Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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94 Chapter Six
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.
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). A 6-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 1 that 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 automotive
zelectrical 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. A larger gauge size is required when aluminum
wiring is used, because aluminum is not
as good a conductor as copper. Similarly, 6-volt
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.
E
E
Correct Wire for Load Easy Current Movement
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 (mm 2 ). 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 = Radius 2 × 3.14. A wire with
E
E
E