By Al Campion


So you have planned how you are going to build your car. You have decided on  type of suspension, the engine and drive train components, the paint and upholstery.

But have you planned the Electrical System?

It is best to start planning the electrical system well before the car is complete. Many people think of the electrical system about the time they install the engine and want to hear it run for the first time. Usually the installation of the wiring is one of the last things to do just prior to installing the seats and upholstery. You also might want to run some of the wiring prior to installing the sound proofing materials. Many times people find themselves installing the wiring in the car after its painted and find that they need to drill a hole in the firewall or kick panel to route the wires or they didn't consider the size of the fuse panel. If you plan ahead as you assemble the cars mechanical components you can save yourself some grief during the wiring job.

Give some thought to where and how you will run the wiring. Will you be hiding the headlight and signal light wiring up under the front fenders where the splash shields or inner fender wells attach? If you are you will need some type of clamps (such as Adel clamps or welded tabs) to hang the wiring on. If you are hiding the wire to the taillights in a boxed frame, how will you get it into the boxed frame and how will you repair it if something happens down the road? What size is the fuse panel and where will you mount it? Will you need to fabricate and weld a mounting pad for the fuse panel? What holes will you need, where and what grommets will you use?

Here are some other ideas to make you think about the wiring as you assemble the car.

Number one in my book is will it be 12 volt or 6 volt? Most cars built or rewired today are going to use a 12-volt system. But if you are rewiring an older car and keeping it 6 volts you will need to use larger wire than you would in a 12-volt system. DO NOT make the mistake that a few have made and use a wiring kit designed for 12 volts on a 6-volt system. Can you say Fire? Why in the world would anyone want 6 volts? Well if you are building a period correct nostalgia car you will probably want original 6-volt equipment. And if you're wanting to keep that genuine '40 Ford dash intact and use the original gauges you have think about converting the gauges to operate on 12 volts or figure out how to supply 6 volts to them.

A few thoughts on converting from 6 volts to 12 volts for those of you who are working on an older 6 volt vehicle and converting to 12 volts. Most original 6 volt generators can be converted to 12 volts. 6-volt starters will work just fine on 12 volts.  Sometimes you will have to change the solenoid to 12 volt but I have found that the 6 volt unit will work for quit a while.  Dash gauges are another story. You basically have two choices, 1) send the gauges out and have them converted to modern 12-volt operation (very expensive) or 2) use voltage reducers or resistors to reduce the voltage to 6 volts at the gauge.  If you are changing from 6-volt positive ground to 12 volt negative ground you will also have to wire the gauges opposite of the original wiring for them to function properly. There are several ways to reduce the voltage to the gauges. The old way was to use a resistor to reduce the voltage but for this to work properly you had to know the amount of power the gauge would draw in amperes and do a little calculating to figure out the correct resistance. Today we have the modern semiconductor and there are Zener Diodes and solid state transistorized voltage reducers that work very well. and Centech sell the diodes for $ 3.00 each.  Speedway Motors sells a transistor called a "Runtz" that also works but they are a bit more expensive. For reducing the power to the 6-volt motors for things like the heater, electric wiper motors, window and seat motors the best bet is to use 1 to 1.5 ohm ceramic resistors specifically built for the job. also sells these.  Sometimes you will need to use two resistors in a parallel configuration to get the proper resistance. If you need help figuring this out please contact me and I can help.

Most rodders and custom builders want the battery hidden out of sight but accessible. For heaven sake do yourself a favor while you're building the rest of the car - Locate the battery and test fit it and the cables before painting the car. There is nothing I hate worse than having to cut up a floor pan or battery box after the car is finished because nobody checked to see if the battery or cables would actually fit.

Battery size (as in Amp Hours) is another thing to think about along with the output (Amperage) of the alternator you plan on using. The number of electrical options you are installing determines both. High output ignitions, stereos with monster amplifiers, halogen light bulbs, power seats, power windows, power antenna, power door locks, power trunk opener. All of these things need to be considered. If the alternator's capacity is not large enough you will end up with a dead battery.  The size of the cable between the alternator and the battery has to be sized to the output of the alternator.  If you put a 140 amp alternator in a car with a 10 gauge wire between the two you will burn up the wire if the battery goes dead and the alternator goes to full charge.

On most GM vehicles the charge wire, ignition feed wire, fuse panel feed wires and the alternator sense (# 2) wire are spliced together in the wire harness and a fusible link at the starter solenoid protects the entire system.  On most aftermarket kits and home built harnesses the charge wire is run from the alternator to the starter solenoid without any protection.   I like to install a 16 gauge fusible link at the alternator to protect the charge wire to the battery solenoid.  Also DO NOT wire the car without some sort of fuse or fusible link between the starter solenoid and the wire to the fuse panel and ignition switch.  All it takes is a short to burn the car to the ground.  I personally prefer a fusible link as they work like a slow blow fuse but others have their own preferences. 

