We will now move on to looking at specific components now we understand how to design and install wiring looms correctly, this section will cover the common components used in vehicle electrics, and their configurations.


Switches are the building block of vehicle electrics, they come in a multitude of styles, types, and varieties, it is these we need to understand, and how they work, or can be made to work in our circuit designs.

Basic switches are called single pole, they have two connections on the back and simply switch an electrical component on or off, they will have a maximum current rating. Single pole is often the confusing term, it simply means a switch switches one wire, obviously it is the live or positive in our application.

Double pole switches have four connections on the back, they can switch two circuits simultaneously, this is usually the live and earth feeds to a circuit we are supplying, equally it could switch two live circuits simultaneously. Care needs to be taken when wiring double pole switches, it is easy to wire them incorrectly and short circuit the circuit if it is wired incorrectly, especially if we are switching live and earth poles simultaneously. A simple test with a multi meter will confirm which poles switch to which outgoing terminal.

Multi pole switches are the most complex; they may switch a number of outgoing terminals in specific sequences, it is important to select these with care for their intended application/s.

Changeover switches have three terminals on the rear, they simply switch a common feed to two possible outgoing terminals, they do not switch to both simultaneously, and they will switch to one or the other outgoing terminal.

Switch applications will be indicated by their working sequence, this is:


Off/on means the switch has one position other than the off position, it is on.

On/off/on means the switch has three positions, off is the central position, up or down switch it on.

Off/on/on means in the first position the switch is off, the second position is on, and the third position is on; but when the switch is in the third position it switches both position two and three on, this is the type for vehicle headlights.
Position two would be the sidelight switch position and position three would be the headlight switch position, if this only switched position three on, and position two off we would switch on our dipped headlights, but extinguish the sidelights.

Mom/off/on means the switch is off in the central position, if it is pushed up it will only operate while it is held in this position as once released the spring loading would push it back to the central position, pushing down means it is on permanently.

Push button switches are basically momentary switches, they only operate while they are held, they are useful for horns, windscreen washers, or even starter switches if you want a more modern vehicle layout.

Switch styles are wide ranging, they may be basic lever switches, rocker switches, dolly switches, toggle switches, rotary switches, or pull switches. They come in a wide variety of styles.

Illuminated switches incorporate an additional terminal on the rear, this is the earth connection terminal, this is necessary as the switch needs an earth supply to complete the internal illumination circuit and light the switch when it is switched on.

Many other types of switch are available and tend to be for specific applications, these may be indicator switches, isolator switches, ignition switches, hazard warning switches, and key switches for security purposes.

Power ratings are essential pieces of knowledge, all switches come with a maximum amperage or current they will handle, the higher the rating, the longer they will last. Switches are often used in conjunction with relays, a switch should never be used to handle a high current, even though it is within its designed power rating, relays should always be used to handle any load higher than a couple of amps.
We have all seen the botched wiring jobs, a boy racer with considerable enthusiasm and very little knowledge attaches two driving lights to the front of his car, he runs a cable from the battery to a dash mounted switch, then to the lights. Never run any high powered application through a dash mounted switch, never run any high power item to the dashboard, full stop.


Relays are nothing more than an electrically powered switch, they are used in conjunction with an ordinary switch using a low power output, to switch on a high power output item; relays come with outputs ranging from 20 amps to in excess of 120 amps.
Relays have a minimum of four electrical connections, most aftermarket relays and most manufacturer fitted relays follow a standard pattern, irrespective of the make or type of relay, some come with more than four pins.

Two pins are a live and earth, these use a low power to operate an electromagnet or solenoid inside the relay, this closes a set of contacts to make a circuit to control or switch a high current such as a set of high powered auxiliary lights.

Pin 85 is always the live from the controlling switch, pin 86 is always connected to earth to complete the circuit, these are the industry standards although some car manufacturers use different numbers to confuse, and ensure a vehicle returns to the dealer for repairs.
Pin 30 is always the high power supply to the relay, this is sourced from our fused supply, pin 87 is always connected to our load or application such as our auxiliary lights.
These are called four pin make and break relays, when power is applied to the control circuit it pulls the solenoid closed to make the contacts, this switches the relay on, and when the control circuit power is switched off a spring opens the contacts, breaking our lights.

Five pin variants are essentially the same as the four pin relay, except instead of pin 87 we have two output pins, these are pin 87 and pin 87, or pin 87 and 87b both these are powered when the control circuit solenoid is operated, this gives us two individual outputs. These are preferable to the basic 4 pin relay as two outputs means we can run each output to two individual fuses, this is beneficial for applications where we have lighting which is always in pairs. Each light having its own fuse and individual wire means if a fault develops such as a blown fuse, or a wiring fault; means it is considerably easier to diagnose the fault and we only lose one light instead of both.
These types of twin output relays are called 5 blade double make and break as they have twin output pins.

