The initial footcandle table gives the footcandlevalue for each isofootcandle curve at a specificmounting height. The values for each letter are thesame on each set of curves. That makes it possible tocompare diagrams directly and interpolate betweencurves for different aiming distances.The mounting heights given in the initialfootcandle table are representative of the wattage andbeam pattern associated with the floodlight. Toconvert to other mounting heights, use the followingformula:OLDNEW(FROMCHART)(FROMCALCULATION)(fc)(MH^{2})(fc)(MH^{2})For example, a 5-footcandle level at 50 feet(isofootcandle curve F) would have a value of 4.13 at a55-foot mounting height.(5)(50^{2}) = (fc)(55^{2})fc = 4.13In figure 6-27 (aiming point MH x 2), thefloodlight is aimed a distance of two mounting heightsaway from a point on the ground directly below thefloodlight. That would be 80 feet for a 40-footmounting height.UTILIZATION GRAPHThe luminaire utilization data graph (fig. 6-27)gives the percentage of the initial lamp lumens that fallinto the area being lighted. Knowing this, you caneasily determine the average lumens per square foot, orfootcandles.The number beside each curve identifies theaiming point, so that the utilization curve can beidentified with the associated isofootcandle diagrams.In the example, for instance, the floodlight aimed twomounting heights away from the pole would have autilization of 35 percent if it were lighting an area threemounting heights wide. The same floodlight aimed atone mounting height away from the pole would have autilization of 45 percent for the same area.MAINTENANCE FACTORLighting efficiency in floodlighting, as instreetlighting, is seriously impaired by blackenedlamps, by lamp life. and by dirt on the reflectingsurfaces of the luminaire. A maintenance factor (MF)must be applied in the lighting calculations tocompensate for the gradual losses of illumination onthe lighted area.The following maintenance factors have beenwidely used in industry when manufacturer’sinformation is not available:Enclosed flood lamps, 0.76Open flood lamps, 0.65LIGHT INTENSITY CALCULATIONSThere are a number of ways by which to determineluminaire requirements. Since most methods wouldrequire an engineering background, we will onlydiscuss the basic area lighting design considerationsthat you, as a Construction Electrician, can perform inthe field if engineering assistance is not available. Tobetter 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 NEMA6 x 5 HLX 1,000-watt floodlights.Solution:1. Apply the 2X-4X rule (fig. 6-28) to determinespacing and mounting height. A 40-foot mountingheight 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 =(N)(LL)(CU)AREAFigure 6-28.—Material yard sample problem.6-25

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