overhead or underground. Single-family and small multifamily dwellings have the kilowatt-hour metes installed on the outside of the building. From the kilowatt-hour meter, the conductors are connected to the main disconnect.
Three separate disconnecting means are used with one common 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 air-conditioning 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 combination 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 complex than small residential installations. Large apartment complexes and condominiums, although classified as residential occupancies, often use commercial-style services .A single-phase, three-wire service or a three-phase, four-wire service may be brought into the building, generally from underground. The service-entrance conductors terminate in a main disconnects. From this point, the conductors are connected to the individual kilowatt-hour meters for each apartment and then to smaller disconnecting means and over-current protective devices. Branch-circuit panels are generally installed in each apartment. Feeder conductors connect the individual disconnecting means to the
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branch-circuit panels. Commercial and/or industrial buildings may have more than one kilowatt-hour meter, depending upon the number of occupancies. The service is usually a three-phase, four-wire 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 recommended that the service be installed near the center of the building. This arrangement minimizes line loss on feeder and branch-circuit conductors. Some utilities supply a three-phase, three-wire or three-phase, four-wire delta system. The common voltages that may be obtained from the three-wire delta system are 240V, 440V, or 550V. With this arrangement, a transformer must be used to obtain 120V. The usual voltages supplied from the four-wire 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. Three-phase 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.
3.3 Consumer Loop Systems
Although the radial system of distribution is probably the most commonly used system of transmitting power on the consumer’s property, the loop system is also employed.
When installing any system, over-current protection and grounding must be given primary consideration. Electrical personnel who design and install these systems must comply with the NEC and local requirements.
3.4 Secondary High-voltage Distribution
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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. Step-down transformers are installed in strategic locations to reduce the voltage to a practical working value.
Sometimes the high-voltage system may be radial, and the low-voltage 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 over-current 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 over-current devices when a ground fault occurs. Current should not flow though the grounding conductor during normal operation.
Direct-current systems generally have the grounding conductor connected to the system at the supply station, and not at the individual service. Alternation-current 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.
3.5 Grounding of Electrical Equipment
Metal conduit and cases which enclose electrical conductors must be grounded. If the ungrounded conductor comes in contact with a metal
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enclosure which is not grounded, a voltage will be present between the enclosure and the ground. This presents a potential hazard. Persons coming in contact with the enclosure and ground will complete a circuit.
All non-current-carrying metal parts of electrical installations should be tightly bonded together and connected to a grounding electrode. Good electrical continuity should be ensured though all metal enclosures. The current caused by accidental grounds will be conducted though the enclosures, the grounding electrode to the earth.
If the current is large enough, it will cause the over-current device to open.
（1） Ground-Fault Protection
A ground-fault protector is a device which senses ground faults and opens the circuit when the current to ground reaches a predetermined value. A ground-fault circuit interrupter is a device which opens the circuit when very small currents flow to ground.
There is no way to determine in advance the impedance of an accidental ground. Most circuits are protected by 15A or larger over-current devices. If the impedance of a ground fault is low enough, such devices will open the circuit. What about currents of less than 15A? It has been proven that currents as small as 50mA though the heart, lungs, or brain can be fatal.
Electrical equipment exposed to moisture or vibration may develop high-impedance grounds. Arcing between a conductor and the frame of equipment may cause a fire, yet the current may be less than 1 ampere. Leakage current caused by dirt and/or moisture may take place between the conductor and the frame. Portable tools are frequently not properly grounded, and the only path to ground is through the body of the operator.
The ground-fault circuit interrupter was developed to provide protection against ground-fault currents of less than 15A. The GFCI is designed to
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