Interior Lighting for Designers

resistance or interrupted by an open switch,
there will be little or no current (amps) even
though the potential to produce it (volts) is
high.

Wiring
Materials that electricity flows easily through
are called conductors. Materials through
which it does not flow easily are calledpoor
conductors, or insulators. All metals are
good conductors: silver is the best conduc-
tor, but it is too costly for most wiring pur-
poses; copper is an excellent conductor and
is used widely.
Almost all wire is encased within an
insulator, which confines the current to its
metallic conductor. Wire that is wrapped
with a poor conductor, such as rubber or
synthetic polymers, is calledinsulated wire.
Before connections are made with insulated
wire, the wrapping is removed from the ends
of the wire.
Insulated circuit wires are sometimes
covered by a mechanically protective con-
duit for installation in buildings. Flexible,
nonmetallic sheathed cable (“romex”) and
flexible, metal sheathed cable (“BX”) are
often used in single-family homes. Commer-
cial installations use wires inserted in flexible
metal conduit (“greenfield”), or in rigid elec-
trical metal tubing (“EMT”) for long runs.

Circuits
Direct current(dc) is electric current that
always flows in one direction. Alternating
current(ac) also moves in a single direction;
however, that direction is reversed at regular
intervals. Alternating current is the prevailing
electrical current in use today (figure 11.2).
Acycleincludes the complete set of
values through which the alternating current
passes. The unitHertz(Hz) is used to mea-
sure the number of times the cycle occurs
each second, which is also called thefre-
quencyof the cycle. Power distribution sys-

tems operate at 60 Hz in the United States

and 50 Hz in most other parts of the world.

Series circuit

If one lamp fails in an inexpensive strand of

Christmas tree lights, the remaining lamps in

the strand go out. When the tungsten wire in

one lamp breaks, it causes a break in the cir-

cuit because its filament is part of the con-

ductive path carrying current to other lamps.

Lamps connected in this way are wired

inseries. All lamps in a series circuit must be

of the same wattage; if a lamp of different

wattage is substituted, the remaining lamps

will grow brighter or dimmer due to the sub-

stituted lamp’s resistance. A series circuit is

therefore said to beload-sensitive(figure

11.3).

Parallel circuit

If one lamp in figure 11.4 goes off, all of the

others remain lighted; the current still flows

to the other lamps and the circuit remains

complete. These lamps are wired inparallel.

Since the voltage of the circuit is present

across all branches of the circuit, several dif-

ferentloads(for example, a 60 W lamp and

a 100 W lamp) may be connected to the

INTERIOR LIGHTING FOR DESIGNERS

Figure 11.2Alternating current.