Battery cable size is another thing to think about along with where it will be located and mounted to the body or frame. I tend to go with heavier than needed cable on everything I do. On short runs I might use something like a number 2 gauge battery cable but if I'm running cable all the way from the trunk on a race car I will probably use a number 1/0 welding cable. I use welding cable because it has more individual wires for the size of the cable (less resistance) and is more flexible than battery cable. Just remember the farther you run a cable (wire) the larger it needs to be.

Battery cable connections MUST be clean and tight. I always install internal and external star lock washers on all connections made to a chassis ground after properly cleaning it to bare metal. Then coat with dielectric grease or paint.  Most of the electrical problems that I find on street rods, hot rods and customs are related to poor grounding. Always ground the battery to the engine block and then ground the engine to the chassis and to the body. If you are set on using the frame for a ground cable, (battery mounted in the rear of the vehicle) make very sure that you properly connect the cable from the battery to the frame and another cable from the frame to the engine. Down the road when the car doesn't want to start check the cables where they bolt to the frame and the engine. They will be loose, dirty and/or corroded. One good way to use the frame for a ground is to weld a 3/8" stainless steel bolt to the frame. Tape the threads off when painting or coating the frame. When you're ready to connect the cable. Apply some dielectric grease to the threads, run a nut down to the weld, then install a heavy flat washer followed by an internal/external star washer, followed by the battery cable, followed by another star washer, flat washer and nylock nut all liberally coated with dielectric grease.

Wiring Harness and Fuse Panel

Ok now the good stuff. What should you choose for a wiring harness. If you're thinking of going out to the local wrecking yard and using something off a donor vehicle you're sick. This is admittedly the cheapest way you can wire a car. But your time has to be worth something. By the time you drive to the yard, find something that might work, spend an hour or more getting every thing you think you might need and drive home, you will have spent half a day and how many ?? bucks and all you have to show for it is a pile of spaghetti. Plus it will take you another 4 hours to sort it all out before you can even start to install it.  You can also buy new wire by the roll and build your own harness but you will have a lot more money invested than a kit if you purchase multiple colors and do it right.

There are way too many, nice aftermarket wiring harnesses available that make the job a simple one. I like to use a fuse for every major circuit in the vehicle. I admit this is a little over the top, but when a light bulb shorts out and blows a fuse the entire car doesn't blackout. So again do the planning and decide what you need. 

The next option is the less expensive panels and harnesses most of which are already pre-terminated at the panel. Most of these use an ATO/ATC or mini blade fuse panel with the turn and hazard flasher on the panel. Painless Wiring, Ron Francis, EZ Wiring, Centech, Haywire, and Enos Custom Components all come to mind and have good instructions on how to install their specific kits or panels.

A few thoughts on some of the wiring kits available on the market. The available kits mainly fall into two categories: Modular and Pre-terminated. The modular kits take a little bit longer to install because you have to terminate both ends of each wire. The nice thing about these kits is they allow you to run the wiring anywhere you see fit. I like these systems for this very reason. Ron Francis Wiring has been building these kits longer than anybody in the business. Centech Wiring also has a very nice 13 fuse 23 circuit kit with a high quality printed circuit board with a good looking stainless steel cover that only measures 3.5 X 6 inches. These kits are about as complete as they come.

On the less expensive end are the GM style fuse panel kits with the wires pre-terminated in the panel like Painless Wiring and Centech's GV-16.. These panels can be installed a little quicker because of this design. We stock the Centech GV-16 with and without switches. The balance of the wiring, which is not connected to the fuse panel, has the necessary terminals machine crimped and dip soldered in place with heat shrink tube installed.

In the last few years there has been an influx of import kits made in China and everything from soup to nuts available on Ebay.   These kits are inexpensive but  beware of quality control and complete kits.  If you are buying a kit off Ebay make sure the seller will refund all of your money including the shipping if you are not satisfied with the product.  The majority of the kits I see on Ebay are not complete in any way.  Remember the old saying " You get what you pay for".

So which way do you go? I can't tell you. You will have to decide for yourself. I hope this information helps. I can say this much, plan out your wiring as well as you plan out the design of the rest of the car. Be safe and install quality products. Most of all have fun and drive careful.

Thanks for visiting and putting up with my opinion.



Need any help with your wiring problems just call or e-mail me. Any and all comments are welcome.
If you feel something should be added please let me know.


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