I think it's time we sat down and talked about what is possibly the single most crucial component to keeping our four-wheel-drives safe -  fuses.

Sure, putting in a UHF, installing some lights, or swapping out the plug on the back of our Travel Buddy 12-volt ovens or fridges from the ciggy plug to an Anderson plug is pretty straight forward. Red goes to positive, black goes to negative, and things light up and start working as they should. There is much more to it than that if you want to do things safely, and stop your rig from burning to the ground. 

My old man was an electrical engineer, so he explained to me the value of how a fuse in the right spot could keep you from burning your vehicle to the ground when you're installing 12-volt bits and pieces. So with that in mind, I’m going to offer up some of these lessons and tips to hopefully stave off disaster for you.

Despite offering up some magic tips and tricks articles over the years, I want to re-iterate if you're not comfortable playing with your 12-volt system, leave it to an expert. Auto-electricians are a lot cheaper than having to replace your four-wheel drive.

A little bit of knowledge and a fuse will save your pride and joy burning to the ground, and it could save your life.

WORDS BY WES WHITWORTH

Image credit: Flickr @mickyj_photos

12-VOLT ELECTRICS: THE BASICS
The first thing we need to understand is how the electrics in our four-wheel-drive work which is a 12-volt Direct Current (DC) power system. That means power flows in one direction only (unidirectional) and we've only got to deal with positive and negative; simpler than Active, Neutral and Earth in an Alternating Current (AC) system (like your house).

When your engine is running, it's also turning the alternator, and this generates AC power, that is then converted to DC power (by a bridge rectifier), which in turn charges your battery and provides additional power for the vehicle's electrical accessories; so far, so simple. 

On your battery, you'll have a Positive (+) terminal, and a Negative (-) terminal. Negative is usually connected to the chassis, body and motor by way of earth cables or straps. All the positives end up back at the positive terminal of your battery.

When your alternator is generating power, it is earthed via the alternator housing to the engine, via an earth strap, back to the battery. In contrast, the positive is directly back to your battery by a rather heavy gauge wire to carry the current.

These are fused usually at the fuse box, or inline at the battery terminal with a fuse-able link in most cases, or sometimes with an 80+ Amp fuse at the battery.

That power is unidirectional, meaning it runs from the positive on your battery, through to an electrical item (such as your fridge), and then back to the negative on your battery (or to the chassis/body that's connected to your negative terminal).

A quick word on insurance
If you happen to decide to wire in an accessory, incorrectly, and do not include a fuse in it, and your four-wheel-drive burns to the ground, there's a 95 per cent chance the insurance assessor will work out how the fire started. From there, the assessor will work out you've installed wiring or an accessory without a fuse, and that by your ignorance (or stupidity?), your expensive four-wheel-drive is now a burnt mess. Chances are, they'll either write you off or settle a reduced claim, as directly through the owner's negligence there has been a loss – this is why auto-electricians have business insurance; if they screw up, their insurance covers your four-wheel-drive. If you screw up, you have to pay for the damage.

WHAT CAUSES ELECTRICAL FIRES
Next up, let's look at what causes electrical fires. In a nutshell, a contacting of positive and negative without an appropriate load on it will create a short circuit. Think, touching the positive wire to the chassis or body which will create a short circuit (and some impressive sparkly 'arklys'), whether on purpose or by accident. Also, if something goes drastically wrong inside an accessory you've wired into your four-wheel-drive they can internally short circuit, and achieve the same result.

The simple analogy is to think of your average halogen/incandescent light globe. Positive attaches to one side; negative attaches to the other. If there wasn't a resistance (load) on the circuit, everything would melt. The globe itself is an appropriate load, in that it is essentially a controlled short circuit – the filament within the globe is basically a resistor; it receives power from the positive and negative. It starts to heat up, the amount of energy it uses is controlled by its resistance, and it glows white-hot – creating light.

If you remove the globe and connect the positive to the negative wire, the wire in the system attempts to do the same thing - become a resistor, and it glows white-hot. As you may well have guessed, this isn’t ideal.

When you have a short circuit across a wire, it will burn out whatever has the least ability to carry that current – namely a fuse. As an example, say you've got 25A wire and a 20A fuse. That is the perfect equation, as the 20A fuse will burn out before the 25A wire will. When a fuse blows, the burn is contained within a bit of plastic, and it happens (usually) pretty quickly (think microseconds); no time for everything to get hot over the space of a few seconds. When the same wire doesn't have a fuse, the wire will become red hot, and eventually blow out (as you would have seen in the video hereabouts). Chances are it will have melted and set alight the wiring insulation that will continue to burn, and also burn anything flammable around it – think carpet, seats, other wire, etc.

USE A FUSE!
Repeat after me: USE. A. FUSE.

You’d be surprised how many folks start wiring things up and forget to put a fuse on the main power wire. We'll get into which fuse and amp rating a little later, but the most critical thing is to get a fuse on the positive.

If you’re going to rewire your fridge or 12-volt oven ciggy plug with an Anderson plug, something you may not know, is that the ciggy plug you're cutting off has a round glass fuse inside of it (sometimes they're plastic, but work the same way). So by cutting it off, you've just removed a fuse from that line. Next thing to think about, the ciggy socket you were plugging it into, also has a fuse at the fuse box (if it's a factory socket). If you run 50A wire directly from your battery to an Anderson plug, you've just removed the factory fuse too. So now you've got a lead from your battery to your fridge with no fuse(s) at all.

