Next to the fuel system the electrical systems is possibly just as dangerous to the average kit car builder. This is the part that most builders fear the most as the possibility of something going wrong is very high. Years ago the electrical systems on a kit car basically just ran the lights and the ignition and that was it, nowadays with fuel injection it has to take on just as much as a modern production car. One of the popular misconceptions is, 'it's only a 12V battery what's the big deal'.

The deal is it's not the voltage but the current that matters. A car battery is capable of delivering over 100A easily. Using ohms law for heating, W(watts) = I (current) Squared X R (resistance) you can work out the heating effect when a battery is shorted out. For example say a battery can deliver 100A, that is (100X100)X 1 (short circuit) = 10000W. A cable will heat up instantaneously melting its insulation and possibly the insulation of other cables around it, in turn causing more short circuits and more current flow. Hopefully the wiring won't be able to take it anymore and burn through completely or you are able to disconnect it in time, the worst case being that the fuel line is ruptured and it causes a fire and/or it causes the plates in the battery to buckle and the battery explodes sending lead and acid in all directions. That is possibly the worst case scenario, but it can and does happen.

I'm not trying to put you off doing the wiring in your car but as long as you follow simple rules and treat your car's electrical system as you would a household mains system everything should be safe and sound. In this section I will cover wire identification, how to handle wire, how to join wire and making a wiring loom.

Common sense tells us that the larger in diameter a wire is the more current it can handle. This is true when dealing with solid copper wire but you can get stranded thinner diameter wire that can handle more current that it's thicker copper counterpart. Why is that? If you look at a cables data sheet you often see the following line of information '75/0.19'. That refers to the number of conductors and each conductors diameter. In cabling it's all about cross sectional surface area. If you think back to a solid copper wire they are often talked about being 1.5mm sq and 2.5mm sq etc. If you look square on at the end of the conductor it would be 2.5mm sq end on but being a solid core it is heavy and also quite inflexible. Stranded wire achieves the same cross sectional area but uses many finer cores each with their own cross sectional area. Looking at the original example of 75/0.19 it means each finer conductor has a diameter 0.19mm and there are 75 of them, 75/0.19 is actually 2.12mm sq but length for length they are lighter in weight and easier to handle. The most popular cable sizes for automotive use are;

0.5mm sq to 1mm sq - side lights, relay feeds etc - usually rated between 6 - 12A

2mm sq - headlights, fog lights, ignition coils, injectors etc - usually rated between 17 - 25A

4mm sq to 6mm sq - charging cable for alternator - usually rated between 40 - 50A

15mm sq to 40mm sq - Battery leads, starter cables - usually rated between 100 - 300A

Depending on the supplier, the rating of the cables differs. To work out the cross sectional area from say 19/0.15 use the following formula, 0.15/2= 0.075 (radius of the conductor), (0.075X0.075)X 3.14 = 0.0176 (cross sectional area) X 19 (number of cores) = 0.33mm sq in this case not large enough even for side lights.

The cable insulation is another confusing factor. Standard cable is usually insulated with PVC which is fine under normal operating temperatures but PVC becomes brittle at lower temperatures and at high temperatures becomes soft or melts it also 'dries out' in high temperatures becoming brittle over time. A car can experience a wide variation of temperatures from as low as -5 to as high as 40 deg C and sometimes very quickly, normal PVC insulation would decompose very quickly and could lead to short circuits (remember those, not good). Automotive cable also uses PVC insulation but of a higher specification sometimes known as arctic flex and can handle high and low temperature extremes.

The new buzz word in wiring circles is now THINWALL, this uses even higher grade PVC insulation allowing it to be thinner thus reducing the cables overall diameter making it more flexible and easier to handle whilst being more resistant to abrasion damage. It also has higher grade copper cores boasting greater current carrying capabilities. Most modern production cars use Thinwall and would be cable of choice for use in a kit car. Thinwall cable can be more expensive than standard PVC insulated cable and it's not worth sacrificing cable for cost (remember short circuits) and is often worth buying it in reels instead of by the metre as you will be surprised about how much you will use especially for the smaller sizes.

The wire can come in many colours which is important as you may want to set a certain colour for a common purpose ie black for negative. When it refers to TRACER it means that there is a single line of colour running down the length of the wire on top of the base colour. You could use this for indicators, say yellow base colour with a red tracer for the left hand side and a green tracer for the right hand side it would be easier to identify later.

Don't be fooled into buying OFC cable. OFC or Oxygen Free Copper cable goes on having a low oxidisation rate on the conductors. Oxidisation on the copper increases it's electrical resistance when it's joined to connectors so eliminating heating effects and power losses. For it's cost it is not worth it besides if you ask the guy why it is better than normal copper I can guarantee he won't be able to tell you why it is supposed to be better!! It's just overpriced wire that appealed to the boy racer in-car stereo crowd, it's all about bling.

The secret to handling wire is make sure it knows who's boss, no really if you pussy foot around with it it'll take you for a ride and won't do what you want it to do.

