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Race Car Wiring for Beginners

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Before we consider designing the electrical circuits we are going to use in a vehicle and its attendant wiring loom, fuse-box and relay requirements, we need to look at some (basic) definitions:



Energy is defined as the work done when a force acts on any system. A force of one Newton acting over a distance one Metre produces one Newton-Metre of work, or one Joule of energy.


Power is defined as the rate of change of energy with time. One Watt = One Joule per second.


The Coulomb is a measure of electric charge and is approximately 6 ×1018 electrons.


Current is a measure of the displacement of electric charge. One Ampere represents the rate of one Coulomb of charge per second.


Voltage is defined as the potential difference across a conductor. One Volt occurs when a current of one Ampere dissipates one Joule of energy per Coulomb of charge.


Resistance is a measure of the degree to which an object opposes an electric current passing through it. Assuming a uniform current density, an object's electrical resistance is a function of both its physical geometry and the resistivity of the material from which it is made.


R = (L x P) / A


Where: L is length, A is cross-sectional area, P is the resistivity of the material.




It therefore follows that if one Amp is a Coulomb per second and one Volt occurs when one Joule of energy is dissipated per Coulomb, then the power generated is one Watt. Therefore:


P = I x V


Where P is Power, I is current, V is voltage (the easy way to remember this is Watts = Amps x Volts).




Ohm's Law states that, in any electrical circuit, the current passing between two points through a conductor is directly proportional to the potential difference (voltage) across the two points, and inversely proportional to the resistance between them. The mathematical equation that describes this relationship is:


V = I x R


Where V is voltage, I is current, R is resistance.





Using these 2 equations we can always work out the current requirement(s) for any given electrical component in a car. If we know the current requirement we can use the appropriate grade wiring, fuses, and/or relays.


This way, we can wire the car in such a fashion that we don’t:


1. Use over graded wiring (which is a weight penalty.

2. Use under graded wiring (which will lead to excessive current flow, overheating and, eventually an electrical fire.




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As stated earlier, in order to prevent wiring looms becoming overloaded (which eventually leads to a fire) it is essential that the current capacity of the wiring and fuses matches (or exceeds) the current requirement of the electrical component in question. Another thing that is essential in my view is to ensure that the current flow through any switches is minimal.


Unfortunately 60�s and 70�s cars weren�t really built with this in mind. For example in the headlight circuit the master lighting switch was placed in series with the lights. Now, this wasn�t too bad with respect to the lights of the day; sealed beam units were generally rated at 45/40W. Running a pair of headlights at full beam would give a current flow of (45/12) x 2 = 7.5A. This is all well and good given that standard 14/0.30mm.1mm2 cable has a current rating of 8.75A. (although I believe 7.5A through a grotty Lucas switch is a poor idea).


However, what happens when the vehicle owner decides that sealed beam headlights offer too little illumination and replaces them with halogens?


Generally speaking, halogen H4 bulbs are rated at 60/55W. In this case if we run a pair of headlights at full beam we get a current flow of (60/12) x 2 = 10A ! This is well above the current capacity of the wiring and will cause it to overheat, the insulation to break down, and the system to fail. But it gets worse than that, running on dipped beam gives a current flow of (55/12) x 2 = 9.17A which still overloads the wiring! I�ve lost count of the number of cars of Spitfire/Midget/G15 era where the behind dash wiring is heat damaged.


So how do we stop this occurring, and how do we keep the current flow through any switches to a minimum?



The answer is to use a relay, which is basically an electromagnetic switch. Anytime you want to switch a device, which draws more current than is provided by an output of a switch or component, you need to use a relay. A relay is basically an electromagnet switch that that uses a coil (when energised) to connect two contacts together to allow a current flow that is independent of the switching circuit.  The coil of most automotive relays draws very little current (less than 200 milliamps), whereas the amount of current that you can pass through a relay's common, normally closed, and normally open contacts will take up to 30A or 40A.


There are, fundamentally two types of relay


1. SPST Relay (Single Pole Single Throw).

2. SPDT relay (Single Pole Double Throw).


There are others, but I will discuss them later. For most basic vehicle applications all you need is a SPST relay.




The SPST relay consists of a coil (terminals 85 & 86), 1 common terminal (30), and one normally open terminal (87). Thus when current flows through terminals 85 and 86 then the coil energises and current can flow from terminal 30 to the output (terminal 87).




A variation of this is the fused SPST relay:




Using a SPST relay we can ensure that the high current to an item, eg headlamps, is independent to the switching circuit. Generally speaking, when wiring a relay like this I take a common feed from a dedicated fuse to both terminals 30 and 85 and I put the switch in the earth line to terminal 86. That way there is commonality in the wiring of every relay.



