Lighting Systems 337
energy. A measure of how well this is done is called efficacy. Efficacy is the
lumens of light produced per watt of electrical power converted. A com-
parison of some different types of light sources is shown below:Efficacy
Lamp Type (Lumens/watt)
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200 Watt incandescent lamp 20
400 Watt mercury lamp 50
40 Watt fluorescent lamp 70
400 Watt metal halide lamp 75
400 Watt high-pressure sodium lamp 110
——————————————————————ELECTRICAL LIGHTING CIRCUITS
There are several types of electrical circuits that are used for electri-
cal lighting control. We will study some of the circuits used for incandes-
cent and fluorescent lighting systems. Electrical lighting circuits for dif-
ferent areas of a building must be wired properly. Some lighting fixtures
are controlled from one point by one switch, while other fixtures may be
controlled from two or more points by a switch at each point.
Incandescent lighting circuits are a typical type of branch circuit. The
most common type of incandescent lighting circuit is a 120-volt branch
that extends from a power distribution panel to a light fixture or fixtures
in some other area of a building. The path for the electrical distribution is
controlled by one or more switches that are usually placed in small metal
or plastic enclosures inside a wall. These switches are then covered by
rectangular plastic plates to prevent possible shock hazards.
Switches are always placed in a wiring circuit so that they can open
or close a hot wire. This hot wire distributes electrical power to the light-
ing fixtures that the switches control. These switches are referred to as “T-
rated” switches. They are designed to handle the high, instantaneous cur-
rent drawn by lights when they are turned on. A switch that accomplishes
control from a single location is a simple, single-pole, single-throw (spst)
switch, such as that shown in Figure 13-8. A pictorial view of the same cir-
cuit is shown in Figure 13-9.
Control from two locations, such as near the kitchen door and inside
the garage door, is accomplished by two 3-way switches. This circuit is