342 Electrical Power Systems Technology
branch current
units = ———————
load current
20 amperes
= —————
3 amperes
= 6.67 units (round off at 6)
- Find the number of branch circuits needed.
lighting units
circuits = ——————————————
No. units per branch circuit
125
= ——
6
= 20.8 (round up to 21)
Twenty-one 20-ampere branch circuits will be needed.
Solution for total power:
- Find the total current drawn by the lights.
I = 125 units × 3 amperes
= 375 amperes
- Calculate the total power.
p = V × I × pf
= 120 × 375 × 0.75
= 33,750 watts (33.75 kW)
An even more common type of branch circuit lighting problem is
given in the following discussion. This problem begins with the minimum
number of lights required in a room. The number of branch circuits that
are required must be determined.