The SPDT relay consists of a coil (terminals 85 & 86), 1 common terminal (30), 1 normally closed terminal (87a), and one normally open terminal (87).


When the coil of the relay is not energised, the common terminal (30) and the normally closed terminal (87a) have continuity. When the coil is energized, the common terminal (30) and the normally open terminal (87) have continuity. Therefore we can use this relay as a changeover switch for applications that need change between two circuits � for example 2 speed wiper motors.



Again I tend to wire these the way I wire SPST relays with a common feed to terminals 30 and 85 and the switch in the earth line to terminal 86.


Relays come in a number of power ratings, the most common being rated at 30A and 40A, although you can get them rated at 20A as well as 70A, 100A and 120A, so you can always match the relay to the current requirements of any given circuit. One should note that there are two types of terminal layout and they should not be confused:



There are some other relay types available:


5-blade double make and break, to power two circuits which are independent when off:



5-blade twin make and break. Similar to the 4 pin SPST relay but with two blades at 87 feed to power 2 separate circuits which are not independent when off:


I see no need for these types of relay in a race car.


Finally you can get SPST and SPDT relays which incorporate a diode:


The diode is connected across the coil to provide a path for current when the current path to the relay is interrupted (i.e. the coil is no longer energised). This allows the coil field to collapse without the voltage spike that would otherwise be generated. The diode protects switch or relay contacts and other electronics sensitive to voltage spikes. In this case you would have to connect the 86 terminal to a +ve feed and the 85 terminal to earth so as to allow the diode to provide the return current path to kill any voltage spike when the switch is placed to the off position.


Again I see no need for these types of relay in a race car.



In the next instalments I shall consider  the selection of switches, wiring, and fuses.




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Before we turn to grades/types of cable, it is worth pointing out that there is a British Standard (BS-AU7) that details the colours to be used for various circuits.


This can be found here:




When converting a road going car into a race vehicle I try (wherever possible) to stick with the common wiring colours. Obviously if I’m building a single seat race car then this goes out of the window since there are far fewer circuits involved. In any case I draw up a wiring diagram, along with colour codes, for each circuit as I go along.


When addressing the grades of cable there are fundamentally two types, the older or ‘Standard’ grade cable, and the newer or ‘Thin Wall cable’. Both have advantages and disadvantages.


Standard Grade Cable


The disadvantages of this are that it is:


1. Heavy (a major consideration on a race vehicle).

2. Expensive.


The advantages are that it is:


1. Readily available (eg Halfrauds).

2. Comes in a wider range of current ratings than does Thin Wall.


Standard Grade cable comes in the following sizes (note the figures break down as: Number of Strands of wire in the core/Diameter of each strand in mm, Cross-sectional area in mm2, continuous current rating in Amps):


14/0.30mm, 1mm2, 8.75A

28/0.30mm, 2mm2, 17.5A

44/0.30mm, 3mm2, 27.5A

65/0.30mm, 4.5mm2, 35A

84/0.30mm, 6mm2, 42A

97/0.30mm, 7mm2, 50A

120/0.30mm, 8.5mm2, 60A

80/0.40mm, 10mm2, 70A


Thin Wall Cable




1. Lightweight.

2. Cost.




1. Fewer sizes (current ratings) available.


Thin Wall is a high performance cable, which is now OEM on most new vehicles.


So a bit of a ‘no brainer’ really!


The only reason I can think of to not using Thin Wall are its limited availability outside the trade and the cost of buying multiple reels to totally re-wire the vehicle.

Bearing this in mind, it is possible to strip down the original loom and re-use parts of it (although it is a messy job getting the tape off!). The problem here is in assessing whether the original (possibly 30+ year old) cable is still fully serviceable. On the grounds of cost, this is what I did with the race Midget – given that we have so few circuits on a race car (and a far smaller loom) the weight saving using Thin Wall is probably not that significant vs the savings in ££££.




With regards to high current capacity cabling, ie battery cables, starter motor feed, generator output, then my preference is to use:


37/0.71mm, 15mm2, 130A


This is good for most applications – especially if using something like a ‘Power Lite’ or ‘Edge’ high torque pre-engaged starter motor (or similar) which only draw around 60A.


If you are going to stick with the execrable Lucas 2M100 type starter motor (and I can’t see why you would on a race car since these are heavy and, more importantly, struggle to start high compression motors, especially when they are at working temperature) then you need to check, since on a 4-pot you will be close to the maximum working load of this cable. On a 6-pot you might need to use the heavier 196/0.40mm, 25mm2 170A cable.


