344 Electrical Power Systems Technology
total bulbs required
branch circuits = —————————————
number of bulbs per circuit
200
= ——
11
= 18.18 (round up to 19)
Nineteen branch circuits will be needed.
It should be pointed out that, in these problems, we determined the
minimum number of branch circuits. But room for flexibility should be
provided for each branch circuit. This can be done by designing the circuit
to handle only 16 amperes (80 percent) rather than 20 amperes. Also, room
should be provided on the power distribution panel to allow for connect-
ing some additional lighting branch circuits. This will allow additional
loads to be connected at a later time.
LIGHTING FIXTURE DESIGN
Lighting fixtures are the devices that are used to hold lamps in place.
They are commonly referred to as luminaires. Luminaires are used to ef-
ficiently transfer light from its source to a work surface. The proper de-
sign of luminaires allows a more efficient transfer of light. It is important
to keep in mind that light intensity varies inversely as the square of the
distance from the light source. Thus, if the distance is doubled, the light
intensity will be reduced four times.
Many factors must be considered in determining the amount of light
that is transferred from a light bulb to a work surface. Some light is ab-
sorbed by the walls and by the light fixture itself; thus, not all light is effi-
ciently transferred. The manufacturers of lighting systems develop charts
that are used to predict the amount of light that will be transferred to a
work surface. These charts take into consideration the necessary variables
for making a prediction of the quantity of light falling onto a surface.
Each luminaire has a rating, which is referred to as its coefficient of
utilization. The coefficient of utilization is a factor that expresses the per-
centage of light output that will be transferred from a lamp to a work area.
The coefficient of a luminaire is determined by laboratory tests made by
the manufacturer. These coefficiency charts also take into consideration