MAINTENANCE FACTOR

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 manufacturer's information is not available:

Enclosed floodlamps, 0.76

Open floodlamps, 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 160-foot material storage yard using two NEMA 6 5 HLX 1,000-watt LucaloxR floodlights.

Solution:

1. Apply the 2X-4X rule (see fig. 1-26) 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:

Figure 1-26.\Material yard sample problem.

where:

N = number of floodlights

LL = initial lamp lumens

CU = utilization of the floodlights

From the utilization data (fig. 1-25), you can find that the utilization for the HLX luminaire aimed at two mounting heights across an area 160 feet or four mounting heights wide is 38 percent. The initial lumens for the 1,000 watt lamp is 140,000 lumens, obtained from the manufacturer's literature. Substituting in the formula,

The maintained light level is obtained by multiplying the initial light level by the maintenance factor.

fc = (4.2)(0.75) = 3.15 fc

Using the isofootcandle diagram, we obtain point by point footcandle values. For example, the center of the area occurs just inside isofootcandle line E. From the initial footcandle table, the 1,000-watt LucaloxR HLX at 40 feet produces 3.1 footcandles at line E and 7.8 at line F. Since the point is approximately one-fourth of the distance between the two isofootcandle lines, the value will be about 4.0 foot-candles. With the two floodlights contributing, the value in the center will be 8.0 footcandles. Note that the corners of the area will have very little light. This is why two or more floodlights are recommended at perimeter locations.

Another design method that will yield sufficient accuracy is the quick selector design method. The general layout considerations shown in figure 1-26 should be followed. The watts per square foot obtained from the graph in figure 1-27 produce an average lighting level accurate to

Figure 1-27.\Lamp watts per square foot chart.

within 20 percent of desired value. This is close enough since the difference between the luminaire requirement obtained from the graph and the number that will actually be needed to satisfy the physical requirements of the job involve adjustments greater than 20 percent. It is not unusual, for instance, to need two poles instead of one or to require three luminaires per pole instead of two. This calculation method should not be used for sports lighting or where the poles are set back from the area to be lighted.

Before determining the number of luminaires, you should work out the size of the lighted area location of the luminaires. Also, you should determine the maintained illumination level. The following rules of thumb provide some guidelines to help in these decisions.

Approximate required lamp watts/square foot.

1. From table 1-3, you find that the minimum average footcandles recommended for industrial yard/material handling is 5.

2. Read up the left side of the graph in figure 1-27 until you come to 5.0. Follow this line across until you intersect the dark diagonal line representing LucaloxR.

3. By reading straight down from this intersection to the value at the bottom of the chart, you find 0.095 lamp watts/square foot of the area is required to light the yard to 5 footcandles.

4. Area to be lighted is (160)(160) = 25,600 square feet.

5. Multiply 25,600 by 0.095 = 2,432 lamp watts.

6. By using the general layout considerations, you will find that the most economical floodlight installation will use the 400-watt LucaloxR lamps, mounted on 40-foot poles as shown below.