Electrical Power Systems Technology

Lighting Systems 343

Sample Problem:

Given: a large room in an industrial building needs to have 40,000

watts of incandescent lighting. It is decided that 120-volt 20-ampere branch

circuits will be used. The lights will use 200-watt bulbs.

Find: the number of branch circuits required for the lighting.

Solution:

1. Find the amount of current each lamp will draw.

P

I = ——

V

200 watts

= —————

120 volts

= 1.67 amperes

2. Find the maximum number of lamps in each branch circuit.

amperage of circuit

lamps = ——————————

amperage of lamps

20 amperes

= —————

1.67 amperes

= 11.97 (round down to 11)

3. Find the total number of bulbs (lamps needed.

wattage of room

bulb required = —————————

wattage of bulbs

40,000 watts

= ——————

200 watts

= 200 bulbs required

4. Find the minimum number of branch circuits needed.