【文章內(nèi)容簡介】 ial establishments. For residential occupancies, the service conductors are installed either 13 overhead or underground. Singlefamily and small multifamily dwellings have the kilowatthour metes installed on the outside of the building. From the kilowatthour meter, the conductors are connected to the main disconnect. Three separate disconnecting means are used with one mon ground. From the main disconnect, the conductors supply power to the branch circuit panels. For dwelling occupancies there are three basic types of branch circuits: general lighting circuits, small appliance and laundry circuits, and individual branch circuits. The individual branch circuits are frequently used to supply central heating and/or airconditioning system, water heaters, and other special loads. （ 1） Grounding Requirements All AC services are required to be grounded on the supply side of the service disconnecting means. This grounding conductor runs from the bination system and equipment ground to the grounding electrode. For multifamily occupancies it is permitted to use up to six service disconnecting means. A single grounding conductor of adequate size should be used for the system ground. （ 2） Commercial and Industrial Installations Commercial and industrial installations are more plex than small residential installations. Large apartment plexes and condominiums, although classified as residential occupancies, often use mercialstyle services .A singlephase, threewire service or a threephase, fourwire service may be brought into the building, generally from underground. The serviceentrance conductors terminate in a main disconnects. From this point, the conductors are connected to the individual kilowatthour meters for each apartment and then to smaller disconnecting means and overcurrent protective devices. Branchcircuit panels are generally installed in each apartment. Feeder conductors connect the individual disconnecting means to the 14 branchcircuit panels. Commercial and/or industrial buildings may have more than one kilowatthour meter, depending upon the number of occupancies. The service is usually a threephase, fourwire system. The available voltages may be 120/208V or 277/480v. If the system provides 277/480V, a transformer must be installed in order to obtain 120V. If the building covers a large area, it is remended that the service be installed near the center of the building. This arrangement minimizes line loss on feeder and branchcircuit conductors. Some utilities supply a threephase, threewire or threephase, fourwire delta system. The mon voltages that may be obtained from the threewire delta system are 240V, 440V, or 550V. With this arrangement, a transformer must be used to obtain 120V. The usual voltages supplied from the fourwire delta system are 240V, three phase and 120V, single phase. Many large consumers purchase the electrical energy at the primary voltage, and transformers are installed on their premises. Threephase voltages up to 15 KV are often used. The service for this type of installation generally consists of metal cubicles called a substation unit. The transformers are either installed within the cubicle or adjacent to it. Isolation switches of the drawer type are installed within the cubicle. These switches are used to isolate the main switch or circuit breaker from the supply during maintenance or repair. Consumer Loop Systems Although the radial system of distribution is probably the most monly used system of transmitting power on the consumer’s property, the loop system is also employed. When installing any system, overcurrent protection and grounding must be given primary consideration. Electrical personnel who design and i nstall these systems must ply with the NEC and local requirements. Secondary Highvoltage Distribution 15 Large industrial establishments may find it more economical to distribute power at voltages higher than 600V. Depending upon the type of installation and the load requirements, voltages as high as 2300V may be used. Stepdown transformers are installed in strategic locations to reduce the voltage to a practical working value. Sometimes the highvoltage system may be radial, and the lowvoltage system may be connected into a loop. Another method is to have both the primaries and secondary connected to from a loop. （ 1） Secondary Ties Loop System It is frequently convenient to connect loads to the secondary conductors at points between transformers. These conductors are called secondary ties. Article 450 of the NEC gives specific requirements regarding the conductor sizes and overcurrent protection. （ 2） Grounding of Electrical Systems In general, most electrical systems must be grounded. The purpose of grounding is to limit the magnitude of voltage caused by lightning, momentary surges, and accidental contact with higher voltages. System grounds must be arranged to provide a path of minimum impedance in order to ensure the operation of overcurrent devices when a ground fault occurs. Current should not flow though the grounding conductor during normal operation. Directcurrent systems generally have the grounding conductor connected to the system at the supply station, and not at the individual service. Alternationcurrent system, on the other hand, must be grounded on the supply side of the main disconnect at each individual service. For specific information on the location and method of grounding, refer to NEC Article 250. Grounding of Electrical Equipment Metal conduit and cases which enclose electrical conductors must be grounded. If the ungrounded conductor es in contact with a metal 16 enclosure which is not grounded, a voltage will be present