148 Modern Commercial Wiring
Copyright Goodheart-Willcox Co., Inc.
Table 220.12 of the Code contains minimum general
lighting loads (in VA/ft2) for various types of buildings.
A condensed version of this table is shown in Figure 12-3.
The general lighting load is calculated by multiplying
the floor area (in ft2) by the unit load (in VA/ft2). If the
load is continuous, the calculated load is multiplied by
1.25 (the inverse of 80%) to determine the circuit
requirements.
Sample Problem 12-1
Problem: A 25,000 ft2 office building is being
designed. What is the general lighting load and what
load does the circuit need to supply?
Solution: From Table 220.12, the unit load for an
office building is 3 1/2 VA/ft2. The general lighting
load is determined by multiplying this value by the
square footage of the building:
3 1/2 VA/ft2 × 25,000 ft2 = 87,500 VA
The general lighting load is 87,500 volt-amperes.
However, the load is continuous and can only be
80% of the load supplied by the circuit. This value
must be multiplied by 1.25 to determine the circuit
requirements:
87,500 VA × 1.25 = 109,375 VA
The circuit is designed to supply 109.375 kilovolt-
amperes.
The general lighting load is not required if the load
for each lamp is determined separately. If the individual
load is continuous, it must be multiplied by 1.25. When
determining the current draw of fluorescent fixtures, use
the current rating of the ballast, not the tube wattage.
Sample Problem 12-2
Problem: A 4′ long, two-lamp fluorescent fixture
ballast draws 0.7 amps at 120 volts. How many of
these fixtures can be connected on a 20-amp circuit?
Solution: This is a continuous load, so the current
used by the lights can only be 80% of the circuit
current rating:
Allowable current = 20 A × 0.80
= 16 A
By dividing the allowable load by the load of each
lamp, the total number of lamps is determined:
16A
= 22.8 fixtures
0.7A
The maximum number of fixtures on the circuit is 22.
Show-window lighting
The show-window lighting load is not considered as
part of the general lighting load. Section 220.43(A) of the
Code requires that show-window lighting be computed as
200 volt-amperes per linear foot or as the maximum volt-
ampere rating of the equipment and lights, whichever is
greater.
NEC NOTE 100
Show Window: Any window used or designed to
be used for the display of goods or advertising mate-
rial, whether it is fully or partly enclosed or entirely
open at the rear and whether or not it has a platform
raised higher than the street floor level.
The Code includes several sections applicable to
show-window lighting:
• Section 220.43(A)—Show-window lighting.
• Section 220.14(G)—Load computation.
The Code also requires at least one receptacle outlet
for every 12′ of show-window space measured horizon-
tally with the load computed at 180 volt-amperes per
outlet. This receptacle load is in addition to the show-
window lighting load.
Figure 12-3. Minimum general lighting loads are dependent on
the type of area being lit.
General Lighting Loads
Type Building Unit Load (VA/ft2)
Auditoriums 1
Banks 3 1/2
Barber shops 3
Churches 1
Dwelling units 3
Hospitals 2
Hotels 2
Office buildings 3 1/2
Restaurants 2
Schools 3
Stores 3
Warehouses 1/4