Wiring harness question.

My buddy wants to sell me a 21 circuit wiring harness with fuse box from a hot rod project he never finished. Is a 21 circuit harness enough for a GT40? I will be running electric radiator fans of course, electric fuel pump, electric oil pump and possibly electric oil cooler fans.
Shawn I would think so, we start with a basic 7 circuit system. I always add a marine block with additional circuits because it has a grounding block attached. It allows me to isolate easily if I am at the track and have shut everything down except cooling fans.............m
RCR supplied a basic front engine Hot Rod loom with my car kit. I ended up taking it all apart. The fuse block was good but to route the wiring neatly around the car proved too great a compromise so I built my own loom using much of the material. This route means that I have been able to include plugs to easily disconnect dash and front and rear bodywork. Also you can mount all the relays in best positions and include any extra features you may need. BUT be prepared for many weeks of design and build fun! BUT the joyful feeling when it all works is why we do all this!!
Cheers Roger Allen


Before you buy, it may be wise to lay the harness out on your chassis and see if the lengths will accommodate a rear engined car and your desired placement of the fuse box etc.
Hi Shawn,
I've got my fuse box on the right hand side of the driver's floor just forward of the gear shift. I mounted the battery on the right hand side in the rear, the RCR has a box in the chassis sponson just to the rear of the right hand fuel tank. The main loom runs from the battery area through the lower right hand side of the seat back bulkhead through a bank of three heavy duty toggle switches up to the fuse box. From the fuse box the loom goes into multi plugs in the right hand inner wheel arch area to feed the dash and the car. A shield in the footwell keeps the plugs away from my legs and feet.
My main strategies in the layout were short runs, good access and serviceability.
Cheers Roger
Thanks Roger! That seems like a sensible way to make your main wire runs. I will definitely keep that in mind when I begin to map out my wire runs. I hope I don't have to do too much splicing into this harness.
Good point Pat! Where is a good place for the fuse box?
I did all the wiring on my car.
I would do it differently know.

A power supply at the front middle and rear.
Same with the fuse boxes X3, front middle rear.
I think it would be the only way I would do it next time because it uncomplicates things.

It would have made things way easier, you end up with a lot in one place and its less wiring and cleaner to break it into 3 modules .

So...Jim would you run three main power wires from one battery to these three fuse boxes? Or run one wire inline to all three power points?

Howard Jones

I wired my car myself. So I really don't have much to say about modifying a painless harness to work. My experience is that modifying an existing harness is more work than doing one from scratch.

In any case be sure you don't bury any splices in the side pods where you would have to take it all apart to fix a bad one. Have a look at my build log and you can see how I ran conduit in the side pods so you can replace/add wires easily. They have already saved me a couple of times.

Count up circuits. lights front high, low and rear (3), dash, ignition, fuel pump, fans, That's seven. You get the idea. Look up current requirement for each and add about 40%. Example would be a 6 amp fuel pump would be fused at 10 amps or use the same fuse rating that comes with the device if they supply a inline fuse holder.

Add up the fuses and size the main feed wire for the total.

Here's a chart for 12volt DC

12 Volt Wiring Tech: Gauge to Amps | Offroaders.com

I’m building a Tornado GT40. It came with a wiring harness but it had circuits I didn’t need and I needed some circuits that it didn’t have. All they provided was an illustrated wiring diagram that showed how the wires connect to the switches and loads.They did not have a circuit schematic; so, I drew a circuit schematic to better understand the wiring that I had to do. In addition to the basic lights, etc. for the car, I also needed to wire the ECU so I also drew a schematic for the ECU.


Howard Jones

If I was doing a street car with a engine management computer and other non race car components I would segregate the standard circuits like the lights, horn, dash power from the computer systems. Then run a third power block for the pumps, AC coil and ignition starter coil.
Run a separate power cable to the ignition box from the main 12+ point at the master breaker output.

So now you have three separate power cables (the forth being the ignition power) running from the battery thru their circuit independent protection (I like circuit breakers instead of fuses).

Why? A car battery provides a huge amount of surge protection. The inductive components like the starter coil, starter motor, AC coil, and pump motors can induce voltage spikes in their wiring circuits. Running separate power feeds (and keeping the wires apart) will eliminate this to a very large degree.

This would look something like this: One big cable running from the battery + to the hot side of the Master breaker. Then from the output side, three separate smaller cables running to each of the common + for the circuits breakers or the + of the fuse blocks. Then each fuse block would feed their responsibility's.

The alternator is treated like a second battery. It's output is tied into the + side of the master breaker (same electrical point as the + side of the battery). This configuration will cut ALL voltage sources from the rest of the system.

The above is the basic main power system for just about any car you could build.