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 formula:
|(FROM CHART)||(FROM CALCULATION)|
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 mounting height.
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 footcandles.
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
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:
fc = (N)(LL)(CU) / AREA
Figure 6-28. - Material yard sample problem.Continue Reading