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You can see from the formula above that the CURRENT
(I) varies inversely to the RESISTANCE (R).
To maintain the required current flow disconnecting
the service conductors from the supply source.
It may consist of a single manually operated switch
or a circuit breaker. The NEC sets
a minimum size for entrance
switches at 60 A for the fuse type
and 50A for the circuit breaker type. A CIRCUIT
BREAKER is a protective device that automatically
opens the circuit, rather than burning out
like a fuse, when the amperage exceeds that rated for
the circuit breaker. The NEC recommends
a minimum size of 100-A service for
individual residences. However,
when not more than two two-wire branch
circuits are installed, a 30-A entrance switch
may be used. Panelboard A PANELBOARD (fig. 9-10) is defined by the NEC as a single panel, or a group of panel units designated for assembly in the form of a single panel, including buses. It comes with or without switches and/or automatic overcurrent protective devices for the control of light, heat, or power circuits of small individual as well as aggregate capacity; it is designed to be placed in a cabinet or a cutout box and placed in or against a wall or partition and is only accessible from the front.A BREAKER PANEL uses a thermal unit built into the switch with the breaker being preset at the factory to open automatically at a predetermined
Figure 9-11.-Lighting panel. ampere setting. It maybe reset to the ON position after a short cooling-off period. LIGHTING PANELS (fig. 9-11) are normally equipped with 15-A single-pole automatic circuit breakers, while the power panels may have one-, two-, or three-pole automatic circuit breakers with a capacity to handle the designated load. In most buildings, the entrance switch and panelboards can be mounted close to each other; however, they must be placed where service and maintenance can be easily performed. They should not block any passage that is supposed to be open, and they should not be in a place where exposure to corrosive fumes and dampness is imminent. Panelboards should be located as near as possible to the center of the electrical load. Conductors ELECTRICAL CONDUCTORS generally consist of drawn copper or aluminum formed into wire. They provide paths for the flow of electrical current. Conductors are usually covered with insulating materials (fig. 9-12) to minimize the
Figure 9-12.-Types of single insulated conductors. chances for short circuits and to protect personnel. Atmospheric conditions, voltage requirements, and environmental and operating temperatures are factors to consider in selecting the type of insulating material for a particular job. |
 
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