I am going to start with some basics and then we can work on to more complicated things. So, first, time to go back to school. For those that are heavily involved in electricity/electronics, things are somewhat simplified to allow those who don't know anything about electricity to gain a grasp. So, I am going to attempt to put out the data as accurately as possible without getting into the nitty gritty details. All things I am talking about are specific to direct current (DC) systems like our trucks. You get into alternating current (AC) systems like in your house and some of the equations I am going to talk about have different symbols. So, if you want to know about this, let me know so if you are cruising the internet and see these equations with different symbols, that is probably what you are seeing.

So, now, how does electricity flow? For those that don't understand electricity, I am going to relate it to a mechanical stuff. The first point is voltage. Think of voltage simply as a pressure that is being applied. Higher voltage means more pressure. Kinda like with a hose, if you press up the pressure high enough, the water will push through anything. The same can be said for voltage. Get the voltage high enough and it can push its way through wiring insulation (more will be said about this later). When it comes to trucks, this normally is not a concern due to the low voltages (except for HIDs, but this will be discussed later in a different topic).

Now, like with a hose, there has to be a high pressure and low pressure side. When it comes to electricity, think of the positive as your high pressure side and the negative your low pressure side. Electricity will always flow from the positive to the negative side. This is known as conventional current flow. Most people use this to talk about how electricity moves.

The next topic is resistance. Like the name implies, it resists the movement of electricity. You can think of it like putting in a valve into a hose. You close the valve, it resists the movement of water. In the case of electricity, this resistance is known as ohms. For the most part, you simply need to know if something has no resistance or has a lot of resistance. The amount of resistance varies from item to item. It is a function of how big, what it is made of, etc. Just understand that everything has some amount of resistance, even wiring.

Finally, you have what is called current. This is what some call amps. Think of this as the amount of electricity flowing past a given point. You can relate this to gallons per minute of water flowing. As with water, if you can get a high flow rate, you can get things done faster. Same is true with electricity. You have more current, you can have a higher power item, more power means it can get the same job done faster.

If you know anything about mechanical systems, you can relate the pressure in a system (voltage) to the size of the pipe (resistance) and be able to figure out how much flow you have (current). With electricity, this is expressed as Ohm's Law, or Current (I) = Voltage (V) / resistance (ohms). So, putting this to practical terms, if you know that the truck is running at 14 volts and you have an item that has a resistance of 7 ohms, using the equation, you can find the current, or current = 14 volts / 7 ohms = 2 amps. If you are good with math, as long as you know 2 of the values, you can find the third.

Along these same lines, you hear things talked to in the way of power (watts). Watts is simply the ability to do work. For electricity, it is simply the amount of voltage times the current, or power (watts) = voltage x current. So, for example, if you know that your truck uses 55 watt bulbs for the headlights and the truck runs on 14.4 VDC, then you can find the current going through the headlight bulb. in this case, you rearrange the power equation to be current (amps) = power (watts) / voltage (VDC). So, you have current (amps) = 55 watts / 14.4 VDC, or 3.8 amps. Since you have 2 headlight bulbs, each bulb will pull 3.8 amps, so, when you flip on your headlights, you will pull 7.6 amps (3.8 amps x 2) of electricity.

You can take things even one step further and pull Ohm's law into this. Since you now know the headlight bulb runs on 14.4 VDC and it will pull 3.8 amps, you can rearrange Ohm's law to resistance = voltage / current. So, the resistance of the headlight bulb is 14.4 VDC / 3.8 amps, or 4 ohms. You can prove this to yourself by simply getting a cheap multimeter, putting it on the ohms setting (the funny looking 'O' that has feet) and going between the two pins of the bulb. As long as the value is close, call it good enough. There's a lot that plays into the resistance of a light bulb and now is not a good time to be getting into that.

This will give you a good start to understanding what is going on.

 

Ok, as you can see, this really is not such a hard topic once you understand a few basic concepts here. Now we are going to move on to how to choose a properly sized wire for a project. Again, you know a few basic things and all the other information is easy to figure out.

