Wire Size

wire is made by braiding or twisting a number of solid wires together into a single conductor insulated with a covering of colored plastic, as shown in Figure 6-6. Most automotive electrical system wiring uses stranded wire, either as single conductors or grouped together in harnesses or looms. For more information about wire types, see the section on “Copper Wiring Repair” in Chapter 6 of the Shop Manual.

Printed circuitry is a thin film of copper or other conductor that has been etched or embedded on a flat insulating plate (Figure 6-7). Acomplete printed circuit consists of conductors, insulating material, and connectors for lamps and other components, and is called a printed circuit (PC) board. It is used in places where space for individual wires or harnesses is limited, such as behind instrument panels.

Figure 6-6. Automotive wiring may be solid-wire conductors or multistrand-wire conductors. (DaimlerChrysler

Corporation)

Automotive electrical systems are very sensitive to changes in resistance. This makes the selection of properly sized wires critical whenever systems are designed or circuits repaired. There are two important factors to consider: wire gauge number and wire length.

Wire Gauge Number

A wire gauge number is an expression of the cross-sectional area of the conductor. The most common system for expressing wire size is the American Wire Gauge (AWG) system. Figure 6-8 is a table of AWG wire sizes commonly used in automotive systems. Wire cross-sectional area is measured in circular mils; a mil is one-thousandth of an inch (0.001), and a circular mil is the area of a circle 1 mil (0.001) in diameter. Acircular milmeasurement is obtained by squaring the diameter of a conductor measured in mils. For example, a conductor 1/4 inch in diameter is 0.250inch, or 250 mils, in diameter. The circular mil cross-sectional area of the wire is 250 squared, or 62,500 circular mils.

Figure 6-7. Printed circuit boards are used in automotive instrument panels and elsewhere. (DaimlerChrysler

Corporation) Figure 6-8. This table lists the most common wire gauge sizes used in automotive electrical systems.

(DaimlerChrysler Corporation)

Figure 6-9. This figure shows the relationship between current capacity and resistance as the crosssection of a conductor changes.

Gauge numbers are assigned to conductors of various cross-sectional areas. As gauge number increases, area decreases and the conductor becomes smaller (Figure 6-9). A6-gauge conductor is smaller than a 3-gauge conductor, and a 12-gauge conductor is smaller than a 6-gauge conductor. You learned in Chapter 1that as the cross-sectional area of a conductor decreases, its resistance increases. As resistance increases, so does the gauge number. Also, because the currentcarrying ability of a conductor decreases as the resistance increases, a conductor with a higher gauge number will carry less current than a conductor with a lower gauge number.

Remember that the wire gauge number refers to the size of the conductor, not the size of the complete wire (conductor plus insulation). For example, it is possible to have two 16-gauge wires of different outside diameters because one has a thicker insulation than the other. Twelve-volt automotivezelectrical systems generally use 14-, 16-, and 18-gauge wire. Main power distribution circuits between the battery and alternator, ignition switch, fuse box, headlamp switch, and larger accessories use 10- and 12-gauge wire. Low-current electronic circuits may use 20-gauge wire. Lighting other than the headlamps, as well as the cigarette lighter, radio, and smaller accessories, use 14-, 16-, and 18-gauge wire. Battery cables, however, generally are listed as 2-, 4-, or 6-AWG wire size.

The gauge sizes used for various circuits in an automobile are generally based on the use of copper wire. Alarger gauge size is required when aluminum wiring is used, because aluminum is not as good a conductor as copper. Similarly, 6-volt electrical systems require larger-gauge wires than 12-volt systems for the same current loads. This is because the lower source voltage requires lower resistance in the conductors to deliver the same current. Generally, 6-volt systems use wires two sizes larger than 12-volt systems for equivalent current loads. Future 42-volt systems will not require as large a wire diameter as the current 12-volt system. Generally, a 42-volt system will use two sizes smaller than 12-volt systems for equivalent current loads.

Wire Size Matters The following drawing shows how a large wire easily conducts a high-amperage current, such as you would find going to a starter motor.The heaviest wires are often called cables, but their purpose is the same.On the other hand, a comparatively light wire tends to restrict current flow, which may generate excess heat if the wire is too small for the job.Too much current running though a light wire may cause the insulation to melt, leading to a short circuit or even a fire.

Correct Wire for Load Easy Current Movement

E E E E E E E

More Heat Wire Too Small; Restricted Current Movement

Metric Wire Sizes

Look at a wiring diagram or a service manual for most late model vehicles, and you may see wire sizes listed in metric measurements. Metric wire sizes have become the norm in domestic automotive manufacturing due to the global economy. For example, if you look at a wiring diagram for an import or late-model domestic vehicle, you will see wire sizes listed as 0.5, 1.0, 1.5, 4.0, and 6.0. These numbers are the cross-sectional area of the conductor in square millimeters (mm2). Metric measurements are not the same as circular-mil measurements; they are determined by calculating the crosssectional area of the conductor with the following formula: Area = Radius2 × 3.14. Awire with

a 1-mm cross-sectional area actually has a 1.128-mm diameter. The following table lists AWG sizes and equivalent metric wire sizes.

AWG Size Metric Size Table

AWG Size (Gauge) Metric Size (mm2) 20 0.5 18 0.8 14 2.0 12 3.0 10 5.0 8 8.0 6 13.0 4 19.0

Wire Length

Wire length also must be considered when designing electrical systems or repairing circuits. As conductor length increases, so does resistance. An 18-gauge wire can carry a 10-ampere load for 10feet without an excessive voltage drop. However, to carry the same 10-ampere load for 15feet, a 16-gauge wire will be required. Figure 6-10is a table showing the gauge sizes required for wires of different lengths to carry various current loads. Wire lengths are based on circuits that are grounded to the vehicle chassis.

Figure 6-10. Wire gauge table: As wire length increases, larger-gauge wire must be used to carry the same amount of current.

Special Wiring

Although most of the electrical system is made up of low-voltage primary wiring, special wiring is required for the battery and the spark plugs. Since these wires are larger in size than primary wiring, they are often called cables. Battery cables are lowresistance, low-voltage conductors. Ignition cables are high-resistance, high-voltage conductors.

Battery Cables

The battery is connected to the rest of the electrical system by very large cables. Large cables are necessary to carry the high current required by the starter motor. Figure 6-11shows several kinds of battery cables. Twelve-volt systems generally use number 4 or number 6 AWG wire cables; 6-volt systems and some 12-volt diesel systems require number 0 or number 1 AWG wire cables. Cables designed for a 6-volt system can be used on a 12-volt system, but the smaller cable intended for a 12-volt system cannot be used on a 6-volt system without causing too much voltage drop.

Battery installations may have an insulated ground cable or one made of braided, uninsulated wire. The braided cables or straps are flat instead of round; however, they have the same resistance and other electrical properties of a round cable of equivalent gauge. Most battery cables are fitted at one end with a lead terminal clamp to connect to the battery, although many import cars use a spring-clamp terminal. The lead terminal is used to reduce corrosion when attached to the lead battery post. Atinned copper terminal is attached to the other end of the cable to connect to the starter motor or ground, as required.

Ignition Cables

The ignition cables, or spark plug cables, are often called high-tension cables. They carry current at 10,000 to 40,000 volts from the coil to the distributor cap, and then to the spark plugs. Because of the high voltage, these cables must be very well insulated.

Years ago, all ignition cables were made with copper or steel wire conductors. During the past 30