I'm planning to install an auxiliary fuse box under the dash, and I'm looking for a safe way to do it. I currently have the fuse box and a relay box mounted. For the time being I want it provide power to the radio, gauges, and a CB radio. I plan to initially use one terminal for continuous power, one for switched power, and the others will be used for future additions. Probably things like a pre-oiler, and an electric fuel pump. The two things critical things I trying to figure out is where to get positive power, and where to put a ground. I want to have a general purpose ground wire to send the negative power from the gauges and other things. I'm going to need a proper ground for the CB antenna anyways. My first though was to get the positive from the battery side of the starter solenoid, but I read in another forum that this would cause a disaster. I can't find post again. I think it was in a Mustang forum. They said it would be fine, until you tried to crank the engine, then the fuse box will be juiced with the full starter motor amperage. I look around some more and I found this: http://mmb.maverick.to/resources/auxiliary-fuse-box-for-aftermarket-add-ons.254/ This is what I based my original plans on a few years ago. I bought all my parts and planned things out years ago, but too many other car problems popped up that took priority. I see that he's getting the main power from the batter side of the starter solenoid. If your going to do this would you need some sort of addition relay between the power and the fuse box that turns it off while cranking the starter? Right now all I have is a fuse holder that'll take up to a 100 amp fuse that I planned to put between the main power and the fuse box.
I added a small Painless by the solenoid. https://www.mmb.maverick.to/threads/my-1972-comet-gt-clone.95746/page-7
I've had aux. fuse boxes and stero amp hooked up to battery side of solenoid for many years w/o any disasters for many years.
For my aux. accessory fuse block, mounted under the dash, I ran a 10 AWG wire from the battery to a 30 amp relay mounted beside the fuse block. The NO relay terminal connects to the fuse block. The relay coil is energized by the radio power wire when the key is turned on.
For ground I have a common point in the engine bay where the battery (-) cable connects to the starter solenoid bracket. All of my grounds except for gauges go there. A 12 AWG wire from that point attaches to a dash mounting screw. That's where I attach any ground wires for inside the car, again except for gauges. For those I have a wire from the engine to the aftermarket gauge cluster. Ground for the gauge lights are picked up at the dash ground screw mentioned previously. Whoever said that is clueless about electricity. The battery connects to the solenoid with a short cable. It doesn't matter which end of the cable you get your +12 volts from.
All of this has reassured me enough to move forward. This is how I decided to hook everything up. I'm using 8 gauge wire and an 80 amp fuse. I still have a lot of wiring to before I want to start sending power through everything.
If this is the case, I'll just use a smaller fuse. This is the fuse that the fuse holder came with. Looking over wire length/gauge/amp charts, the results are all over the map. Some charts say that 80 amps would be fine. I haven't been totally sure what to make of it all.
There's more to choosing wire than just gauge and ampacity. You didn't mention what type of wire you're using, that is, what insulation is on it. Some wire insulation is rated at 60* C, some at 90* C, some higher. 60* wire is most common. The more current you draw through a wire the hotter it gets plus you have to consider the surrounding temperature, if the wire is in free air, or bundled, or in conduit. Your wire is located in an engine compartment that can get pretty hot so the ampacity has to be derated (lowered). In any circuit like this the fuse's only function is to protect the wiring, not the devices connected to it. By choosing the low value fuse you're playing it safe.
I looked up the 8 gauge wire I got and is says it's good up 55 amps at 90C. An 50 amp fuse was the closest I could find, so I got that. The fuse box install seems to be a success. All the new gauges and radios are working. I'm going need a few days for my hands to recover from this job. I did enough crimping over the weekend to bruise the palms of my hands.
