Vehicle Electrical Distribution System

The increased application of electrical and electronic systems in automobiles has neces­sitated the use for complex electrical distribution systems. A typical mid-priced, medium-size European car of the 1990s uses more than 1.5 km of wiring and more than 2000 terminals, connectors and relays, and the weight of such an electrical distribution system exceeds 30 kg. The wiring harness is the major component of the electrical distribution system. This contains bundles of cables, which connect all of the electrical parts, sensors and actuators with electronic control units in a vehicle. It serves two primary functions. These include (i) to act as a power distribution network, and (ii) to act as an information distribution network.
The majority of vehicle breakdowns are caused by electrical failure, so vehicle reliability is critically dependent upon good wiring harness design and implementation. Generally, the wiring harness is divided into a main harness that runs the length of the vehicle connecting the battery to the charging system, vehicle interior, lighting and accessory circuits, and various sub-harnesses, such as door wiring, tailgate wiring, and roof wiring. For facilitating vehicle assembly and servicing, connector blocks are used for connecting the sub-harnesses to the main harness.


Electrical cables used in automobiles contain several stands of annealed copper wire bunched together and encased in an insulating covering (generally PVC-polyvinyl chloride) of 0.2-0.4 mm thickness. Each strand of copper wire is typically about 0.32 mm diameter. The number of strands determines the size and hence current-carrying capacity of the cable. Applications requiring greater flexibility, such as in a tax-door sub-harness, flexible cable made from 0.18 mm annealed copper strands, are used. For high temperature applications (usually in the engine compartment) ordinary PVC insulation is not suitable, and hence special plastics such a PTFE, PFA, FED or X-ray treated cross-linked PVC or polyethylene are used.
Generally, cables are specified as per the strand diameter and number of strands. A cable with specification as 7/0.3 is made from seven strands each of 0.3 mm diameter. If an insulation thickness of 0.35 mm is provided, a cable normally has a finished diameter of about 1.6 mm and is suitable for carrying current up to about 4 A.
To reduce costs, manufacturers use the thinnest possible cable for a particular application in a vehicle, without causing too much voltage drop. As a rule of thumb, a maximum voltage drop of 5% (i.e. 0.6 V in a 12 V system) is allowed for general lighting and control circuits. Normally, a current rating of about 8.5 A per square millimetre of cable cross-section is assumed. A reduced rating of about 6 A per square millimetre is considered for continuously loaded cables. Table 13.1 summarizes typical sizes, current ratings and resistances for commonly used wires in automobiles.

Table 13.1. Sizes and Current Ratings of Wires used in

Automobile Wiring Harnesses.
Wire Size Current Rating (A) Resistance per metre Application
7/0.3 4.0 0.032 Side/tail lamps,
9/0.3 5.5 0.027 signals, music
14/0.3 9.0 0.017 system
28/0.3 17.5 0.009 Light, heater, motors
44/0.3 25.0 0.006
65/0.3 35.0 0.004 Alternator, main
84/0.3 42.0 0.003 supply to fuse
97/0.3 50.0 0.002 box
120/0.3 60.0 0.002
37/0.9 170.0 0.001 or less Starter motor

Harness Routing.

Generally the bundle of cables with a diameter in the range 10-30 mm is used for the wiring harness. This bundle is carefully routed around the vehicle, avoiding the locations having trim screws, mounting bolts and extremes of temperature (exhaust system, air-conditioner com­ponents). At regular intervals, clamps are used to secure the harness properly to the body-shell so that it is against stressing of the wires. Rubber grommet is fitted at the places where the harness passes through a metal panel thereby protecting the harness against chafing and preventing the ingress of moisture and dirt.
The places where many cables run (for example, the rear light clusters), ribbon cable is preferred to use. Ribbon cable is simply a number of conductors laid side-by-side to make a wide, flat, harness and it is easy to conceal this type of cable underneath carpeting and along flat body panels.

Cable Colour Coding.

Some form of cable colour coding is provided with the automobile wiring harnesses for fault diagnosis and repair works. The colour codes used invariably vary from one manufacturer to another and sometimes between different models from the same manufacturer. Therefore, it is important to refer to the vehicle repair manual before taking up any electrical work.
Most coding systems use a base cable for each cable and then add a contrasting tracer. These colours then are indicated by a letter code on the vehicle wiring diagram, which is usually in black and white. For example, a cable marked ‘BO’ on a wiring diagram indicates black with an orange tracer. To bring down costs, the ‘tracer’ can be a thin painted line added only where the cable enters a connector. Some manufacturers use just a few base colours and then code the wires by adding small coloured sleeves at each end. Table 13.2 provides some of the main colours used for the principal circuits.

Table 13.2. Colours for Circuits.

Circuit BSI Letter Code British Letter Code German
Earth connections black B SW
Ignition circuits white W WS
Main battery feed brown N BR
Side lamps red R RT
Auxiliaries controlled by ignition switch green G GN
Auxiliaries not controlled by ignition switch purple P VI
Headlamps blue U BL

In view of the different standards used by manufacturers, it is wise to consult the wiring diagram for the vehicle whenever a particular cable or circuit has to be identified.

Circuit Numbering.

In addition to colour coding some manufacturers also use numbers to indicate the circuits. Table 13.3 provides the main numbers used as recommended by the German DIN standard.

Table 13.3. Terminal Marking According to DIN Standard.
Circuit Number Application
1 Ignition, earth side of coil
4 Ignition, HT output
15 Ignition, feed (un-fused)
30 Feed from battery
31 Earth
51 Alternator output
54 Ignition, feed (fused)
56 Headlamps
58 Side/tail lamps
75 Accessories

Sub-circuits are identified by adding a number or letter after the main number; e.g. 15-4 is a sub-circuit based on circuit 15, the ignition feed circuit.

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