relay.
Another great thing about a relay is it does not care what end of a circuit it completes, positive or negative. Power is applied and it electromagnetically closes, what goes through its contacts does not matter. This can be very useful for turning one accessory on with multiple ways. Examples of this include:
Additional reverse lights that you want turned on by the factory reverse lights and/or a switch.
Cooling fans you want controlled by a thermostatic switch and/or a dash switch.
You can not easily run multiple positive power sources to the same item with multiple switches. What will happen is either it will double the volts and fry something or it will flow back through a switch and fry something. You can do it with some more complex electrical that never works out for me or you can do it quite simply and switch the ground with relays. You run your normal fused positive wire directly to the accessory and you use multiple relays or switches (depending on your choice of switch) to control when the accessory is grounded. When its grounded it completes the circuit and turns on! Use as many relays as you need switches.
We will cover wiring a relay a little bit later, its not hard, trust me.