or metal support structures. In either case, the fixture
should be firmly attached to the structure so that precise
aiming for light distribution can be made.
As we stated earlier, a number of light systems are
in use today, such as streetlights, floodlights, and
security lights. These systems are either series or
multiple (parallel), depending on how they are used and
the equipment available.
SERIES AND MULTIPLE CIRCUITS
The series circuit is supplied by a regulating
transformer that gives a constant current, usually of 6.6
amperes, to the lighting circuit. If a higher amperage is
required, autotransformers are available for stepping up
the current to 15 or 20 amperes. This higher amperage
permits the use of more rugged lamp filaments that give
longer life for lamps of equal candlepower and higher
The multiple (parallel) circuit consists of a number
of streetlights supplied by a distribution transformer,
delivering a constant low voltage to a circuit or
secondary main that also supplies other loads; however,
running secondary conductors any great distance to
supply a parallel connected lamp or a group of lamps is
impractical because of the excessive voltage drop.
The cost of the multiple luminaire is low compared
to the series type because the low voltage allows for the
elimination of other luminaire accessories. This saving
is largely offset, however, by the increased requirement
for control devices and the copper wire cost. Lamp life
and efficiency are comparatively low, and the
illumination is not as uniform as in a series circuit.
In choosing a system, here are a few suggestions
that may aid in your selection.
If the total wattage of the circuit exceeds 2
kilowatts or more than 15 lights, consider a series
When extending an existing system, use the
If low-voltage capacity exists at the proposed
location, use a multiple system, even though the load
exceeds 2 kilowatts.
When several small lights are to be spaced rather
far apart and no low-voltage secondary exists along the
route, use the series system regardless of the load size or
the number of lights.
When estimates show that one type of system
will save money and time, use the more economical
Let us consider a series streetlight system. The
power for the circuit will be supplied from the base
primary distribution lines, through fuse cutouts, to an
oil switch, and from the oil switch to a constant-current
regulator (fig. 6-16). The constant-current regulator
will supply power to the series loops and, thus to the
While the current (normally 6.6
amperes) remains constant, the voltage of the circuit is
equal to the sum of the voltages of all the lamps plus the
voltage drop in the wire. With enough lamps connected
in series, the circuit can become a high-voltage circuit.
The series circuit is easily controlled, but any break,
such as a burned-out filament in a lamp, interrupts the
Figure 6-16.Series street-lighting circuit.