in small buildings where the primary requirement is
lighting. It can also be used to operate 120-volt
appliances and motors. The two-wire system consists
of one ungrounded, insulated conductor and one
identified (grounded) conductor, which is called the
neutral. This system is limited to the operation of 120-
volt equipment and relatively light loads of 50 amperes
or less. Larger loads can be better served by another type
of wiring system. The two-wire system requires the use
of an equipment-grounding conductor that may be a
separate conductor, conduit, or other recognized means
Figure 5-2 shows the three-wire, single-phase
wiring system, used both for lighting and power. This
system, commonly known as "220," "110," (single
phase), uses two ungrounded conductors and a neutral
conductor, which is grounded. It provides 110 volts
between each ungrounded conductor and the neutral. It
also provides 220 volts between the two ungrounded
conductors. This system is used for lighting and power
loads, such as air conditioners and heating equipment.
The three-wire, single-phase system provides up to
twice the power that is available from a two-wire system
with conductors of the same size provided that the load
is balanced between the two ungrounded conductors.
The 110-three-wire, 220 volts is the most common
system used in residences today. This system also
requires the use of an equipment-grounding conductor.
A second type of three-wire system is the three-
wire, three-phase system shown in figure 5-3. This
system is used to furnish power, usually 220 volts, to
installed equipment. If some lighting is needed, 220-
volt fixtures and bulbs can be installed, but be aware
that there are certain restrictions placed on lighting
circuits exceeding 120 volts. When substantial
amounts of power are required, higher voltages, such as
480 volts ac, maybe provided with this type of system.
Figure 5-2.Three-wire, single-phase system.
Figure 5-3.Three-wire, three-phase system.
The last type of wiring system in common use is the
four-wire, three-phase system (fig. 5-4). This system
has three ungrounded phase conductors plus a grounded
neutral. This system is a combination of light and
power and offers quite a cost reduction over a three-
wire, single-phase system for the same amount of
power. The usual voltages are 120/208 or 120/240
VAC, depending on the type of transformer connections
The service entrance serves to bring power from the
service drop to the panelboard inside the building. One
of the components of the service entrance is the
conductors through which the current flows. The
conductors may consist of individual wires run through
a protective raceway, such as rigid metal conduit,
electrical metallic tubing, or rigid nonmetallic conduit.
The raceway provides the conductors with protection
from both physical and weather damage. Power may
also be brought into the building by means of service
entrance cable. This cable does not need raceway
protection unless it is likely to be physically damaged
by abrasions or by being struck by passing equipment.
Figure 5-4.Four-wire, three-phase system.