See: http://www.powerlite-units.com/case.htm



With regards to cable connections/terminals you can either crimp or solder, again, both have advantages and disadvantages.


Soldering is generally stronger and more vibration resistant. However this pre-supposes you actually can solder properly without making ‘dry joints’, or increasing the cable resistance at the joint by over-soldering and/or introducing contaminants. If you are going to solder, use a high grade electronics flux cored solder for its freer flowing qualities, but also use a separate acid flux paste or liquid. I tend to use a liquid phosphoric acid flux, which I make up out of ‘Jenolite’ rust remover by watering down (with distilled water) to a 10% concentration. Believe it or not you can (at a pinch) actually solder using Coca-Cola as a flux owing to its phosphoric acid content (no wonder it rots your teeth)! Either way, ensure that the iron is ‘tinned’ properly!


The major problems with soldered joints are ensuring the joint is insulated (I use heat shrink synthetic rubber tube) and the fact that any connector (such as a spade terminal) will be of the non-insulated type. Soldering every connector also takes shed loads of your time!


For this reason, I only use solder for things like battery terminals and when joining wires together (I don’t like crimped ‘Butt Connectors’).


The advantages of crimped terminals are: speed of use and the fact that you can use insulated connectors. Beware! insulated terminals are colour coded as to their maximum current rating:


Red = 5A

Blue = 15A

Yellow = 30A


As with cabling ensure you match the terminal to the current draw of the circuit in which it is placed!


If you are going to crimp then whatever you do, do not use those cheap nasty lightweight crimping pliers sold in places like Halfrauds. The chances are that, using crap tools like this, you will never get a good crimp and the terminal will eventually fall off with (possibly) disastrous results. Get yourself a good, heavy duty, ratchet action, professional crimping tool with built in ‘forming dies’ – if you are going to crimp, this is the only way to do it!




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A word on Circuit Protection:


In the foregoing we discussed the use of relays to protect the switching circuit to any electrical device. Now you might think this will give you protection against over current, but a moment’s thought shows that not to be the case.


Firstly, while it is true that a relay will keep the switching circuit separate from the power circuit, if an earth line fault occurs in the switching circuit (terminals 85 and 86), then the relay will go live and feed current to the power circuit irrespective of switch position. You can reduce the risk of this happening by placing the switch in the relay’s earth line and by keeping the wiring run between the relay and switch as short as possible (thereby reducing the risk of the wiring chafing through). Additionally you should always use a high quality switch (qv later).


The main risk comes from an earth line fault in the output side of the power circuit (terminals 87, 87a). Say, for example, you get a 20A current in the feed to the fuel pump (which normally draws less than 5A), the relay will quite happily pass this current (since the chances are it is rated at 30A or even 40A), at best you will fry the wiring to the pump, at worst you will fry the pump. Now if this happens, your shiny race car will come to an ignominious halt mid circuit. Failing to finish a race is bad enough, but an over heating fuel pump is a recipe for disaster!


Worse still might be an earth fault in the feed to your Electric Water Pump (EWP), these usually draw around 7A. Hit the EWP with 20A and it’s not just the pump that will fry, but your mega ££££ sooper dooper race motor!


So, we need to protect the power circuit of the relay. We can do this in one of two ways, either by using a fuse, or a circuit breaker. As ever, both have their advantages and disadvantages.


Note: Whichever circuit protection you choose, I suggest you put it upstream of the relay (that is to say in the live feeds to the relay) since , not only will this protect the relay output, but also the switching circuit.






The advantages of fuses are:


1. Lightness.

2. Cost.

3. Wide availability.


The disadvantage of a fuse is in the very nature of how it works. Basically all a fuse is, is a fusible link that is designed to melt when heated to a given temperature.


Now, we have already shown that P = V x I. So, for a given voltage, as the current rises so does the power, hence there is a heating effect with current rise. Knowing this, we can set our fusible link to blow at a given current flow.


The problem comes in that the Temperature rise is not directly proportional to the Current, but worse still, it takes a finite amount of time for the fusible link to heat up and then melt. As a result fuses are given a ‘Continuous Current’ rating and a ‘Fusing Current’ rating. It is, therefore, imperative when choosing a fuse for any given application, that it is matched to that application not only in terms of the application’s current draw, but also in terms of the maximum (if momentary) current that application can withstand.


