Lighting Systems 339
in order to provide the type of lighting control desired for each room. You
should study the diagrams of Figures 13-8 through 13-13 to fully under-
stand how these lighting circuits operate.BRANCH CIRCUIT DESIGN
The design of lighting branch circuits involves the calculation of the
maximum current that can be drawn by the lights that are connected to the
branch circuit. Many times, particularly in homes, a lighting branch circuit
also has duplex receptacles for portable appliances. This makes the exact
current calculation more difficult, since not all the lights or the appliances
will be in use at the same time.
The National Electrical Code (NEC) specifies that there should be at
least one branch circuit for each 500 square feet of lighting area. The NEC
further specifies a minimum requirement for lighting for various types of
buildings. Table 13-3 lists the number of watts of light per square foot re-
quired in some buildings.
The solution of a typical branch circuit lighting problem follows.Figure 13-10. Circuit for controlling a light from two locations: (A) Schematic
diagram, (B) Positions of three-way switches