First of all is getting off the reel. There is a right way and a wrong way. the wrong way is to put the reel on the floor and pull the wire of it side ways. If you do this it puts a twist in the wire along it's length and you will never get it out and it'll knot up and generally annoy you. It's best to take it off the reel the same way it was wound on. Fix your reel on a horizontal pole either in a vice or between axle stands or make a purpose built frame. The advantage of doing this you can mount all your reels of wire on it and pull it off (fnarr,fnarr) as you need it. The first photo shows the wrong way and the second the correct way.

Say you have length of wire that is all kinked and got twists in it like this one

It can be straightened out. The way to do it is to clamp one end in the vice and wrap the other end around a pair of pliers.

Then by giving the pliers a hard yank a couple of times you will slightly stretch the wire and take the kinks out then cut the ends off. You may want to leave it to settle before you use it as the insulation may want to pull back a bit.

To straighten heavier pieces of cable they too can be straightened between two pieces of sheet wood. Roll the top piece over the cable and it will take any kinks out, this also works with brake pipe tube as well.

Occasionally you may be called upon to use a twisted pair, especially for engine sensors. A twisted pair is essentially two wires twisted together. The reason this is done is to eliminate interference by allowing the wires to cross couple and give a capacitive effect. Making twisted pair is really easy. Take two lengths of wire, clamp one pair in a vice and knot the other end.

Then take the other end and clamp in the chuck of a drill and wind the cables together until there is a really tight twist.

To stop the wires from un-twisting give the wires a sharp tug whilst still in the drill chuck. It's best to make a long length of twisted pair as it's easier to do and you can use for all the sensors rather than making it up as you need it.

In an ideal world I wouldn't have any joints at all. Joint are a weakness in the system they can deteriorate over time and can introduce resistance which can make your lights dim and draws excessive current causing all kinds of problems. However in the real world that we live in we need to join cables at together, there are may ways in which this can be done including crimps, soldering and I've even seen wires twisted together and covered in tape (I wouldn't recommend that!). Below I will cover the two main ways of joining cables and wires together.

Soldering is NOT the preferred method of joining cables in automotive use. The reason is when a solder joint is made solder wicks up the cable under the insulation adjacent to the joint and over time vibration can cause the cable to fracture at that point and eventually fail. If you must solder, the joint detailed below gives the strongest and most reliable joint of all and is the only one used in the aerospace industry, it is called the hook joint.

The tools you will need are as follows. Soldering Iron - preferably 25W or greater, solder, side cutters, wire strippers and pliers - I use circlip pliers as it gives a rounder hook.

First of all you need to strip about 12mm of insulation off the wire and twist the cores tightly together.

Then you need to tin the exposed copper cores. The best way is to ensure the tip of the iron is clean by wiping it on a damp sponge and not your jeans, then tin the tip of the iron with a small amount of solder. Apply the iron to the copper to be tinned and heat it for about 2-3 seconds and then start to feed the solder into the cable.

Making sure that it is throughly tinned all the way around and it is even. Don't over do it and end up with a fat blob of solder on the cable or under do it there should be no exposed copper left. At the same time make sure that you don't melt the insulation either! When you are satisfied that the cable is fully tinned pull the solder off first and then 1-2 seconds later the soldering iron, you should end up with something like this and then repeat it for the other cable.

Then using the pliers form a small hook on the part you have just tinned and repeat it on the other piece of cable.

Then hook the two pieces of cable together and using the pliers bend the hooks over each other. This joint is now secure and mechanically strong but still need to be soldered together.

Wipe the tip of the iron and re-tin the iron tip and again apply it to the joint and heat it up for 2-3 seconds before feeding in the solder and then feed in just enough to create a smooth shiny joint.

If the joint is dull or not shiny re-heat it again and let the solder flow and remove the heat again and hold it still until it sets. You will only get about 2 more tries at this until the flux in the solder boils off and you may need to use a bit more solder to allow the joint to flow properly.

To prevent the joint from possibly breaking it needs to be sleeved with some heat shrink. The best kind to use is the adhesive lined variety. Sleeve the joint with enough heatshrink to cover at least 15mm of insulation either side of the joint and shrink it down with a hot air gun. When it shrinks the adhesive melts and provides support for the cable and joint, it also makes it waterproof too.

If you have never used a soldering iron before be careful as it gets mighty hot and if you drop it don't try to catch it, as you will probably catch the hot bit!! You might want to wear safety glasses as well if you want. Solder is made of mainly lead so wash your hands after using it lead poisoning ain't fun, plus you might end up firing blanks as a result. Making the perfect joint also takes practice so have a go on some off cuts first or alternatively stick to crimping.

Crimping is by far the best and the quickest way to join cables. When choosing crimps go for ones that have been plated. Un-plated crimps tend to oxidise and will cause joints to fail over time as they age making your lights dim. There are two kinds of crimps that are used Insulated and Non-Insulated.