Now, you wonder why having the right sized wire is important? This is a balance between having a wire that is small enough to fit where you need it to, not adding additional weight to the vehicle, yet big enough to safely handle the power going through the wire. With that being said, if in doubt, go with a bigger wire. The additional weight is of little concern normally, the additional size is normally very minimal, for the safety of not making your truck a big BBQ (not a pretty sight).

Now, you ask, how do I know what size the wire is? All wiring is normally marked with a "gauge" number. When people talk about wiring, they will call wires by their gauge numbers, for example, 14 gauge wire. The gauge number is simply the area that the metal part of the wire occupies if you were to look at the end of a wire. The smaller the number, the more surface area it has (I didn't make the scale, that is how it is). So, if you were to compare an 18 gauge wire to a 10 gauge wire, the 10 gauge wire would be bigger.

I bet you see where I am going with this, like with a hose, the bigger the wire, the more current it can handle. Like with a hose, the bigger the hose, the more water that can flow through it. KINDA!!!!!! Remember me talking about not having the correct size wire can turn your truck into a big BBQ? Wiring is kinda unforgiving. It will attempt to do whatever you wire it up to do. For those that work around higher pressure mechanical systems, have you ever felt a pipe with a lot of pressure drop across it? Did you notice the section of pipe was kinda warm when compared to larger piping? That is because the fluid inside the pipe is scraping along the inside of the pipe and converting the mechanical flow into heat through friction. The same thing happens with wires but to a larger extent. You start pushing more electricity through a wire and the electrons (the actual part that you could measure to get the current) will start scraping against the wiring, forming heat. As you can imagine, heat is not a good thing. This basic concept is how a toaster works inside your house. As you can see with that, get the wiring hot enough, it will start glowing red and emitting a lot of heat. The trick with wiring is using a big enough wire for the given load to maintain the heat generated to a safe level since all wiring will generate some heat if any current is passed through it.

So, now you are asking, how do I know what size wire to use for my project? As a general rule, you need to first know either how much current the item pulls or the power the item uses (which you can figure out the current using Ohms law). When trying to determine the wire size, you then need to know how far is it between where it is being powered from and where the load is. Don't think straight line distance, think of the twists and turn that the actual wire is going to be making. Once you know the distance and current, you can look up on a wiring chart for the proper size wire. Most wire charts have the distances broken down to say 5, 10, 50, 100, 200, 300 feet for example. Very rarely will you be right on with a distance. So, always use the next higher distance to what your wire length is and then round up on your current to the next higher level. That will keep you on the safe side of things. if you don't have a wiring chart handy, make an educated guess (after awhile, you will get a pretty good idea of what size wire goes where). Then, you can pull out a multimeter and you can measure the voltage difference between where the wire gets its power (ie, fuse box) and the input to the load (the power wire going in). If you are over 1.000 VDC, then the wire is too small, go up in wire size. This is a national standard/rule. I tend to shoot for a max of 0.5 VDC drop between the power source and load.

 

I mentioned in the last post that by passing current through a wire, it causes it to get warm. This is because all wiring has a finite amount of resistance to it. We are talking on the scale of fractions of an ohm per foot, but it does have some. For example, 18 gauge wiring (commonly found in vehicles) has a resistance of 21 ohms for 1,000 feet of wiring. So, in your truck, if you had a 10 foot run of wire (about that needed to go side to side in your truck due to bends), it would have a resistance of 0.21 ohms. That is pretty small (can't be measured by most multimeters you can buy accurately). Where if you used say 6 gauge wire for the same run, it has a resistance of 1.3 ohms for 1,000 feet, or 0.013 ohms for that same 10 foot run.

