If you own a modern Jeep Wrangler – YJ, TJ or JK, chances are at some point you’re going to want to add a light bar, bumper lights, winch, CB radio, aftermarket stereo or some other accessory that will need additional fused power. This DIY add-on fuse box example shows how you can run two separate sets of fused switches (or up to 6 fused circuits total) for an LED light bar on a Jeep Wrangler Unlimited JK, but really this example will work for any Jeep or 4×4. This DIY waterproof auxiliary fuse box can be set up for up to six fused accessories and can be built for less than $50 in parts and in about one hour.
The biggest obstacle I had with the JK Wrangler was there’s very limited space under the hood for additional accessory wiring. The only sizable gap I could find was directly on top of the battery. I measured the space and searched for a waterproof box of similar size and came across a BUD Industries waterproof economy box which measured roughly 10.5″ long by 4″ wide by almost 3″ tall. This is a nearly perfect fit for on top of the Jeep Wrangler JK battery, but also a nice size to mount to a firewall or fender for other Jeep Wrangler models or 4×4 applications. Best part is it costs less than $15 and is of very nice quality. I chose to spray paint the box black so it would blend in better under the hood, but it comes in the grey color shown in the photo.
Going off the internal dimensions of the waterproof box which are provided via detailed CAD drawing at Amazon I selected a six blade fuse panel for all of my positive wires that would fit well in the box and still leave room for relays, negative grounds and possible future expansion. I chose this six fuse panel with LED indicator lights to show if a fuse is blown/bad. The other nice part of this terminal is, like the waterproof box, it’ll cost you less than $15. The LEDs provide an easy way to see if you’ve got dead fuses without actually having to go through the box and pull each one to look. There are cheaper panels without LED indicators or a cover if you so choose.
This fuse box was capable of 100 total amps or 30 amps per circuit. Obviously at 30 amps per circuit multiplied by six circuits you cannot wire each one to maximum or you’d exceed the 100 amp maximum rating. For the lights I was wiring I only needed one 15 amp and one 10 amp fuse – leaving 4 positions open for future electrical add-ons. I wired 5 of these LED lights at 51 watts each on two different switches. To determine amps you need to divide the watts by the voltage. Each light was 51 watts, so three lights on one circuit equals 153 watts divided by 12 equals 12.75 amps. Two lights on a separate circuit equals 102 watts divided by 12 equals 8.5 amps. To wire the panel you simply run one lead (I used 14 gauge red wire) to the primary terminal (shown in photo below, post with nut on center right portion of the block) direct from the positive battery terminal. Plug the appropriate standard blade fuse in, then run off the positive pose back to your lights/winch/CB or whatever you’re powering.
For grounding I mounted a simple four terminal bus (also called a minibus or busbar) and ran a black 14 gauge wire from one terminal direct to the the negative battery post. That left three posts open for terminating negative grounds for any application you might need, including my lights. Once again, this is a less than $15 part from Amazon.
I mounted all of the components – fuse block and bus bar – in the waterproof box. I used small hex head screws with nuts and rubber washers to secure them in the box. Next I drilled 1/2″ holes in the box and fitted it with waterproof plastic cable glands to keep it watertight where the incoming positive and negative wires enter and exit the box. I noticed over time the cable glands have a tendency to loosen up and need re-tightening. It might be a good idea to hit them with a little silicone, caulk or even a tiny bit of blue loctite to keep them snug.
In the picture below you can see the negative wire leaving the waterproof box through the bottom gland and linking directly to the negative battery post. I mounted the minibus on the same side as the negative post to keep a nice short run. Likewise the fuse block terminal is mounted on the opposite (top) side and you can see the 14 gauge wire running directly to the positive battery terminal through the top gland. The three middle glands serve is the inputs for the auxiliary lights served by two different switches and a pair of relays.