The initial footcandle table gives the footcandle
value for each isofootcandle curve at a specific
mounting height. The values for each letter are the
same on each set of curves. That makes it possible to
compare diagrams directly and interpolate between
curves for different aiming distances.
The mounting heights given in the initial
footcandle table are representative of the wattage and
beam pattern associated with the floodlight. To
convert to other mounting heights, use the following
For example, a 5-footcandle level at 50 feet
(isofootcandle curve F) would have a value of 4.13 at a
55-foot mounting height.
(5)(502) = (fc)(552)fc = 4.13
In figure 6-27 (aiming point MH x 2), the
floodlight is aimed a distance of two mounting heights
away from a point on the ground directly below the
floodlight. That would be 80 feet for a 40-foot
The luminaire utilization data graph (fig. 6-27)
gives the percentage of the initial lamp lumens that fall
into the area being lighted. Knowing this, you can
easily determine the average lumens per square foot, or
The number beside each curve identifies the
aiming point, so that the utilization curve can be
identified with the associated isofootcandle diagrams.
In the example, for instance, the floodlight aimed two
mounting heights away from the pole would have a
utilization of 35 percent if it were lighting an area three
mounting heights wide. The same floodlight aimed at
one mounting height away from the pole would have a
utilization of 45 percent for the same area.
Lighting efficiency in floodlighting, as in
streetlighting, is seriously impaired by blackened
lamps, by lamp life. and by dirt on the reflecting
surfaces of the luminaire. A maintenance factor (MF)
must be applied in the lighting calculations to
compensate for the gradual losses of illumination on
the lighted area.
The following maintenance factors have been
widely used in industry when manufacturers
information is not available:
Enclosed flood lamps, 0.76
Open flood lamps, 0.65
LIGHT INTENSITY CALCULATIONS
There are a number of ways by which to determine
luminaire requirements. Since most methods would
require an engineering background, we will only
discuss the basic area lighting design considerations
that you, as a Construction Electrician, can perform in
the field if engineering assistance is not available. To
better understand how the calculations are performed,
solve this sample problem:
Determine the average, initial light level in a 160-
foot x l60-foot material storage yard using two NEMA
6 x 5 HLX 1,000-watt floodlights.
1. Apply the 2X-4X rule (fig. 6-28) to determine
spacing and mounting height. A 40-foot mounting
height provides MH x 2 or an 80-foot aiming distance.
2. The formula used to calculate the average,
initial light level (fc) is as follows:
f c =
Figure 6-28.Material yard sample problem.