Generally speaking, automotive fuses fall into three categories:


Blade Fuses:


These are my preferred type. Standard Blade Fuses are 14mm and are colour coded as follows:




Continuous Current     Fusing Current       Colour

1A                              2A                    Black

2A                              4A                    Grey

3A                              6A                    Violet

4A                              8A                    Pink

5A                              10A                   Tan

7.5A                             15A                  Brown

10A                             20A                  Red

15A                             30A                  Light Blue

20A                             40A                  Yellow

25A                             50A                  White (Clear)

30A                             60A                  Light Green



Also available are ‘Automatic Indicating Glow Fuses’. These are standard blade fuses with a bulb built into the fuse. If the fuse blows the bulb lights, giving a clearly visible indication of which fuse has blown. To be honest this seems to me to be a bit of an indulgence!


Mini Blade Fuses (11mm) are also available in the same ratings/colours as Standard Blade Fuses. These may be an advantage on a race car (less weight), albeit they are more difficult to source.


Also available are Maxi Blade Fuses. These are 27mm and colour coded as follows:




Continuous Current     Fusing Current       Colour

20A                                      40A                   Yellow

30A                                      60A                   Green

40A                                      80A                   Orange

50A                                      100A                  Red

60A                                      120A                  Blue

70A                                      140A                  Tan

80A                                      160A                  White (Clear)



Again, these are harder to source, and, given their extreme current ratings I see no need for them on a race car.



Glass Cartridge Fuses


These will be recognisable as the OEM fitment on 60’s and 70’s Sports Cars. They come in two sizes, 25mm and 30mm.





Continuous Current     Fusing Current

7A                             14A

10A                            20A

15A                            30A

20A                            40A






Continuous Current     Fusing Current

1A                             2A

2.5A                            5A

5A                             10A

7.5A                            15A

12.5A                           25A

17.5A                           35A



I don’t like this type of fuse. Firstly they are not colour coded which leads to the possibility of the wrong fuse being inadvertently used. Secondly, the available fuse boxes tend to be of dubious quality at best. Most importantly, however, they do not resist vibration well, so, unless you mount the fuse box with some sort of damping, your race car will suffer all sorts of electrical ‘gremlins’. In my view, this sort of fuse has no place in a race car unless ‘Historic Regulations’ demand it.



Ceramic Fuses


Also known as ‘Bosch Fuses’, these are cylindrical in shape but have the fusible link running along the outside of the fuse. They come rated as follows:




Continuous Current     Fusing Current         Colour

5A                             10A                     Yellow

8A                             16A                     White

16A                            32A                     Red

25A                            50A                     Blue



I have used these in the past, but the problems are (again) availability coupled with limited current ratings.



Circuit Breakers


Unlike a fuse, a circuit breaker operates (almost) instantaneously, this is why they are in widespread use in the Aviation industry.


The disadvantages are price, weight, availability, limited range of rated currents, and the fact that with a circuit breaker people are tempted to just reset the breaker when it trips rather than finding the root of the problem.


Circuit breakers generally fall into one of two categories:


Magnetic Circuit Breakers use a solenoid whose pulling force increases with the current flow. The circuit breaker contacts are held closed by a latch, as the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows the contacts to open by spring action.


Thermal Circuit Breakers use a bimetallic strip, which heats (and thus bends) with increasing current, as it does so it releases the latch.  


Automotive Circuit Breakers are generally of the ‘Thermal’ type.


The commonly available ones fit straight into standard blade fuse boxes and are rated as follows:







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Switches and other bits �n bobs:


We have discussed at length how we must match our wiring, relays and fuses to the electrical load they will experience. What about switches?


We need to ensure that the switches we use are not going to fail internally and are not going to arc (which will eventually cause failure), we also need to ensure that they are vibration resistant so that they remain in the selected position. Having your fuel pump switch turn off of its own accord because you have bounced across a corner apex curb is not conducive to winning races!


The best switches available are those used in aircraft. To this end I always junk the crappy OEM Lucas switches and utilise Aerospace quality �toggle switches� rated at 20A or 25A. These have a nice, positive, �throw� and are resistant to vibration.


Originally I would beg/borrow/steal them from a contact who worked as an Aerospace engineer. If you don�t have such a contact you could try the maintenance section at your local airport, or alternatively, your local flying club (if they are Certificated to carry out �C of A� examinations/maintenance).


Similar type switches are available from �Vehicle Wiring Products�:




Use the 20A or 25A rated ones and beware of using cheap copies!


Generally speaking you will only need single pole OFF-ON switches for single relay circuits. External lighting (headlights) may need a single pole OFF-ON-ON switch, windscreen washers (if fitted) utilize a OFF-MOM switch (momentarily on when pressed and held), two speed windscreen wipers (if fitted) require a ON-OFF-ON switch to control the pair if speed circuits.