Five pin changeover relays are exactly the same as our twin output relay except they change from one output pin to the other, it has no off switch, it merely powers one or other output pins depending upon whether control circuit power is on or off. These relays are used where a two speed item is to be controlled, these may be two speed windscreen wipers, or an auxiliary or supplementary two speed electric fan.

Micro relays are essentially the same as regular relays except that they are smaller and rectangular, rather than square; they follow their own standard pin configurations and have a low power output handling capacity when compared to conventional relays.

High power relays are exactly the same as conventional relays, higher rated relays will have pins 30 and 87 replaced with a thread and a nut, and this is to allow ring connectors to be fitted to handle the higher loads with a greater contact area.

Relays may be purchased with a mounting bracket attached, this is simply a metal bracket with a hole, a hole is drilled in a panel and the relay is bolted to the panel and fitted with ordinary 6.35mm female blade connectors. Some relays come with loose brackets, these snap into an aperture in the relay body if required, or left off if they are mounted in a relay holder, some do not come with brackets at all.

My preference is to mount the relays in relay holders, these are similar in operation to our wiring multi plug connectors, the un insulated female blades are crimped to our cables and snapped into the relay holder, these are all five pin holders. If we use a four pin relay these fit, but will need the fifth aperture left unconnected, basically these holders are universal. Relay holders come as individual single units, single interlocking units which can be snapped together to form a block of relay holders, or a dedicated relay box; there are trailing plugs which are similar to relay holders, but connect to relays mounted with brackets. Relay mounting boxes will be covered in more detail later in another chapter as they are available as modular units.
I always advocate using relay holders, these are fastened by bolting them to a solid surface, this ensures not only a professional finish, but also allows them to me mounted into waterproof boxes for additional protection and security on off road vehicles.

Fuses and Fuse Boxes

Fuses are often called thermal links in technical circles, as this is basically how they operate by using the heat created by an overloaded circuit to blow this fuse or thermal link, thus protecting our circuit and vehicle from damage.

Fuses come in many different types and have differing names, the first types are called a strip fuse, or mega fuses; these are the very high power fuses ranging to in excess or 100 amps rating which connect directly to a battery. These sit in their own holder and provide a main fuse for a number of circuits which will be fused individually; due to the considerable loads on modern vehicles these are deemed a necessity for additional safety.

Blade fuses are the most common fuses in automotive use, they have a plastic top in a variety of standard colours to denote their fused rating and two flat prongs on the bottom, they simply pull out, and then new ones are pushed in to replace them. These fuses are cheap, standardised, and readily available.
Mini blade fuses are identical in every respect except for size, these are obviously smaller, but are now becoming popular in modern vehicles as they allow more fuses to be fitted to a designated space or size due to more compact vehicles. It is not uncommon for a mixture of standard and mini blade fuses to be used in one vehicle.
Maxi blade fuses are again identical, but about twice the size of standard blade fuses, they handle considerably higher loads, and are now replacing the mega or strip fuses in many vehicles as they are more compact than these fuses.

Standard blade fuses ratings are designed to blow at twice their rated capacity, and come in the following sizes and colours:

Colour Working rating Blow rating

Violet 3 amps 6 amps
Pink 4 amps 8 amps
Tan 5 amps 10 amps
Brown 7.5 amps 15 amps
Red 10 amps 20 amps
Light blue 15amps 30 amps
Yellow 20 amps 40 amps
White 25 amps 50 amps
Light green 30 amps 60 amps

Automatic indicating glowing fuses are available for standard blade fuses, these glow if the fuse is blown, so are a useful piece of equipment for a confined fuse box, or those who operate considerable electrical equipment on a vehicle.
Glass cartridge fuses are a clear glass tube with a metallic cap on either end, the fused link is contained within the clear glass tube, these come in a range of ratings and are available in 25 or 30 mm lengths. These fuses snap into circular round spring clips on a fuse board or in a fuse box, they are used mainly on older or classic vehicles, but ancillary wiring may contain these fuses.

Ceramic fuses contain a tubular plastic body with pointed ends and a groove running down its length, the groove contains the fusible link while the pointed ends contain the contact area, these fuses snap into U shaped holders.

