150 Modern Commercial Wiring
Copyright Goodheart-Willcox Co., Inc.
Sample Problem 12-5 Continued
Solution: Each type of lighting load is computed
separately and then combined to determine the total
lighting load. First, the general lighting loads for
the two areas of the store are calculated:
Storage area = 80′ × 40′
= 3200 ft2
Showroom area = 80′ × 80′
= 6400 ft2
Table 220.3(A) lists unit loads for storage and show-
room as 1/4 VA/ft2 and 3 VA/ft2, respectively:
General lighting load (storage) = 3200 ft2 × 1/4 VA/ft2
= 800 VA
General lighting load (showroom) = 6400 ft2 × 3 VA/ft2
= 19,200 VA
General lighting load = 800 VA + 19,200 VA
= 20,000 VA
The show-window lighting load is based on 200 VA
per linear foot:
Show-window lighting load = 45′ × 200 VA/ft
= 9000 VA
The minimum load for the sign lighting is used:
Sign lighting = 1200 VA
Now the total lighting load can be calculated by
adding the parts together:
Total lighting load = 20,000 VA + 9000 VA + 1200 VA
= 30,200 VA
These loads are all continuous, so the total load is
multiplied by a factor of 1.25 to determine the
circuit requirements.
30,200 VA × 1.25 = 37,750 VA
Additional lighting loads
Additional lighting loads should be computed sepa-
rately from the general lighting load and then added to
the general lighting load. Loads for additional lighting
such as security lighting, parking area lighting, sidewalk
lighting, roadway lighting, and stadium lighting are
calculated using the actual load. These are considered
continuous loads where appropriate. The additional
lighting load must be treated separately from the general
lighting load for computation purposes.
Demand factors for feeder loads
Due to the great diversity of lighting loads on commer-
cial premises, Section 220.42 and Table 220.42 allow the
general lighting load to be derated (reduced) for feeder,
panel, or service computations. See Figure 12-4. For example,
it is highly unlikely that every light in a hospital would be
operating at the same time. Of course, there are areas within
a hospital where the derating factors should not be applied as
these areas (such as operating rooms, emergency rooms,
intensive care units, nurses stations, stairways, and cardiac
units) are likely to have lighting units on at all times.
NOTE
Derating factors do not apply to branch-circuit conductor or
branch-circuit overcurrent protective device calculations.
Figure 12-4. Lighting loads can be derated for structures where
all lights are not in use continuously.
Demand Factors for
Lighting Loads
Type of
Occupancy
Dwelling Unit
Hospital
Hotels and
Motels
Warehouses
Porton of Demand
Load (VA) Factor (%)
0–3000 100
3001–120,000 35
Over 120,000 25
0–50,000 40
Over 50,000 20
0–20,000 50
20,000–100,000 40
Over 100,000 30
0–12,500 100
Over 12,500 50
All Others Total VA 100