Question

An overhead lighting system is being assessed during the energy audit of a school. In a classroom with floor area = 85 m^2 , the overhead luminaires are all on a single electrical circuit (with no other loads on the circuit).

The room has 12 identical luminaires, each with three “F32T8” lamps and with electronic ballasts. The system-efficacy of the lamp-ballast combination is approximately 95 lm/W.

From the manufacturer’s specification sheet, the “luminaire efficiency” is estimated to be 78%. The lights are powered on for approximately 1,200 hours per year.

With the lights turned on, an electrician obtains the following measurements for the single-phase lighting circuit: V(rms) = 220 volts, I(rms) = 5.17 amps, and power factor = 0.95.

a) Calculate the power demand [W] of the lighting circuit.

b) Calculate the lighting power density [W/m2 ] of the classroom.

c) Estimate the annual cost [$/year] of electricity used by the lights if electricity costs $0.13/kWh

Answer 1

a floor area = 85m2 Toral lights =12 w 1200 hrs Lofesor Pof=0.95 Noms -2200, Iring = 5.174 ☺ p= Vome i Birmg X6084 = 220X 5.17X0.95 1.08ko Power density - 1.08x10² 12.I w/m &s Annual cost totul Kwhr = 1.08X1200 cost -0.13 Kwh Total cost 1.08X1200 x0.13 = $168.56