Before you jump on Facebook and yell at me that most fridges and accessories have a fuse inside them, that’s just there to protect the accessory. If you have a short somewhere on the positive power wire between the battery and accessory (let's say you've just crushed the power wire in your fridge slide, and the fridge slide is earthed), all of the cable from the battery to where the short is, will burn. This is why when you do add a fuse, put it as close to the battery as humanly possible – within a few inches of the positive terminal is perfect, and absolutely before you go through the firewall, or body, or anything that is earthed to the negative terminal on your battery – body, chassis, bullbar, tray, etc.

Essentially, by adding a fuse, it’s making the fuse the sacrificial lamb, should something untoward happen. Better to blow a fuse, then melt your wire right? One you can replace in 30 seconds, the other may take your pride and joy with it.

USE THE RIGHT FUSE FOR THE JOB
Insofar as the wiring is concerned, know what wire you’re putting in your 4X4. There’s no point grabbing a bit of leftover wire, thinking it’s 25A wire, and placing a 20A fuse in there. Especially so, when the cable is 10A, and your fuse is no longer the sacrificial lamb. If you happen to put a fuse with a larger amp draw rating than your wire, the wire will burn, and the fuse won't blow; making it useless. Know what wire you’re putting in and make sure you have a fuse that is rated LOWER than your wire. If you’re using 25A wire, and you’re expecting to draw 25A, don’t just put a 25A fuse in there. Go to 50A wire, and put a 30A fuse inline.

DON'T PUT A BIGGER FUSE IN THE LINE IF IT BLOWS
One critical mistake you can make is replacing a blown fuse with one that has a higher amp rating. Unless the fuse is older than Noah and it's just died from old age, replace the blown fuse with one that has the same amp rating; 20A to 20A, 5A to 5A and so on.

If once you’ve replaced a blown fuse, the fuse doesn’t blow again, chances are it’s died from old age, you’ve been pushing the limits of the fuse, or there’s an intermittent short on the line somewhere. I would suggest investigating the wiring regardless, to see if you’ve got a cut or crushed wire that may be arcing out somewhere.

If it blows again as soon as you replace it, then you have a hard short somewhere on the line or the accessory you're using is toast (there's a good reason in there to buy quality electric accessories for your four-wheel-drive). If it does blow, you’ll need to go on a fault-finding mission and see if you can find where it's blown. By putting a bigger fuse in there, chances are you'll put a fuse with a higher amp rating than your wire, and we're back at burning your 4X4 to the ground. Don't do it.

FIREWALL
This is the first one because it's the most common. If you're going to drill a hole in your firewall (or any piece of steel really) to run wire through, make sure you use a grommet. It adds a layer of rubber around the sharp steel edges and stops the steel rubbing through the insulation of your positively charged wire.

FRIDGE SLIDE / DRAWER 
There's a particular brand of drawer system getting around that has a steel frame, alloy capping, and is usually bolted into the body of your four-wheel-drive. As soon as it's bolted down, it's an earth / negative point. Have your fridge cable floating around in the back, and sooner or later, you're going to munch it up in the slide. I went through two cables before actually getting it properly out of the way and suspending it (my expensive lesson is yours for free). Some aftermarket slides can be mounted to the body, so they're the same – crush the cable, touch the exposed positive to the slide (or the chassis on some fridges), and instant short circuit.

UNDER CARPET
This one happens when you put an amp in the back of your rig; you’ll run a power cable to run the amp usually under the carpet, down next to the door jam, and people getting in and out can kick it or stand on it. Put a bit of semi-sharp metal in there somewhere (or even plastic), and you can cut through the insulation exposing bare wire to short when it touches the floor pan.

LOOSE WIRES
Our final common one is loose wires. Bouncing around as you drive the corrugations to The Cape, any cable that knocks against a bit of steel long enough, will rub through the insulation and short out on whatever it can. There's a reason you'll see wires secured in pretty lines on top-notch 12-volt installs.

COMMON PLACES YOU’LL FIND SHORTS
Every dad is right now saying out loud, "on your legs". Right, now that the required dad joke is out of the way (I'm an uncle, and still rip out the best dad jokes you'll ever hear), let's look at some places where you're going to get a short circuit in 12-volt systems.

BONUS POINTS – DODGY CONNECTORS / TERMINALS
This one is my pet hate - dodgy connectors. If you absolutely must use spade or bullet terminals to have something semi-removable, make sure you've got a quality set of ratchet crimpers. Those 5-in-1 crimp/striper/cutter jobbies you can get for $5 at automotive shops are rubbish. You'll most often not get a good connection, nor the leverage on them to get a good connection when using them with terminals. Scotch locks, screwdriver connectors, anything like that, is a bad idea too. And that's because they're intended for 240-volt stuff, in your house. Those wires are always (earthquake aside) stationary, and once they're installed, they'll seldom get moved around.

HOW TO WORK OUT AMP DRAW FOR YOUR ACCESSORIES
Time for a bit of math that you'll actually use:

To work out Amp draw of a specific accessory you use the equation I = P / V – where I is current in Amps, P is power in Watts, and V is Voltage in, er, volts. That can also be written as Amps equals Watts divided by Volts (A=W/V). To make it simpler, turn the equation around, so it looks like this:
Watts / Volts = Amps

As an example, let's say we’re installing an LED work light on our roof-cage. The LED work light has a quoted power of 120watts. We know it’s a 12-volt work light, but need to know the amp draw. The equation will look like this:
120W / 12V = ??? (10A for those playing at home).

So now we know, at peak power draw, the LED work light will draw 10A. So to wire this up, we'll want to use 25A cable, with a 15A fuse on the positive, as close to the battery as possible. You're welcome.