Cars are wired with a 12 volt, negative ground
circuit. The car battery is the base of the circuit;
everything is wired through the battery.
However the battery doesn't have enough power
to provide electricity for a running car, it has just
enough to start the engine. The bulk of the
power actually comes from an alternator, which
is spun by the engine's accessory belt.
The voltage isn't a measure of how much
electrical power is produced. The power is
measured by amperage or amps for short. The
whole electrical system is 12 volts, but each
component uses a certain number of amps. A
radio uses more amps than a light bulb.
Most circuits in a car are limited to about 10-20
amps. Each circuit is limited by a fuse, which is
a short piece of wire that melts when it exceeds
a certain amperage, which disconnects the
circuit. If a circuit were to draw too much
amperage it could overheat and cause an
electrical fire. Fuses are cheap protection and
should be used on every circuit. The maximum
amperage is labeled on each fuse.
Cars are wired with separate circuits for each
major function. Each circuit is connected to the
fuse box, which contains all the fuses and serves
as a distribution block for power from the
battery. This way each circuit is independent,
and if one fails the others remain operating as
long as the battery is connected. Every circuit
depends on the battery.
Part of each circuit, the negative or ground
component, passes through the metal body of
the car itself. The body is treated as if it were a
wire, which is connected to the negative battery
terminal. This simplifies wiring, because the
positive wire for each circuit winds through the
car as in normal wiring, but the negative end of
the circuit is simply attached to the closest metal
body part (usually via simple screw).
Blade Style Automotive Fuses