Add Fuse Box - Check my Plan?

Rick2004LJ

TJ Enthusiast
Joined
Jun 11, 2018
Messages
159
Location
Texas
2004 LJ Trail Rig - cleaning up the accessory wiring and just want to ground truth my plan. I will admit I am not an electrician but have a basic understanding of automotive electrical wiring, fusing, use of relays etc.

Plan is to add 2 new small fuse boxes (with negative bus) - 1 always hot and 1 switched with relay. Current needs are just for CB & GMRS radios, USB outlet, and Sirius Radio, but will be adding some lights in the future. Winch is wired direct to battery. For
Firewall penetration I plan to make a new hole on passenger side firewall under the battery per spod instructions and using the 1" Daystar Firewall boot.

Power for both boxes will come directly off the battery with fused wire - like this example:
Jeep Fuse Box Relay 6.2.2021.JPG

Q1 - Plan is to run the negative bus wire from fuse boxes back to neg battery terminal, is that best way? Or should it go to the main ground screw on firewall or to a ground source inside cab?

New Fuse Box mounting location - I would like the new fuse boxes inside the cab for ease of wiring, short wire runs, etc.
Q2 - Is it OK to mount the fuse boxes inside cab instead of under the hood?

Assuming it is OK I was trying to decide where to mount them but space is a little tight behind dash/glove compartment - So I came up with this hairbrained scheme...

We never use the glove compartment so my first thought was to remove the compartment portion from the backside of the door - glove compartment door just becomes a hatch to access new & stock fuse boxes, wiring etc. Still not any convenient spots to mount new boxes. In messing around with the removed compartment I realized I could mount it directly to the frame around the glove compartment so it is stationary - only the door opens and compartment stays put.
Glove Compartment.jpg


Q3 - Any reason I could not use this compartment for mounting the new fuse boxes & relay? I could trim the back and sides a little to make access to stock fuse box and other wiring easier, and create a convenient easy to access location for the new wiring. Probably mount the new fuse boxes on the flat horizontal portion of the newly mounted compartment.

Fire away - any and all advice and ideas greatly appreciated!
 
I'm not sure about you, but my 98 TJ had 1 fused constant hot accessory wire and one fused ignition switched accessory wire tucked in the glove box, if you had those as well you could perhaps use them to feed your fuse boxes.

Im not really sure what you mean by one of the fuse boxes being controlled with a relay, unless that was for the lights and you wanted every light to be controlled with one switch. If you want lights to be controlled separately with separate switches, each light would need its own fused power tap (like you have from these fuse boxes and each light would also have to have its own relay so the power for the light isnt running through the switch.

Q1: For a fuse box like that, I would always run both leads back to the battery, (grounding directly to the chassis is better for just a single appliance).

Q2: Well first, if you look in your picture, Jeep mounts a fuse box inside the cab, so theoretically there would be nothing wrong with following suite, as long as you don't have anything as heavy draw as a winch running through there it will be fine. Personally, I wouldn't run the wiring for lights into the cab. Especially because the will also each need a relay, it will get cluttered in there.

Q3: If you dont mind going through the work, go ahead and modify the glovebox, and show us how you did it.

Additionally, just a tip, if you choose to leave one or both of the fuse boxes under the hood, and only had to run the control wires for the relay(s) through the firewall, you could poke a hole through the foam where the A/C / Heater lines run into the cab (next to the ECU).
 
Just some thoughts as they came to mind reading your post...

- What kind of lights do you plan on running, interior or something like fog or off-road?

- For CB/radio/USB chargers, that seems like a bulky option IMO for a relatively low amount of amperage.

- A solid chassis ground is always a good option and gives a short ground-loop. Recall that the ground side in a modern 12V vehicle is actually the electron source. Long wires - especially grounds wires - can add to EMF interference with radios/antennas/etc.

- Be careful using similar fuses in series (or breakers especially); an ATC-style fuse supplying a fuse box with similarly sized ATC fuses can cause issues with how the fuses act during a full-load short. You typically want a slower-acting fuse (such as a Maxi or similar) closer to the battery which protects the wiring and smaller, faster-acting fuses to protect the circuits.

- Nothing wrong with mounting fuse boxes inside - the factory does it like that for a reason. However, generally speaking, high-amp stuff shouldn't pass through the firewall. For example, you wouldn't want to run a 10 AWG wire for a 12v+ source from the battery to a relay under the dash, to then run back out through the firewall to some high-amp off-road lights. Only your low-amp relay trigger (or trigger-ground) would go through the firewall.

If you don't need many circuits, which it doesn't sound like you would, I really like this Dorman fuse block: https://www.amazon.com/dp/B000CO7I04/?tag=wranglerorg-20

You can even attach a standard Bosch-style SPST relay to the side of the block and can trim the bus-bar as needed to get switched/constant leads. Here's a shot of what I did in my YJ:

2018-04-26 Jeep YJ-8.jpg


It was still "in process" in the photo above, but I used the top 3 fuse taps as a switched-source (via the relay) and the bottom one for 12v+ constant. I used the switched sources for CB, USB 2.1A charger, etc. and the bottom one for a 12v+ cigarette lighter (for charging phones while taking a break on trail rides or running a small electric light).