Research matters - Illuminating Engineering Society

ENERGY ADVISOR
Willard L.Warren,
PE, LC, FIESNA
THE SIZE OF THE ELECTRIC
feeders required for a new building
are determined in accordance with
the National Electric Code’s (NEC)
“General Lighting Loads by Occupancy,”-
Table 220.3(A).
The minimum unit loads for lighting
are given in volt-amperes per sq.
ft, which is watts per square foot
divided by power factor. For most
commercial and institutional occupancies
the power factor is essentially
1.0 per ft, so watts per square foot
equal volt ampere per square foot
(VA per sq ft). There are about 20
occupancies listed in the table; here
are the calculated loads for three of
them:
• Office Buildings-3.5 watts per sq
ft + 1 watt per sq ft for receptacle
outlets
• Schools and Stores-3 watts per
sq ft
• Restaurants, Hospitals and
Hotels-2 watts per sq ft
(Please note that these watts per
square foot are two to three times
higher than the LPDs that the new
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energy code allows.)
The 2002 NEC Handbook points
out that “energy saving type calculations
are not permitted to be used to
determine the “calculated” lighting
load...but can be a useful tool to
reduce the “connected” lighting load.
The “calculated” load, in watts per sq
ft, determines the copper size for the
customer’s service feeders; the “connected”
lighting load, also in watts
per sq ft, theoretically determines
what’s connected electrically through
the feeders, but the real Energy Used
for Lighting is the “actual” lighting
load, in kilowatts, times the hours
used, and is given in kilowatt hours
(kWh), and it’s the “lighting energy
usage” that determines the lighting
portion of the electric bill.
The Diversity Factor
Interestingly, the NEC allows the
consulting electrical engineer to
apply a “demand factor” to allow for
a reduction in the customer’s copper
requirement as the size of the installation
increases. It stands to reason
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that the larger the facility, the less
likely that every electrical and lighting
device is on at the same time.
Utilities also apply a “diversity factor,”
from experience, to reduce the size
of their copper feeders, since not
every electrical device will be utilized
at the same time because of the
diversity of occupancy.
After allowing for a feeder size
reduction for the NEC’s “demand
factor” and then a realistic “diversity
factor,” the utility provides an incoming
service to the customer with
much smaller copper cables than the
NEC calculations require the customer
to buy. Con Ed in NYC did a
survey not long ago and reported to
our NYC Code Committee that
restaurants never exceed 50 percent
of their “calculated” load, so Con Ed’s
feeders are smaller and more realistic
than the NEC requires that the
customer must provide.
PIER Project
In addition, energy usage is much
less when energy saving measures
are taken. As I pointed out in a previous
column, the one-year California
PIER project experiment is near
completion in 12 schools in
California to determine how much
load the diversity factor and energy
saving measures actually reduces
lighting usage.
In these 12 schools there are over
24 classrooms with three lamp
direct/indirect pendent luminaires in
two or three-24 ft long rows,
depending on classroom width. Each
unit has 2-4 ft electronic T8 lamps
uplighting for general illumination,
and 1- 4 ft T8 lamp on a dimmer, for
a downlighting task light to take
notes when audiovisual teaching is
used. The horizontal illumination is
50 fc minimum maintained; the vertical
illumination on the chalkboards is
approx 20 fc maintained. Both levels
are quite uniform.
The classrooms are approximately
750 sq ft, in area. The “calculated”
load for the purpose of sizing feeders
by NEC Table 220.3(A) is 75O sq f t
x 3 watts per sq ft = 2250 watts.
The “connected” load is: 18 units
(for three rows) times 50 watts per
unit (low power ballasts) = 900 watts
for general lighting, plus; 9 times 50
. . . . . . . . . . . . . . . . .