I'm sure you are wondering where I am going with this. Lets put this into practical terms. For your headlights, you have roughly 20 feet of 18 gauge wiring between the fuse box, to the headlight switch, through the steering column, and out to the headlight bulb. So, that has a resistance of 0.42 ohms. We said earlier that each headlight bulb pulls 4 amps (rounded to make the math easier). Using Ohm's law (starting to get tired of this equation yet?), voltage = current x resistance, or 4 amps x 0.42 ohms, or 1.68 volts. Look out, you are walking down a path of a possible fire. What does this truely mean to you? Simple, the alternator (the true source of power) is outputting 14.4 VDC, but the wiring is now dropping 1.68 volts of that. So, in reality, your headlights are only operating on 12.8 VDC. Another way to look at it is that power = volts x current. In this case, the wiring is eating up 1.68 volts x 4 amps, or 6.72 watts. So, instead of the headlight outputting 55 watts of light, it is only outputting 48.28 watts (55 - 6.72). So, by using too small of a gauge of wire, you are minimizing your headlight brightness. Is this on the extreme side, yes. But, do a simple voltage measurement between your battery positive and the positive wire of your headlight. See how much of a voltage difference there is. You might be a bit surprised, especially on the older vehicles where the wiring has been affected by age and the wiring resistance has increased. Where in contrast, if I used the 6 gauge wire for the same headlight bulb, I would only have a resistance of 0.026 ohms, for a voltage drop of 0.104 volts, or a power drop of 0.416 watts. So, the 55 watt headlight bulb would be actually outputting 54.6 watts. Gee, my headlights are brighter. I wonder why?

This situation is even worse when you start talking about either winches or high power stereos. A winch can pull upwards of 400 amps. So, even a resistance as small as 0.001 ohms can have a significant impact on the overall power reaching the winch. In the case of car stereo amps, they are designed to output a finite amount of power. So, if you start putting in too small of wiring, the wiring will start dropping too much voltage. This will cause the amplifier to attempt to pull more power (ie, more current), this intern allows the amp to output the necessary power to the speakers, but at the cost of having an even larger voltage drop on the supply wire. Now, remember, power = voltage x current. In the case of the amplifier, because you raised the current to get the necessary power output and it caused the wire to drop more voltage, you have a compounding problem. The amount of heat will increase exponentially. So, car stereo wiring needs to be looked at especially hard for the main power feeding an amp.

 

So far, I have been talking about the ideal world where you have a single wire running from the source of power all the way to the load (the battery to the headlight from my first example). Sorry to say, but it isn't quite that easy in real life. For power to get from the battery to the headlight, the power leaves the battery, goes through the battery terminal to another wire, on over to the starter solenoid where it makes another junction to the post, through another junction to the wire that runs to the fuse box, which then makes a junction to the bar that powers all the fuses which then has another junction to the fuse. As you can see I'm not even to the headlight switch (about half way there) and I have been through a number of junctions. I would say that for power to reach the headlights, it will go through about 20 junctions.

When I say a junction, all I am talking about is simply a point where one piece of metal that is conducting electricity comes in contact with another to allow the current continue to flow. So, the attaching of a wire to a terminal to be bolted somewhere is 2 junctions, one of the wire to the terminal and then the terminal to the bolt. The way that say 2 wires are joined together is only limited by your imagination. The problem is, each of these junctions is a source for a high resistance. As I showed, you get a high resistance, it can affect things negatively, in some cases, extremely negatively. So, careful selection of how you join wires together is key to having long term, reliable, electrical systems.

So, you ask, how do I make a good junction? It is more simple than you think. If you are crimping a wire to a terminal, you want the end of the wire to be a nice copper color, you want the terminal to be clean and not corroded and a firm squeeze applied (don't want to go too firm on the squeeze as you can start cutting the wiring and this leads to lack of physical strength, ie, the wire breaking into 2 pieces in a little while). A simple tug test is all you need to do. Grab on to the terminal, grab on to the wire and give it a gentle tug. If it stays together, the crimp was tight enough. This simple of a connection is sufficient for a junction that is not exposed to the weather. So, if your connections are inside the cab, you are good using this method.

