Thank you. I needed that. 
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Story by Matt Smith • Photos As Credited
We’ve all been spooked by electrons in one form or another, from harmless static shocks to threatening lightning. However, electrons also have the ability to do some pretty amazing things: light our homes, power air compressors, weld together metal, and perhaps most importantly, provide the spark needed to make our engines purr. Most of us are able to use the factory wiring harness–either as delivered or with some modifications–but what happens when you have to start from scratch? Whether you’re building a formula car, dedicated racer, $2000 Challenge machine or something similar, don’t fret. Wiring an entire car can be an intimidating task, but it’s much easier when you have the knowledge and tools to get the job done right.
Looking ahead is the only way to make this a painless and safe project. Starting with a plan–and it can be created on paper or in pixels–will keep you from having to improvise and tangle with rat nests. Plus, if you ever sell the car, your scrupulous notes and documentation will impress the new owner. Start the planning process by listing everything that needs to be wired into the car’s power supply, and group these items into their respective systems. Typical cars have a starting system, ignition system, dash gauges, switches, a fuel pump, headlights, running lights, brake lights and sometimes an ECU. These systems will be your starting points in designing the circuits. It’s also very important to know the current (in amps) that each of these devices demands. This information will help you select the proper fuses, wire sizes and wire lengths, as well as decide when to use a relay. Some devices may have their current requirements already calculated and displayed. Other devices, such as lights, often require a bit of simple math to determine current requirements. Watt’s Law describes the relationship between work, voltage and current:
In this formula, I indicates current (in amps), P indicates power (in watts), and V indicates voltage (in volts). If you’re running 12 volts–common for most of our cars–and know the wattage of each device, you can use this formula to find their current draw. Other devices will only list their resistance, so you’ll need to use Ohm’s Law for them:
The R stands for resistance (in Ohms). Unfortunately, devices like starters and coils rarely list their wattage, resistance or amp ratings. To determine the electrical demand for these unrated devices, you’ll need to do some digging. After calculating the current requirements for each device, determine how much current each circuit will need. There’s one more calculation to consider before moving on, but this one’s comparatively easy: Can the alternator supply enough electricity for all the circuits, including any future additions?
Now it’s time to sketch out the plans for each circuit in this new electrical system. Graph paper is low-tech and very effective. Computer-aided design programs are more flexible but more difficult to learn. Major items that will need homes in your wiring system: the battery, alternator, coil, fuse panel and kill switch. Don’t forget the various devices–lights and so on–plus the switches needed to operate them. Necessity will dictate some positions. Most alternators and coils, for example, are in the engine compartment. And while the battery can go practically anywhere, don’t forget that its weight can impact chassis balance. Also, be mindful of where you’ll put your fuse panel and how many fuses will be required. Adding fuses may increase complexity, but those fuses serve an important purpose: Should something short-circuit or overload, the fuse will sacrifice itself to protect the rest of the system. A simple fuse can also prevent fires, component failures and major meltdowns. The best way to run power to the fuse panel is directly from the kill switch. This ensures that the ignition, lights, fuel pump, and all other electrical devices can be completely disconnected in the event of an emergency. During this critical planning stage, also be aware of the importance of grounds. The positive side of the wiring receives much of our attention, but a ground is required to make a complete circuit.
Once you have a basic plan and know the requirements for each circuit, it’s time to start linking everything together. The difference between an amateur and professional electrical installation is in the details, so selecting the correct connectors, wires, fuses and relays is critical. Cable ties, loop clamps, high-temperature cable sleeves and firewall grommets can cut down on potential problems while allowing for a nice, clean installation. Use different colored wires for each circuit, just like the real car companies do. This will make later repairs much easier. When using connectors, make sure they’re the right size for the wire you’re using. Many electrical failures can be traced to a connector that’s too small. Neatness counts here, too. Using the proper connectors and shrink-wrap tubing rather than twisting wires together and wrapping them with duct tape can make the difference between a podium finish and a DNF. Your system’s switches, relays, and fuses also need to be up to the electrical demand of each circuit. For example, using a relay rated for 40 amps is a recipe for disaster if the circuit is going to see 80 amps. Race car supply houses typically carry quality electrical components, but car audio stores can help, too. Either one will stock a collection of fuse blocks, distribution blocks, switches, and various sizes of wiring. Avoid cheap connectors: They tend to break, or fail to provide a solid crimp.
Wiring should never be overlooked: It’s one of the most critical elements of building a successful race car. You may have been zapped in the past, but don’t let an electron’s negativity get you down. When you’re finished, you may be shocked at how easy wiring can be.
If you’re wiring from scratch and aren’t sure where to source a fuse box, you can always get one from any car in a junkyard. This has several benefits: The wiring is already terminated, there are tons of fuses and relays to choose from, and they’re easy to mount. Plus, they’re inexpensive, which is perfect if you’re rewiring a LeMons car. Since they’re built to OE specs, they’re often reliable and safe to use. For those seeking a more professional installation, purpose-built fuse boxes are available from various race car supply companies.
As complex as wiring can be, you only need a few tools in your arsenal to do a professional job. Knowing how to use these tools is key to a bulletproof electrical system.
