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How-to: 12 Volt Auxiliary Fuse Box - PBR

27853 Views 27 Replies 16 Participants Last post by  Old Navy
6
Pretty simple installation with some tips.

I ran an Auxiliary Fuse Box up under the Driver-side Dash to run my Ham Radio and my Roof Light Bar. Looking at the Loads for these things I needed at least 55 Amps, if everything is drawing full power. You simply add the Ampere Rating of each device together to come to the Full Load. If you have the Wattage then divide the Wattage by 12 Volts to come up with the Current in Amps.

Example:

300 Watt Pencil Beam Lights divided by 12 equals 25 Amps for the pair
260 Watt Driver Beam Lights divided by 12 equals 21.6 Amps for the pair
Amateur Radio rated at 8 Amps.
25 + 21.6 + 8 = 54.6 Amps total draw

Using this handy chart on this page http://www.powerstream.com/Wire_Size.htm to get the Gauge of Wire needed to conservatively carry the Current Load. Look under Chassis Wiring which is closer to Automotive Wiring where the Wire is exposed to Air rather than the Power Transmission Rating which is for Bundled Wiring.

55 Amps = AWG 10 Gauge.

The length of Cable also falls into the Calculation with shorter Cables creating a smaller Voltage Drop and longer Cables causing the Voltage to drop a bit under demand. So looking that 10 Gauge Wire actually just delivers 55 Amps which is my exact Load I went up a Size to 8 Gauge which will handle 73 Amps.

CAUTION. THE CAR BATTERIES VOLTAGE OF 12 VOLTS DOES NOT PRESENT A DANGER BUT THE MASSIVE AMOUNT OF CURRENT THAT IT CAN PRODUCE AND BE VERY DANGEROUS AND POSSIBLY DEADLY. TOOLS WILL ARC WELD AND POSSIBLY EXPLODE IF CONNECTED BETWEEN THE POSITIVE TERMINAL AND THE CHASSIS OR THE NEGATIVE TERMINAL.

If you are at all uncomfortable about working around electricity, then carefully remove the Negative Terminal (-) from your Battery first and then proceed to work on the Positive (+) side. This may cause problems with your Air Bag or Engine Service Light that may require a Reset by the dealership.

So starting with the Battery, I used a Gold Plated Terminal Ring Rated for 8 Gauge Wire. Pull off the Plastic Cover that protects the +12 V Terminal by lifting each side up in turn. I use a very short 4" Socket Handle to pull the 10 mm bolt on the Engine side of the Battery Post.

CAUTION: DO NOT ALLOW A TOOL TO TOUCH THE POSITIVE TERMINAL AND THE CHASSIS AT THE SAME TIME.

There is a bit of Loctite Compound holding the 10 mm bolt in place, so it takes a bit of a tug to get it going. When dealing with this much Current you have to make sure that your Connections are very tight and metal makes as much contact with the Copper of the Wire. Strip about 3/8th of an inch worth of Insulation off the end of a 12" length of 8 Gauge Wire and run it into the Terminal Lug. Very firmly Crimp the Lug onto the Wire. I used a set of Vice Grips and then a Bench Vice to tighten mine. Slip the Lug onto the Batteries Post and replace the 10 mm nut ensuring that you Torque that thing down tight. I used a bit of Loctite to hold mine in place when I was done.


The other end of this 12 inch long Wire will go into the end of the Master Fuse. Depending on the type of In-line Fuse the Cables attach in different manners. Typically you strip off 3/8th of an inch of Insulation and run it into a hole and Crimp down with a Hex screw. Be very certain that there are no stray threads of Copper sticking out - if so re-do the Connection until you get it right. Firmly twisting the Wire between your Fingers will tend to keep those threads together.

The other end of the Master Fuse will run off into the Passenger Compartment. For the time being leave the Master Fuse out of its container, but do Connect a 10 foot length of Cable to the far end of the Fuse Holder.

The entire Fuse Holder can be Mounted to the Plastic Leg of the most forward of the two under Hood Fuse Boxes. A couple of self tapping screws ought to be enough to hold this light load in place. Run the Cable under the two Factory Fuse Boxes on the Engine-side using Black Wire Ties to hold the 8 Gauge Wire to the Factory Loom. You do not want to over tighten the Wire Ties, as it will cause them to over stress and possibly fail over time.


Run the Wire along the top of the Firewall - the back of the Engine Compartment using a Wire Tie every 6 to 8 Inches. Continue all the way past the Power Brake Booster Pump and over to an area on the far Driver-side. Look down the Firewall and you'll find and Empty Gasket Plugging a handy hole. I pulled the Plug and put in my own Gasket, but you can also "X" the Plug with a Razer-knife and Silicone Glue it afterwards. Leave a bit of slack here as a Drip Loop, so that Water flowing down the Wire will drip off the Loop rather than follow the Wire into the Driver's Compartment.


Run the Wire out from behind the Insulation. Now if you don't have the Satellite Radio Option you ought to have a nice big open area up above the Driver's Kick Panel. This is the area where you can mount an Auxiliary Fuse Box like the type available at a Car Stereo Shop like www.parts-express.com. Drill a couple of holes being carefully not to drill though to the Quarter Panel Skin.


The Light Pairs will draw 25 and 22 Amps so they can use a bit thinner Wire. Since this is going to be long run of about 15 feet, I went stupid large and bought some 10 Gauge Wire Rated at 50 Amps. After stripping the Wires and Clamping them firmly with the Hex screws, I tested them for continuity to Ground, (none is good), and then put the Local 30 Amp Fuses in their Circuits. For the Ground Wire for the Roof Rack I used a 5 foot length of 8 Gauge Wire to run from the Lights down the Driver-side "B" Pillar and to a major mounting screw using a proper size Lug. [Edited to fix AWG Gauge Sizes - Paul];


The Ham Radio comes with it's own Cables and they'll hook into this same Fuse Box. There's a Menu Adjustable Timer, which Shuts the Radio off after a preset amount of disuse. The Radio gets its own Fuse too.

Once everything is installed, look over your Wires one more time before putting in the Master 60 Amp Fuse. If it blows, break out another $2 Fuse only after looking over everything for a third time.

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To address the grounding discussion make sure you pick an nice thick piece of the truck and make good/tight connections. A loose connection can create heat and cause sparks. On a truck with a unibody you can be sure just about everything is a good ground, but with a frame based one like the Xterra the best choices are along the frame.

Did the install last night. After negative battery terminal was connected my vdc and slip lights are on. Grrrrr!
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Typical - an air bag light some times goes off until the system settles down.
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