Fuses may be mounted in many ways, the simplest way is an inline fuse, these are individual fuse holders which simply insert into an accessory line or cable, and many variations are available including interlocking units to form a fuse board.
Fuse boxes are available as a box which houses a number of fuses, these hold varying numbers of fuses, but all come with standard connectors and lids to protect the fuses; these are a better option where space is limited. Fuse boxes offer more flexibility than an inline fuse as they house a number of fuses in one location, this is considerably easier then scrabbling around a wiring loom in the dark should a fuse blow, most come with side or bottom entry points for the wiring. Having this form of fuse box allows for easier installation of a wiring loom when a number of accessory circuits are to be installed to a vehicle as each fuse has its own power entry and exit point which makes testing/isolation of a circuit easier.
Having a fuse box for accessories means they may be made into modular units with relay sockets or dedicated relay boxes, in confined spaces these are mounted to suit the space available, particularly odd shaped spaces.

Individual relay holders are available with three outlets for three fuses; these are one modular moulding which has a variety of uses.

Fuse boxes are available for all types of fuse, this means that irrespective of what type of fuses your vehicle is originally equipped with, your accessory circuits will match the original fuses, so no having to buy and carry more than one fuse type.

Strip or mega fuse holders are also available; this means an accessory circuit can be fitted with the additional security of a high power master fuse adjacent to the auxiliary battery

Modular Fuse/Power Boards

Modular power boards are the latest aftermarket item to come to the market, they offer the flexibility to mount a number of electrical items such as a variety of relays and fuses in differing sizes within one enclosure. This gives a neat, professional appearance to our accessory wiring, or allows us to rewire a vehicle totally, and fit a number of accessories into the new wiring system to form one fully functioning power control centre. This removes all the various add on’s, and allows us to remove the compromised wiring, and build our own circuits up to a quality for very reasonable costs, it also enables us to retain the existing wiring looms in the standard wiring colours.

Modular boards may also be called battery distribution boxes as they are essentially the same thing, but the battery distribution boxes tend to come with a fixed number of pre-determined types of installations, so lack the outright flexibility of modular systems.
Most aftermarket modular systems come as 4 or 5 way base units, this means they have either 4 or 5 apertures to accept the various modules they will hold, and all come with a top cover which will accommodate the modules and items to be fitted. Modular systems come with a bus bar system, bus bars are a common metal strip which connects to the whole module to power it, while being large enough to handle the power requirement of the particular module.
Modules generally come as relay modules which will hold a varying number and type of relays, these may be standard 4 or 5 pin relays, micro relays, or high power relays, and these can connect to the next module in a variety of ways or configurations.
Fuse modules come in different types to house the different fuse types, it may be that you want to house the standard fuses in one module and add mini blade fuses in the next module to fit more into the space to add additional circuits. Should you want to provide the additional security of maxi fuses to protect differing groups of circuits it can be done, this removes the additional maxi or mega fuse mountings and incorporates them into the one modular unit.

Connecting the various modules together is done with the bus bars, they are purchased separately to suit your chosen wiring system, and can be configured to suit whatever layout you choose.

Terminals are selected to fit the module you select, these can be bridged to provide a group of fuses fed from one relay, these are beneficial where a set of four roof lights are installed. One higher power relay switches them all on and off, the outgoing terminal is bridged to one fuse in the module with a bus bar, this is then bridged to three additional adjacent fuses, this gives each light its own fuse and individual feed wire.

Modular systems best feature is there flexibility, and the ability to upgrade or alter wiring to much higher, and safer standards of today’s vehicles wiring systems.

Ancillary Systems

Many electrical systems have to conform to current legislation, this particularly applies to additional lighting which must have warning indicators to inform the driver they are switched on, and full beam headlights have a dashboard warning on vehicles.
Gauges may need supplementary wiring to illuminate them when the vehicle lighting is switched on, or additional power sockets may require adding to a vehicle, due consideration must be given to these when designing a wiring system.

Cable Chart

Standard Cable

Size CSA Maximum Current
14/0.30 1 mm 8.75 amps
28/0.30 2 mm 17.5 amps
44/0.30 3 mm 27.5 amps
65/0.30 4.5 mm 35 amps
84/0.30 6 mm 42 amps
97/0.30 7 mm 50 amps
120/0.30 8.5 mm 60 amps
80/0.40 10 mm 70 amps

Standard cable has now been superceded by the newer high performance thinwall cables, these are the most popular types of cables now sold; it is important to be able to distinguish between them as older vehicles will be wired with the standard cables.

Thinwall Cable

Size CSA Maximum Current
16/020 0.5 mm 11 amps
32/0.20 1 mm 16.5 amps
28/0.30 2 mm 25 amps
44/0.30 3 mm 33 amps
56/0.30 4 mm 39 amps
84/0.30 6mm 50 amps

Back to Section 1 | Proceed to Section 3