A ratcheting crimping tool will guarantee solid crimps every time. However, the cheaper and simpler crimping tools can still provide an acceptable crimp if used correctly. The rounded portion of the crimping tool should clamp on the split side of the connector, with the bump side of the crimp tool pressing in on the solid side of the terminal. Using this orientation can save you hours of frustration, as you’ll create a more reliable crimp. If your connector doesn’t come with its own insulation, make sure to use a small section of shrink tubing to cover the exposed metal.
Any connections that may be exposed to the elements should use Weather Pack Connectors. These seemingly simple connectors keep debris and water away from your connections. Unfortunately, they require a special tool. A Weather Pack tool will crimp those special connectors and their weatherproof insulation onto the wire. These connectors are then inserted into the sealed Weather Pack housings to ensure a corrosion-free connection.
A soldering gun is an electrical device that heats connections to the appropriate temperature so that solder can join them. This method is much more dependable than using butt terminals. Simply strip the wires to be joined and slip a piece of heat-shrink tubing on one of them. Next, twist together the exposed wires and apply heat with the soldering gun. After a few moments of heating, apply solder to the joint. If the solder doesn’t instantly melt, apply more heat. Spread the silver solder along the length of the joint, making sure it’s coated entirely; copper should not be visible. Next, clean the newly soldered joint with flux cleaner, available at any hardware store, to remove the potentially corrosive flux. Finally, slide the heat-shrink tubing over your new connection and shrink it to insulate the joint.
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Thank you. I needed that.
Pro tip: don’t just buy a spool of 12 gauge red and use it for everything including grounds. 
Patrick said:Pro tip: don’t just buy a spool of 12 gauge red and use it for everything including grounds.
This was written for our first Lemons car, back when they would literally penalize you for spending money on different colors of wire.
In reply to Patrick :
Worse would be what my old boss did in a chemical plant (he thought he could fix anything)
He ran out of red, and decided green was just as good to use for hot legs. 
Tom Suddard said:Patrick said:Pro tip: don’t just buy a spool of 12 gauge red and use it for everything including grounds.
This was written for our first Lemons car, back when they would literally penalize you for spending money on different colors of wire.
It wasn’t a shot at you guys, it was a shot at the guy that wired my first WRX. Not even black tape at the end denoting a ground, just berking red everywhere
Patrick said:Pro tip: don’t just buy a spool of 12 gauge red and use it for everything including grounds.
I have read - and I'm not sure of the truthfulness, or of the age from when it came, if it is true - that every wire on F1 cars is the same color, the rational being that it forced those working on the systems to refer to the wiring diagram, and not assume "oh, that red must be a constant +12V." I'm not sure I believe this, but in thinking about some wiring I have coming up in the hopefully near future, I think I might adopt a variation on this. That is, something like all constant 12V are red, all power grounds are black, all sensor signals wires are blue, all computer outputs / commands are purple, etc.
When you're finished with all that, also put together two binders with full wiring diagrams of each system, fuse reference chart, one chart alphabetized by color for wire function, and another chart alphabetized by function for wire color. Strap one into the car and keep the other on the shelf or in your hauler. The amount of time that you can save by simply looking up "black-red" in a chart to find out what the heck that wire goes to is surprising.
In reply to kabohm :
Can I use that to wire fishing lights on my canoe?
NorseDave said:Patrick said:Pro tip: don’t just buy a spool of 12 gauge red and use it for everything including grounds.
I have read - and I'm not sure of the truthfulness, or of the age from when it came, if it is true - that every wire on F1 cars is the same color, the rational being that it forced those working on the systems to refer to the wiring diagram, and not assume "oh, that red must be a constant +12V." I'm not sure I believe this, but in thinking about some wiring I have coming up in the hopefully near future, I think I might adopt a variation on this. That is, something like all constant 12V are red, all power grounds are black, all sensor signals wires are blue, all computer outputs / commands are purple, etc.
FWIW, a lot of those wires have labels stamped on each wire. So you can see what it is when you look closely at it.
We had some test cars built that way, it was was horrible until someone figured that out. Even then, you can tell the lack of skill of the engineering company that did it (who I have bitched about more than once on this board, but that was 20 years ago now).
I've done more than one major wiring harness...
One thing missing- a peg board or a large piece of plywood where you can lay out the harness. On that, you use screws to go around corners, and map out exactly where stuff goes. That way the entire harness is laid out in an easy location. This is how it's done in real prototypes, and is pretty easy.
That does require a good drawing with dimensions of where stuff is and where wires go. From a hole in the bulkhead to corners to where one wire comes off the harness to it's plug. Repeat.
Doing that helps a lot.
Also, it's a LOT easier to NOT design your own harness. Which means if you can use the diagram of the car you have, and just take out what's not needed- do that. Let some other engineer do the hard work. If you have a simple digital copy- then load it into paint brush, and carefully use the eraser to take out circuits you don't need.
For the actual harness- for those of us who have old cars, you can contact Molex https://www.molex.com/molex/home and on that page, find the exact PINS you will use, and ask for an engineering sample of them. They will happily send you 100 of each. I did my GTV race car that way (using the original connector housings).
And while soldering should help, do bear in mind that no OEM solders wires. A good crimp connection is superior. So spend your money on a good crimping tool vs. soldering connectors. BTW, a crappy crimp sucks, and soldering is superior.
Lastly- don't be afraid. It's not as bad as it looks.
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