Interior Lighting for Designers

inductive ballasts, the overall power factor is
50 to 60 percent. With a capacitor, the lead-
ing current drawn by the capacitor compen-
sates for the lagging current in the
remainder of the circuit, improving the power
factor.
Ballasts are classified according to one
of the following three categories:

High power factor: 90% or greater
Power factor corrected: 80 to 89%
Low (normal) power factor: 79% or less
High-power-factor ballasts use the
lowest level of current for the specific amount
of power needed; this reduces wiring costs by
permitting more luminaires on branch cir-
cuits. Low-power-factor ballasts use higher
levels of current—approximately twice the
line current needed by high-power-factor bal-
lasts—allowing fewer luminaires per branch
circuit and increasing wiring costs.
Power factor is not an indication of the
lamp-ballast system’s ability to produce
light; power factor measurements pertain
only to the ballast’s ability to use the power
that is supplied. Thus, power factors are
invalid as a multiplier in determining light
output values.

Lamp-Ballast System Efficacy
The initial lumen and mean-lumen ratings
published by lamp manufacturers are based
on the operation of the rated lamps by aref-
erence ballast. This is a laboratory instru-
ment that is used to establish a baseline, or
reference conditions, so that commercially
available ballasts can be compared against
the same reference. Some commercially
available ballasts operate lamps more effi-
ciently than the reference; others operate
lamps less efficiently.
In practice, when a lamp is operated by
a commercially available ballast, it usually
provides fewer lumens than the rated

amount. Because of the electrical resistance

created by the passage of a current through

the core-and-coil of an electromagnetic bal-

last, some power is converted to heat. This

lost power, calledballast loss, is unusable

for producing light from the lamp.

The disparity between light provided by

the reference ballast and the commercially

available ballast is called theballast factor.

The ballast factor is the ratio of light output

produced by lamps operated by a commer-

cially available ballast to that which is theo-

retically supplied by lamps powered by a

laboratory-reference ballast.

Ballast

factor

Lamp lumen output when operated

with

=

commercial ballast

Lamp lumen output when operated

with laboratory-reference ballast

The term “ballast factor” implies that

this is a property of the ballast; it is actually a

property of the lamp-ballast system. Some

ballasts have different factors for different

lamps. That is, they have one ballast factor

for operating standard lamps and another for

operating energy-saving lamps.

Ballast factors can be greater than 1.0.

A commercial ballast can operate a lamp in

a way that produces more lumens than when

the lamp is operated under reference condi-

tions.

Theballast efficacy factoris a ratio of

the ballast factor to the input watts of the

ballast. This measurement is used to com-

pare the efficiency of various lamp-ballast

systems. Ballast efficacy factors are mean-

ingful only for comparing different ballasts

when operating the same quantity and kind

of lamp.

Ballast efficacy factor =

Ballast factor

Ballast input watts

For a given lighting system, the ballast

factor is an indication of the amount of light

INTERIOR LIGHTING FOR DESIGNERS