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Bracket length, 12 feet

Required average maintained level of illumination, 2 footcandles

400-watt luminaire (50,000 lamp lumens)

Solution:

1. Spacing. The equation to determine correct spacing is

Spacing (S) = (LL)(MF)(CU) / (fc)(W)

Where: LL = MF = CU = fc = W = rated initial lamp lumens maintenance factor coefficient of utilization illumination in footcandles street width. curb to curb

The values are given for LL (50.000), MF (assume 0.70). W (50). and fc (2). After a value for CU is determined, you can solve the equation for average spacing.

To determine the coefficient of utilization, calculate the amount of wasted light on the street side (SS) and the house side (HS) where:

Ratio of HS = Transverse Distance / Mounting Height = 10 / 40 = 0.25

Ratio of SS = Transversel Distance / Mounting Height = 50 - 10 / 40 = 40 / 40 = 1.0

From the utilization curve in figure 6-17, the ratio of 1.0, street side,corresponds to 40 percent. and the ratio of 0.25, house side, corresponds to3 percent. for a total of 43 percent CU. Spacing can be determined as

S = (50,000) (0.70) (0.43) / (2)(50) = 150 feet.

2. Uniformity.

The uniformity of illumination is expressed in terms of a ratio of

Average fc / Minimum fc

It has been determined that one-side spacing of 150 feet will produce an average of 2 footcandles on the roadway surface. The point of minimum illumination can now be determined from the isofootcandle diagram.

The minimum value of the illumination can be found by studying the isofootcandle diagram and tak- ing into account all luminaries that are contributing significant amounts of light. Generally, the minimum value will be found along a line halfway, between two consecutively spaced luminaries. The minimum value can be determined by. checking the minimum foot- candle values at points P1, P2, and P3. as shown in figure 6-18.

The roadway surface can be plotted on the isofootcandle curve by observing the 40-foot mounting height to longitudinal and transverse distance ratios. (See fig. 6-19.) Since PI is located outside the 0.02 footcandle line. it is the lowest total footcandle value. This value would be 0.03 fc (0.015 footcandle from each luminaire).

Figure 6-20 shows a perspective view of the two isofootcandle lines that are considered when determining the illumination value at P1.

The following factors are now applied to this "raw" footcandle value, as shown in the formula: fc min = (fc) (LF) (MF) (CF) Where: fc min = minimum point footcandles fc = raw footcandle from isofootcandle diagram LF = lamp factor MF = maintenance factor CF = mounting height correction factor

The values are given for fc (0.03) and MF (assume 0.70). The value for LF was determined earlier as 50 for the 400-watt lamp. The CF factor can be determined from the correction chart below the isofootcandle curve in figure 6-14. The CF for a 40- foot mounting height is 0.56.

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