Lighting Systems 347
12,000 × 0.83 × 0.85
= —————————
600
FC = 14.11 footcandlesConsiderations for Electrical Lighting Loads
Incandescent lamps produce light by the passage of electrical cur-
rent through a tungsten filament. The electrical current heats the fila-
ment to the point of incandescence, which causes the lamp to produce
light. The primary advantage of incandescent lights is their low initial
cost. However, they have a very low efficacy (lumen/watt) rating. They
also have a very high operating temperature and a short life expectancy.
Incandescent lights are usually not good choices of light sources for com-
mercial, industrial, or outdoor lighting applications; their primary appli-
cations are for residential use.
Fluorescent light sources have a higher efficacy (lumens/watt) than
incandescent lights. They have a much longer life, and lower brightness
and operating temperature. Fluorescent lights are used for residential
(120-volt applications) and for general-purpose commercial and industri-
al lighting (120-volt and 277-volt systems). Disadvantages of fluorescent
lights include the necessity for a ballast and a rather large luminaire. They
also have a higher initial cost than incandescent lamps. It is estimated that
fluorescent light sources provide approximately 70 percent of the lighting
in the United States.
Vapor light sources also have very good efficacy ratings and long
life expectancies. They have a high light output for their compact size.
They are typically used for industrial, commercial, and outdoor applica-
tions, since they can be operated economically on higher-voltage systems.
The initial cost of vapor lights is high, they require a ballast, and they are
a very bright light source. Their very high efficacy ratings have led to in-
creased use of vapor light sources.