INTERIOR ELECTRICAL WIRING SYSTEM

Figure 9-7.-Wiring diagram of the four-wire system in figure 9-6.

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as the resistance increases, one must increase the VOLTAGE, or ELECTROMOTIVE FORCE (E), accordingly. The increase in voltage makes it possible to use smaller wires or cables, thus minimizing the support for aboveground transmission lines, and consequently minimizing the cost of the system. U n d e r g r o u n d D i s t r i b u t i o n The Navy uses UNDERGROUND power distribution systems on most shore facilities for several reasons: underground lines are secure against damage that high winds and storms in- flict on overhead lines in some areas; underground lines leave clear areas and open spaces for the operations of heavy mobile equipment; and underground lines are much more secure against enemy attack than overhead lines. There are three principal categories of underground lines: duct lines, cables buried directly, and conduits located in tunnels. The system most frequently installed by construction battalions is the underground duct system, which consists of manholes, handholes, duct lines, and cables. In general, a representation of the system layout and a list of materials needed to install the system can be found in a standard set of drawings. INTERIOR ELECTRICAL WIRING SYSTEM In general, the term service means the electrical system that brings the power from the pole or other point on the exterior power distribution line to the point on or inside the building from which it is distributed to the building circuits. Service for a building consists of two parts: the service conductors and the service equipment. The SERVICE CONDUCTORS supply power from the pole or other point on the exterior distribution system to the building. These conduc- tors may be SERVICE DROP conductors for overhead service, or they may be SERVICE LATERAL conductors for underground service. From the service conductors, electrical power is brought into the building through a SERVICE ENTRANCE to the SERVICE EQUIPMENT on or inside the building. The service equipment is the necessary equipment, usually consisting of a circuit breaker or switch or fuses, that is located near the entry point of the supply conductors to the building. This equipment is the main control and means of cutting off the power supply to the b u i l d i n g . 9-6 Service Conductors The SERIVCE DROP CONDUCTORS (fig. 9-8) run from the pole to the building. These conductors may consist of an approved multi- conductor cable or individual (single) conductor. In either case, they must have thermoplastic, rubber, or other weatherproof insulation. The current-carrying capacity of the service drop conductors must be sufficient to ensure that ample current for the prospective maximum load may be conducted without a temperature rise to a point high enough to damage the insulation. The NEC® specifies the minimum size conductors that may be used for different load (amperage) requirements. Figure 9-9 shows an UNDERGROUND SERVICE that brings power into a building. the conductors, corresponding to the service drop that Figure 9-8.—Overhead service entrance.

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