Power Distribution Equipment 251
cuit breaker panelboard. Panelboards are metal cabinets that enclose the
main disconnect switch and the branch circuit protective equipment.
Distribution panelboards are usually located between the power feed
lines within a building and the branch circuits that are connected to it.Low-voltage Switchgear
Metal-enclosed low-voltage switchgear is used in many industrial and
commercial buildings as a distribution control center to house the circuit
breakers, bus bars, and terminal connections that are part of the power
distribution system. Ordinarily, a combination of switchgear and distribu-
tion transformers is placed in adjacent metal enclosures. This combination
is referred to as a load-center unit substation, since it is the central control
for several loads. The rating of these load centers is usually 15,000 volts or
less for the high-voltage section, and 600 volts or less for the low-voltage
section. Load centers provide flexibility in the electrical power distribu-
tion design of industrial plants and commercial buildings.
Metal-enclosed switchgear, or metal-clad switchgear, is a type of
equipment that houses all the necessary control devices for the electri-
cal circuits that are connected to them. The control devices contained
inside the switchgear include circuit breakers, disconnect switches, in-
terconnecting cables and buses, transformers, and the necessary measur-
ing instruments. Switchgear is used for indoor and outdoor applications
at industrial plants, commercial buildings, and substations. The voltage
ratings of switchgear are usually from 13.8 to 138 kV, with 1 to 10 MV A
power ratings.THE ELECTRICAL SERVICE ENTRANCE
Electrical power is brought from the overhead power lines, or from
the underground cable, into a building by what is called a service entrance.
A good working knowledge of the National Electrical Code (NEC) specifica-
tions and definitions is necessary for an understanding of service entrance
equipment. The NEC sets the minimum standards that are necessary for
wiring design inside a building.
The type of equipment used for an electrical service entrance of a
building may include high-current conductors and insulators, disconnect
switches, protective equipment for each load circuit that will be connected
to the main power system, and the meters needed to measure power,