But my connection is under the truck or it is inside the engine bay. What do I do for this? You need to provide something that can protect this joint from the elements. Most modern day vehicles have weather tight plugs that don't allow the water to reach these junctions (ie, wire to terminal, or terminal to terminal). So, make sure that the rubber seals are good where they touch the wire or the two halves of the plug. But, if you are having to join 2 wires using a butt splice or other crimping piece, the use of say heat shrink tubing or even electrical tape will be recommended. In short, the less water you can let reach the junction, the better off you are. This is especially true of the northern states and coastal areas as the salt in the air/on the roads is very hard on electrical stuff as you are probably aware of already. You can pick up heat shrink tubing at most hardware stores now or if they don't have it, Radio Shack does. I personally recommend stuff from RayChem as this has a glue on the inside of the tubing that seals the tubing to the wiring. Even standard heatshrink (like you will find in most places like your hardware store and Radio Shack) will still allow some water into the joint (the big thing is you are minimizing it). The RayChem stuff seals it 100%. it also provides an insulative barrier to the joint so you don't develop shorts at that point. The Raychem product is rated to survive in a steam environment at 300F, so the conditions under your hood are much less than that (ie, conditions if a nuclear reactor were to blow up and things were contained in the containment building). You can find this stuff on E-bay for fairly cheap too.

Also keep in mind that your headlight switch is an exposed junction, your turn signal is an exposed junction. These are both prone to developing a high resistance. So, if you are concentrating the resistance into one spot, you are also concentrating the heat into a single spot. This is bad because when you get metal hot, it tends to oxidize faster. Why is this bad? Metal oxide is not very conductive (ie, has a lot of resistance), so, your high resistance joint becomes even more resistive. Another one of those self defeating situations. So, be aware of what sort of environment you are putting a junction in and do what is necessary to make for a junction that will survive the test of time. As the old adage goes, you can pay me now or you can pay me later. You can either take a little bit of extra time now and make a good joint or be fixing it later after you have problems.

So, as you can see, simply saying that I am using a wire of XX gauge for my situation isn't always the end all and the use of a wire table won't garantee a flawless system. Each part of wiring needs to be paid attention to. But, fortunately, the steps for each thing is very easy to ensure long life as long as you take the time as you are assembling.

In short, make sure that your junctions are clean and free of dirt/debris/oxides and you will be fine.

 

One last tidbit on making up junctions. Some people are believers in soldering junctions. While there is some truth to the methodology, the gains are normally offset by the amount of time it takes to do the solder joint. A good crimped connection is going to add a very minimal resistance (normally in the range of 0.001 ohms or so). By soldering, you can normally get rid of this increased resistance. So, for most systems in a vehicle, the gain does not offset the work required.

The one place that I will say soldering is beneficial is with your higher power systems. The connection on the back of the alternator (can be handling up to 250 amps, possibly higher, depending on the alternator you have installed), a winch (up to 400 amps), or even your external fuse for your stereo amplifier (most run in the neighborhood of 100 amps, but can be much, much greater than that). In these cases, soldering the connections can lead to better system operation. The general rule I would say to use is that if you know the wiring is going to be handling above say 50 amps, solder the connections. The gains from this can be substantial and they will also help prevent future hot spots at these junctions.

With that being said, normally the wiring associated with these systems is not going to be small wiring. So, the heat source for melting the solder will need to be capable of adding a fair amount of heat. A 40 watt soldering iron probably will not cut it for this job. Something in the 80 watt range will do. I normally use something like a propane torch to heat up the connector. Just a warning though, try to keep the heat as far from the insulation as possible as the heat can damage the insulation and then lead to electrical shorts later on.

 

Something that I need to point out (probably should have covered this first) is that any amount of electricity has the potential to hurt/kill you. Yes, KILL. Now, all of you are probably wondering why I am using such strong words on what appears to be very safe things. The common thought is: everyone messes with stuff in their cars and no one dies except when their battery explodes in their face or something like that. It is a true statement to make that working on 12 VDC has a very low possibility of causing harm. To put things into perspective, under 1 millamp (0.001 amps), you don't even feel it. Get between 1 and 10 milliamps (0.001 to 0.010 amps) and you can feel the tingle in your body. Get between 10 and 100 milliamps (0.010 to 0.100 amps) and you will loose control of that part of the body (ie, the electricity is overriding the minds ability to control stuff, so, if you are holding on to the wire, you are not able to let go). Above 100 milliamps (0.100 amps) for more than a second can stop the heart, leading to death. Now, look at all your fuses in the truck. Do you see one that is under even 1 amp (1,000 milliamps)? NOPE!!!! So, in theory, everything in the truck has the possibility. So, now that most of you are wondering what you were thinking for even thinking about messing with wiring, let me put some reality back into the situation.

