A 30 ft by 40 ft classroom with 8 ft high ceilings will have an ambient lighting target illuminance of 70 fc at a work plane that is 28 inches above the floor. It is anticipated that the ceiling reflectance is 0.50 and the average wall reflectance is about 0.3. The space will be illuminated with recessed lay-in 2 ft × 4 ft open parabolic troffer luminaires with three lamps, as shown in Figure 20.16. The initial output of the fluorescent lamps is 2850 lumen. The light loss factor will be assumed to be 0.70. Neglecting the spacing criteria, determine the minimum number of luminaires required to provide uniform illumination in the space.

Figure 20.16

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4 Linear Fluorescent Lamps 10 0 O Spacing Criteria (CS) 1.51 1.2|| Surface Prismatic Wraparound Fluorescent Luminaire (Four Lamps) Reflectance Ceiling cavity (Pec) Wall Surfaces (Pw) Cavity Ratio (CR) 10 9 8 7 6 5 4 3 2 1 0 50 80 30 .26 .29 .31 27 .35 .29 .39 .44 .48 10 .21 .17 .23 .19 .23 .26 50 Coefficient of Utilization (CU) 30 30 50 30 10 .24 .19 .26 .29 32 .36 .31 .29 .33 .29 .38 .34 .39 .35 .44 .39 .44 .40 .54 .49 .46 .48 .61 .56 .53 54 .51 .67 .66 .63 .61 .76 .76 .76 50 10 .16 .22 .18 .25 21 22 .25 .21 .28 .24 28 25 .30 .27 .33 .32 .37 .33 .29 .29 .31 .41 .38 .36 .37 .46 .43 .46 .43 .40 .43 .40 .48 50 58 .47 .47 .46 .57 .56 .55 .53 .52 51 .59 .67 .67 .67 .63 .63 .63 .57 .57 .18 .15 .18 50 10 30 10 .21 .17 .23 .20 .21 .26 23 .24 .29 .26 .25 .31 .30 35 .35 .39 .17 .17 .20 23 .26 .29 .33 .38 .44 .50 .57