Interior Lighting for Designers

Electrical Distribution
Electric current generated and delivered by
an electric utility enters a building through a
service panel. In the United States, three
kinds of systems are common:

1. 120/240 V, single-phase, three-wire.


2. 120/208 V, three-phase, four-wire.


3. 277/480 V, three-phase, four-wire.

The 120/240 V, single-phase, three-
wire system is commonly used in single-
family homes and small commercial build-
ings. Wire conductors leading from the
entrance panel distribute the power through-
out the building. Because the wire has resis-
tance, the longer the distance that power is
carried, the greater the voltagelosses, caus-
ing lights to dim and appliances to operate
sluggishly. This is corrected by using larger-
diameter wires, which have less resistance.
Distributing current at higher voltages
reduces losses occurring because of the
wire’s resistance. Therefore, in large com-
mercial buildings, 120/208 V, three-phase,
four-wire and 277/480 V, three-phase, four-
wire systems are used to reduce resistance
losses.
In commercial buildings, running each
circuit from the entrance panel will create a
substantial voltage loss or require the use of
large-diameter, expensive wires. To avoid
voltage loss,feeder circuitsconduct power
from the entrance panel to secondary distri-
bution panels, calledpanel boards, located
throughout the building. The wires that dis-
tribute power locally between the panel
board and the luminaires or receptacles are
calledbranch circuits.

Power Consumption
Awatt(W) indicates the rate at which elec-
tricity is changed into another form of

power—light or heat. Power consumption in

watts is calculated by multiplying volts times

amps (W = V × A).

Theoretically, a 20-amp circuit operat-

ing at 120 V will handle a possible maximum

load of 2,400 W (that is, 20 × 120 =

2,400). In practice, the National Electrical

Code limits the possible load of a branch cir-

cuit to 80 percent of the branch circuit

ampere rating: a 15 A, 120 V circuit to

1,440 W; a 20 A, 120 V circuit to 1,920 W;

a 20 A, 277 V circuit to 4,432 W.

Energyis the amount of electric power

consumed over a period of time; it is mea-

sured inkilowatt-hours(kWh). One kilowatt

(kW) = 1,000 W. Hence, kWh = kW ×

hours used. For example, a 150 W lamp is

equivalent to 0.15 kW. When operated for

40 hours it uses 6 kWh (0.15kW × 40 hrs =

6 kWh). Utility rates are based on monthly

kWh usage.

In estimating the connected load for dis-

charge and low-voltage incandescent sources,

the power consumed by the ballast or trans-

former must be included.

To obtain lightingwatts per square foot

for an installation, divide the total luminaire

watts by the area of the space in square feet.

Life Cycle Costs

The cost of lamps and luminaires plus their

installation is a minor part of the total cost

over the life of a lighting system. The cost of

electricity (operating costs) is the single larg-

est cost in lighting. Except in homes, mainte-

nance (labor costs) to replace lamps and

clean luminaires is the second greatest

expenditure. Lighting systems, therefore,

must be evaluated in terms oflife cycle costs.

A typicalcost analysiswill include initial

lamp and luminaire costs; installation costs;

electricity costs based on burning hours per

year; labor costs, including those incurred

because of dirt conditions; and interest

costs on the original capital investment.

INTERIOR LIGHTING FOR DESIGNERS