Remember that current is a function of voltage divided by resistance (VDC / ohms). Since the average body resistance is up around 500,000 ohms, to even get a tingle in your fingers (0.001 amps, or 1 milliamp), you would need to apply around 500 volts to your skin to feel something. But, if you are only going across say your hand, the amount of resistance is less, if you are sweating, your body resistance is less, if you have a cut, your body resistance is less, wearing wet clothes can lower your body resistance. As you can see, a lot of things can lower your body resistance. In some cases (extreme cases), it can drop as low as about 3,000 ohm. So, now, you are looking at voltages in your vehicle that you can feel if you touch them. So, be smart about how/when you work on vehicles. Dripping water from a very hot day with your hands covered in sweat may not be the best time to be deciding to wire up your battery.

Along these same lines, there are isolated points in a vehicle that have dangerous voltages in them (regardless of your body resistance). The two main points are your ignition coils/wires and the wires between the ballast and bulbs for HID lights. The HID wiring can have voltages up to 23,000 volts (only on initial starting of the lights). As for the coil and spark plug wires, you are messing with up to 50,000 volts (can be higher depending on what you have installed). The saving grace for both of these is that they are only applied for a brief moment in time. So, they will make you jump really good, but not very likely of causing death. Granted, it will still hurt and make you think twice the next time.

Where most people get in trouble with wiring in vehicles isn't with shocking themselves. It is all in getting into a situation that can cause excessive current to be pulled. I'm sure everyone has seen that little flash as they touched a wire where it shouldn't be. made you jump, probably blew a fuse, but no crime, no foul. But, keep this in mind, current will always follow the path of least resistance. You body is a very bad conductor, so, normally it will direct the majority of the current away from your body. This is good. But, the problem is, it is directing current somewhere else.

To put things into perspective, lets look at a situation of pulling the battery terminals off so you can put in a new battery. Most of us have done this job. Pretty simple, not to much chance of danger. But, if you used say too long of a wrench to undo the battery terminals and you started with the positive terminal. In this case, it was quite tight, so, you are giving it some grunt to break it loose. Suddenly it comes loose and the wrench now rotates around, coming in contact with your watch on your wrist. You just happen to be wearing one of those stretchy metallic bands. You have now created a new circuit in the truck (from the positive post, through the wrench, through your watch, to the body of the truck and back to the negative post). The most resistive part of this whole thing is your watch. It has a resistance of say 0.1 ohms. Lets see, I have a 12 VDC battery, a 0.1 ohm resistance, 12 / 0.1 = 120 amps. Damn, that is a lot of current. Now, lets see, power (aka, heat) = voltage x current, or 12 x 120, or 1440 watts of power. You know how hot a 60 watt light bulb gets, now imagine something about 25 times as hot and that is your watch, on your wrist. If you say that it can't happen, tell that to my friend that had to have his watch surgically removed from his wrist because it burned itself into his skin. The same can be said about wedding rings (or rings of any kind). Be smart, take them off if you know you are doing electrical work.

The other thing to keep in mind is that if you get a large enough current surge, it makes a nice flash. When you are welding, you are effectively shorting two wires together. Ever look at the flash that welding makes from this controlled short? Pretty damn bright isn't it. I bet you were wearing a shield over your eyes to dim the light. Well, if you are messing around with wiring, you can create a light that is just as bright. But, where is your welding helmet? Oh, that is right, on the welder or at your friends house. What is shielding your eyes? NOTHING. It has the potential to create what are known as blind spots (the inside of the eyes have been subjected to such a bright light, the interior part of the eye is permanently burned and will not heal itself, leaving you with a dark spot in your vision). Normally this is a very small point and most people don't realize they have them until you get a series of them and they start forming bigger and bigger spots. In some cases, the eyes can become overloaded due to a bright enough light that the inside of the eyes effectively shutdown. You guessed it, you are now blind!!!!! If you are lucky, it will come back in a short period of time (sometimes minutes, sometimes in days, depends on the brightness of the flash). But, if severe enough, it won't come back.