For �critical� circuits that might need shutting down in a hurry (Ignition), I like to use a coloured switch �gate� such as is used to �guard� the weapons switches on NATO aircraft:


The �gate� makes the switch easy to recognise in a hurry. It means you have to take an extra positive action to turn the switch on (which guards against inadvertent switch selection), and by �flipping� it down it immediately turns the switch off, breaking the circuit.


With regards to the engine starting system, I see no reason to use an ignition key, plus if the barrel is in a steering lock it will need to be removed anyway (race regulations). To this end I use a push button starter such as this:



With regards to an electrical �Master Switch�, the following is taken from last year�s RACMSA Competitors� Yearbook (the �Blue Book�). Note, they have revised the various sections for 2008 but I have not yet received my copy:



Racing Cars:


J 20.11. Circuit Breaker and Ignition Components


20.11.1. Be equipped with an externally operated circuit breaker having positive ON-OFF positions clearly marked [Q8]. An external circuit breaker is not mandatory on open cars of periods A to F, but is strongly recommended. An internal ignition switch must be operable by the driver when normally seated irrespective of whether a safety harness is worn or not.



Sprint/Hill Climb Cars:


L 10.6. Electrical


10.6.1. Must be equipped with an Ignition Cut-off switch having positive �On-Off� positions clearly marked. The ignition cut off system must be operable by the driver when normally seated with seat belts secured. It must also isolate electric fuel pumps.


10.6.3. An external circuit breaker to Q8 is mandatory for all cars except open cars of periods A - E (Section P) and cars licensed for road use (when it is recommended.



Competitor Safety:


Q 8. External Circuit Breaker


The circuit breaker, when operated, must isolate all electrical circuits with the exception of those that operate fire extinguishers. The triggering system for the circuit breaker in saloons should be situated at the lower part of the windscreen mounting, preferably on the driver�s side or below the rear window. On Open cars it should be situated on the lower main hoop of the Roll-over Bar on the drivers side or at the lower part of the windscreen mounting (as above).


Alternatively on cars of periods A to F the mounting point may be mounted approximately vertically below the line of the scuttle on the driver�s side. The location must be identified by a Red Spark on a White-edged Blue triangle (12cm base), and the �On� and �Off� positions clearly marked.



From this, the general consensus is that the external �Kill Switch� should also be accessible inside the car. Personally I don�t like attaching �Bowden cables� to the kill switch, since they can be confused with the Fire Extinguisher cable. To this end I wire two switches in series � one external and one internal.


When it comes to these switches, in my view, the only type to use are those by �Autolec�. Cheap copies are known to fail, and if you are racing on the continent then, chances are, your kill switch will need to be FIA Certified.


There are two types:


The first type can be used where you are either running a �Total Loss Wiring System (ie you have no generator) as on most single seaters, and as we do on the Midget (since we run short �sprint� type races of 10-15 laps), or you are running a dynamo (because of �Historic� regulations):


This type is a simple ON-OFF switch that isolates the battery.



If, however, you are running an alternator the simple switch noted above will cause two problems. Firstly the voltage spike that occurs when you turn the switch off (with the motor running) will eventually damage the alternator. Secondly, and more importantly, dependent upon how the alternator is wired, it is possible that when switching off with the engine running, the alternator will continue to power the coil via leakage through the exciter circuit. The net result being that the engine will continue to run. This is a definite way to fail pre-race scrutineering!


For alternator-equipped cars you need this type:


With this type of switch, turning it to the OFF position causes a positive break in the ignition circuit so the engine will die. Additionally there is a resistor wired in series that acts as a �shunt� and prevents any voltage spikes.




My preference here is to use a battery from the Varley �Red Top� range:




Originally developed for Aviation use, these batteries are lightweight (lightness is a good thing for a race car!) and have a high cranking power (essential given that hot, high compression, wide cam duration, race motors can be pigs to start).


The other advantage of these is that they are gel filled, so, unlike a conventional lead-acid automotive battery, they can be mounted in any orientation and, unlike a lead-acid battery, they don�t leak so they can be mounted inside the cockpit without having to use a �battery box�.


I�d suggest using a Red Top 30 on a total loss system.


With a generator I�d suggest a Red Top 15.


From here on I intend to start �putting it all together� with some basic wiring diagrams. In the meantime here is the �switchery� on the race Midget:

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I will make some attempts to repair this thread some time.


It seems Debs, in some bitter, pathetic effort to get the last word has removed the images from her photobucket site...


All a bit pathetic if you ask me.


Any further technical articles will require images uploaded onto my server. We live and learn.


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