SELECTION OF LUMINAIRE

The National Electrical Manufacturer's Association (NEMA) has classified floodlighting luminaires into four classes according to construction:

1. Class HD (Heavy Duty)\Enclosed with an outer housing into which is placed a separate and removable reflector, or an enclosure in which a separate housing is placed over the reflector.

2. Class GP (General Purpose)\Enclosed with a one-piece housing with the inner surface serving as a reflector and the outer surface being exposed to the elements.

3. Class O (Open)\One-piece housing without cover glass.

4. Class OI\Same as Class O except with an auxiliary inner reflector to modify the beam.

The suffix letter B should be added to the above class designations to indicate an integral ballast when required.

Example: A heavy-duty floodlight with an integral ballast would be designated as a Class HDB floodlight.

The beam spread can be described in degrees or by NEMA types (fig. 1-19). The beam spread is based on the angle to either side of the aiming point where the candlepower drops to 10 percent of its maximum value. The lamp and floodlight NEMA type is given in the upper left-hand corner of each isofootcandle diagram.

The NEMA type should only be used as a reference. It does not describe the shape of the light pattern the floodlight produces or the peak illumination level (footcandles). Symmetrical floodlights have the same horizontal and vertical beam spread and are classified with one NEMA number. Asymmetrical beam spreads have a horizontal (H) and a vertical (V) designation. The horizontal value is always given first.

Figure 1-19.\Luminaire designations.

MOUNTING HEIGHT AND SPACING

The size of the area to be illuminated has a direct effect on determining the number and spacing of the poles. The suggested area that can be covered by a single pole is four times the mounting height. This is known as the "2X-4X" rule.

Areas lighted from interior poles or other central locations (fig. 1-20a) can be more economical, but perimeter locations are also desirable to provide needed visibility at entrances and exits. In the case of perimeter poles (fig. 1-20b), if corner locations are not used, the distance from any side location to the edge of the area should not exceed twice the mounting height. If building-mounted luminaire locations are limited to only one side of the area to be lighted (fig. 1-20c), the system will be effective for a distance of only two mounting heights unless glare is not a determining factor.

According to the 2X-4X rule, the spacing is determined to be, from the corner to the first pole, two times the mounting height (X). The next pole is set four times this mounting height (X), and the CE will continue in this manner until reaching the last pole, which also is to be set two times the mounting height from the far corner. This rule can be used to calculate the minimum number of poles. For long, narrow areas, it is better to choose several short poles than one tall one, especially since pole costs increase substantially above 40 feet. It is wise to consider several alternatives, however, to determine the system with the lowest cost.

Figure 1-20.\2X-4X mounting height rule.

If the pole is located inside the area to be lighted, there should be at least three floodlights or two streetlights per pole. For one side perimeter mounting, there should be two floodlights or one streetlight per pole.