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 of grounding.
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 used.
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.Continue Reading