Insulated crimps come in three flavours red, blue and yellow. Each colour is for a different diameter cable. Red is for 0.4 to 1mm, blue is for 1.1 to 2.6mm and yellow is for 2.5 to 6.5mm. You can get rings, spades, butt splices - for joining wire, bullets and 'lucar' connectors. their use is pretty self explanatory choose the right crimp for the diameter of wire and the application you want and fit it. When stripping the wire you only need to expose about 8mm of the cores to get a satisfactory crimp. The most important factor is what you crimp it with. If you can try to buy a ratchet crimp as they give to most consistent results. Whatever you do DO NOT buy the cheap multi crimp tools that have a thousand and one uses. They are utter, utter crap if you've got one throw it away and buy a ratchet crimp tool you won't regret it. Maplin do one, order number is JH19V and it's only £20.

Uninsulated crimps invariably only come in the 'lucar' style for fitting on 1/4 inch terminals. Try to use these wherever possible as the crimping action is far superior to the insulated style crimps. Again buy a proper ratchet crimp tool as this will fold the connector in on itself providing a secure connection. Don't be tempted to use solder as well as this can weaken the crimps' join like when a solder only joint is made, after all crimping is a solder free connection. You also buy insulated covers for this style of crimp and will prevent accidental short circuits.

You may need to fit larger crimps on battery lead etc. A crimp tool is available for these but it is massively expensive and you may net be able to justify the cost for just a few crimp connections. Two options here for fitting them. One, find somebody who has one of the crimp tools get get them to do it for you. Or two, put your prepared cable inside the lug and use you bench vice. Take a bolt about 2/3 smaller than the diameter of your crimp and place on top of the crimp and use your vice to drive the bolt into the crimp thus crimping the wire in place don't go overboard as you might drive the bolt all the way through the crimp. It's not the ideal solution but it could be used if you are stuck. You might want to try it with a scrap piece first to see if it works for you.

A couple avoid though. Snap Locks that allow you to splice into a cable, avoid, they are in the same league as the cheap crimp tools. Terminal blocks, if you were even to consider using them, shame on you.

Making a wiring loom for your kit car is a really involved process and it's not something I would undertake lightly. I suppose if you were going to race your car then a loom could be justified. You could split the loom into three sections, the central core, rear section and front section all connected together with multi-way water proof connectors. It would ideal if you damaged your car you could just unplug the damaged section and replace it.

I've never made a loom for a car as such but I made up looms for equipment when I was an apprentice and the theory should carry over. The first thing I would do is make a loom for the car in situ. Running all the cables to their respective components and fitting it in place as how I would want it to be and temporally taped together. Then obviously I would test it in the car making sure that everything worked and it wasn't going to get damaged during normal use.

Then I would have to remove it from the car in one piece making sure that all the wire ends were identified. You would use this loom as your template to make the actual loom for your car. You would then need a piece of 6' x 8' sheet of chip board and some panel pins. I would lay your loom out so that the central core where all the wires went from the from of the car was in the middle of the board running length ways and then pull out all the lengths that go to the components at right angles to the central core.

Using the panel pins I would nail them in where the wires broke out of the central core either side of the wire and then nail in another pin where the connectors are maybe moving the pin an extra 50mm out to allow for some slack. I would repeat this all the way along the loom until every break-out and cable end was pinned.

Then I would write on the board what every wire does and where it goes, possibly giving it a unique number so I could see what end goes where and on a piece of paper the number, wire size, colour and what connector goes on the end. Once this was done and I was sure that every wire had been identified and checked again the template loom could be removed from the jig. What would be left would be a board with a lot of pins and a load of numbers.

Next I would start running in the wires in numerical order to make sure I didn't leave any out and checking them off the list as I went. The way I would do this if to wrap, with a couple of turns the wire around the pin where the connector goes and then up into the core and along and back down to the terminating pin where I would wrap a few turns or wire on before cutting it off. I would repeat this all the way along the loom until I had inserted every wire in place. After checking that every wire was in place and again I would start to bind the loom together.

Loom tape is like insulation tape except it has no adhesive on it and relies on friction to hold it together and in time it self bonds together. I wouldn't use self almagamating tape as it would make the loom unnecessarily heavy and when it comes into contact with oil turns into a messy, snotty goo. I would try as much as possible to bind the loom while it is still in the jig to keep it together.

Then I would snip the loom away from where the wire ends are wrapped around the pins and start to fit the required connectors and re-label the loom so I could identify where each wire goes. When it is free from the jig it then needs to be re-installed into the car threading it back in the same way it came out and then reconnecting it back to all the connections.

If that sounded like a lot of work then yes it is, you've got to be bloody sure that you've got it right as the only way to add another wire is to remove from the car and strip it down and start again. If that sounds like your bag then knock yourself out and carry on but for one off cars it's far more cost/time effective to run in the wiring as you go and try your best to make it look as clean and tidy as you can. If you want to make looms to sell to other builders then by all means make a jig and start knocking them out. It used to take me 15 hours to make a loom to fit in a 19inch rack so I would allow at least 120 hours to make a one off loom for your own use.