What Size Wire to Use?

One of the mods that all of us seem to get around to doing is wiring up some kind of electrical device to our Jeep.  Off-road lights, power inverters, and stereos are just a few of the things that we often times add to our vehicle.  One question that you should ask before starting on your electrical project is “What size wire should I use?”

I got into a discussion about this vary question with some other Jeepers.  Rather than quote details from my own personal experience, I turned to one of my reference books that I used in years gone by for electronic construction projects.  The data below is pulled from one of the reference tables in this handbook I had sitting on the shelf.

The book says we can draw the following loads through the wire if it is a single wire with open air cooling.  As long as the ambient air temp is 135 degrees F. or less, we can use:

14 gauge – 32 amps
12 gauge – 41 amps
10 gauge – 55 amps

But, usually we bundle our wires together, and if you are like me and try to slip them into that flexible plastic wire sheathing that keeps them from rubbing the insulation and causing a short, we have to de-rate the current rating.  So, we end up with a continuous duty rating of:

14 gauge – 17 amps
12 gauge – 23 amps
10 gauge – 33 amps

It is obvious that by bundling the wires together, we reduce the current carrying capacity because the wire can not dissipate the heat and stay cool like they could when ran as a single wire.

So….we now take these same wires and run them through the engine compartment of our beloved Jeep. Granted, a couple of feet, maybe even half of the wire run, is probably down around the front of the Jeep where it is cooler, or where ever it is you have those big old off-road lights mounted. But some of that wiring is under the hood, making a connection to your aux fuse panel or the battery itself.

Anyone want to venture a guess how much the current capacity drops when the engine compartment temperature is applied to the wires? (the table didn’t cover that) I can guarantee you that the capacity will drop even further.  The above ratings are for a maximum wire temperature of 212 degrees F.  The ambient temp under the hood might darn near get that hot, and for sure it is more than 135 degrees.

Are you still with me? Lets continue….lets plug in some real world values and see what we come up with.

Two 150 watt lights will draw 25 amps. 300/12=25

If we ignore the engine temp issue (and we really can not) and just run our 12 gauge wire with a bundle of other wires to the front bumper, we are already exceeding the safe operating range of the 12 gauge wire, which has been de-rated to 23 amps.  It is less considering the added temp of the engine compartment.

Sorry folks….anyone that says 10 gauge wire is overkill doesn’t understand all of the facts, IMO. You might have a different opinion, but I for one am not going to risk burning my Jeep to the ground because of an extra $5 of wire. It just isn’t worth it.

We could go on and talk about resistive losses in the various wire gauges….the fact that 12 gauge wire has a 60% higher resistive value per foot than does 10 gauge, and that 14 gauge wire is about 250% higher….but there is no need to. It will only support what was just shown above.

Be safe….use 12 gauge for the smaller current runs, 10 gauge for those current eating lights, and 6 or 8 gauge if you are making a heavy current connection at the rear of your vehicle (such as a Quick Air2 or similar).  For the really big loads, like a Warn 9000 lb winch, use #2 or larger (Warn ships #2 with their xd9000i).