Like I said at the beginning, I am not trying to make you not want to do electrical work, but one must be aware of the dangers. I don't hear people saying "don't work on your engine because you might get your hands caught in the spinning fan or under the serpentine belt". The same applies here. Be aware of the possibilities and do what is necessary to protect yourself. Ever wonder why the mechanic put those nice blankets over the fenders of your car? A lot of it is to protect the paint, but part of it is to prevent shorting themselves out through the body of the car.

 

Something that I have noticed I left out of this discussion which is very critical in preventing BBQ'ed Ford is the discussion of fuses. Sure, running undersized wiring has the ability to cause this same thing, but normally it will raise its ugly head within moments of you doing something, so, you are right there to catch it. In the case of fuses, they are there for when things go awry moving on down the road and you are not there and/or haven't done anything to the truck.

Are fuses needed to make anything work? NO. But, what is the cost of a fuse/fuse holder compared to the cost of having an electrical fire inside of your truck? To me, I can install more fuses than you can think of an still be under the cost of the repairs for an electrical fire. So, spend the extra few bucks on a fuse/fuse holder whenever adding any additional circuits. At a minimum, use an installed wire in the truck (see later discussion).

This leads into what I really want to talk about, selecting the correct size fuse for a new circuit that you are adding. This is a very simple process. First, you need to know the maximum current that your circuit can pull. In the case of lights, you calculate the current like I have mentioned. In other cases, most components talk about the current draw right on the component. If you are running multiple components, add up the currents from all the components (don't forget to base your wire size on this value too, don't need BBQ'ed Ford). The value you come up with is the minimum size fuse that you should be using. But, in a lot of cases, the number that you come up with is not going to relate to a fuse size. Also, things are not exact, so, it is possible that you can pull slightly more current than that. So, you really want a fuse that is rated at slightly more than the maximum amperage you calculated. Most fuses are based on 125% of the maximum current. So, take your maximum current you calculated, multiply by 1.25 and then round up a little bit to find the next higher fuse.

This will lead you to find out that fuses come in many shapes and sizes. You need to be aware of 2 things: 1) maximum interruptable voltage and 2) maximum interruptable current. The voltage rating is normally not a problem in your truck. So, this isn't of interest. The second one is probably having you scratching your head. If the fuse is rated at XX current, doesn't that mean it will stop anything more? Kinda. Yes, when the current gets too high, the fuse will fail and open the circuit. But, you can reach a point where the current can be so high that as the fuse fails, it draws an arch and the circuit will continue to be energized. This is where selecting the correct physical size is important. Normally, what you find in the stores has taken this into consideration and if you are using an appropriately sized wire, this is not a problem. So, if you have a small fuse, you should be using fairly small gauge wire. Medium size fuse, medium gauge wires, etc. if you are having to fight to place the wire inside the clips for the fuse holder, you are using the wrong size fuse/fuseholder. Also, the mini-fuses are normally limited to under about 30 amps. The medium fuses can go up to 80 amps or so. The larger fuses are only limited by what you can find.

As for location of the fuse/fuseholder, follow this very simple rule: place the fuse/fuse holder as close as practical to the source of voltage without causing other issues. So, if you are running a wire from the battery to your amps, the fuse should be just off of the battery. If you are tapping into the fuse box for power, your fuse should be in the fuse box or just outside of the fuse box. The basis behind this is the fuse is what is protecting the vehicle. if you place the fuse away from the power source, the likelihood of having a problem with the wiring causing a fault increases and if the fault occurs this way, you have nothing protecting the vehicle.

 

Sorry for any references to trucks. I pulled this from my posts on a truck site that I also assist in moderating for. I'm sure some of you are sitting there with your heads spinning from information overload. Take it bit by bit, part by part. If you don't understand anything, post it here because if you have a question, others have the same one. If there is something from this that you think needs more detail, speak up. I will explain these topic 20 different ways so that hopefully one of them will make sense to you. I spent 15 years teaching mechanics how electricity works. If I can make those knuckle draggers understand it, I'm sure I can figure out a way for you to understand it.