Figure 4-28.American wire sizes for bare copper.
concerned with the construction, maintenance, and
repair of the distribution system. Depending on the
system, it will consist of a combination of the following
components, substations, distribution transformers,
distribution lines, secondary circuits, secondary service
drops, and safety and switching equipment. The
distribution system may be underground, overhead, or a
combination of the two.
Distribution substations change the transmission or
generator voltage to a lower level, providing voltage
sources for the distribution circuits supplying power to
Distribution transformers are installed in the vicin-
ity of each customer to reduce the voltage of the distri-
bution circuit to a usable voltage, usually 120/240 volts.
Distribution circuits (primary main circuits) are
circuits that originate from the distribution substation.
Primary mains are circuits carrying over 600 volts, but
generally they operate between 2,400 and 34,500 volts.
Primaries can be found in single-phase or three-phase
configurations and generally operate as three-phase,
three-wire or three-phase, four-wire circuits. Two types
of primary circuits are in use today. One, the delta type
(A) system, (fig. 4-29) is used when most of the load in
Figure 4-29.Three-phase delta (A) primary and secondary system.