Consulting-Specifying Engineer - December 2008 - (Page 48)

Case study: clashing codes By Ken Lovorn, PE, Lovorn Engineering Assoc. Inc., Pittsburgh ome municipalities have specific regulations regarding lighting levels in parking garages to improve the safety of the garage patrons. One city requires every point in the parking garage to have a minimum of 5 foot candles at 5 ft. This requirement covers any and all areas of a garage in which garage patrons park, drive, or walk in their use of the facility. Application of this regulation to a typical parking garage can result in an average lighting level of 10 to 12 foot candles and a maximum lighting level that can exceed 20 foot candles. Also applicable to the lighting system in this garage is the lighting power density (LPD) of 0.3 W per sq ft as indicated in the International Energy Conservation Code (IECC). Some states consider the IECC LPD as a code that should be followed like any other code governing a facility’s design and construction. However, in Pennsylvania, the IECC has been adopted by law so that enforcement of its requirements is mandatory. For this particular installation, the garage was existing and the lighting was being retrofitted to meet both the city requirements for minimum lighting levels and the IECC requirements for maximum power density. After attempting to achieve both the maximum LPD and the minimum lighting levels with known types of parking garage lighting fixtures, designers found that it would be impossible to achieve both requirements with one fixture type. The IECC maximum LPD was easily met, given the poor reflectances, low ceiling heights, and structural obstructions, by simply using more fixtures with a lower lumen output. The minimum lighting level of 5 foot candles was easily met by using a similar fixture with a higher lumen output. In every case, however, meeting the maximum LPD and the minimum lighting level at every point was not possible using one fixture type with the same lumen output. S be turned off entirely, even on cloudy days. Light fixtures in the area of 30 to 60 ft from a perimeter wall opening could be turned off if a photocell mounted on an interior column at 5 ft above the floor measures adequate daylight infiltration (more than 20 foot candles). Those lights would turn back on if the daylight infiltration fell below 5 foot candles. Lighting selection Parking structures are typically lighted 24 hours per day, 7 days a week, or 8,760 hours per year, which makes the electrical utility cost the first or second largest expense for parking structure operation. The typical parking facility lighting system consists of 150 W metal halide or high-pressure sodium fixtures with a total input wattage of approximately 190 W per fixture, including the ballast power consumption. These metal halide fixtures may be placed on a spacing of approximately 30 ft by 30 ft (one every 900 sq ft). A 1,000-car parking structure will have approximately 360 light fixtures in the general parking areas. The total annual power consumption for this sample parking facility would be approximately 600,000 kWh. With an energy cost of $0.08 per kWh, the annual utility cost would be about $48,000 and could vary considerably in areas where energy costs are higher, such as on the East and West Coasts where utility costs are nearly double the national average. Substituting more energy-efficient light sources also can reduce lighting operational costs. A light fixture with four 4-ft-long T8 lamps or two T5HO lamps will have nearly equivalent maintained illuminance as a 150 W metal halide or high-pressure sodium fixture. The energy consumption of the fluorescent fixture is 112 W for the T8 fixture or 123 W for the T5HO fixture, compared to 190 W for the metal halide or high-pressure sodium fixture. Therefore, there is a 41% energy savings for the T8 fixture or 35% savings for the T5HO fixture. However, fluorescent lamps lose light output in cold weather and can be used only in enclosed fixtures equipped with low temperature ballasts, where the temperature is frequently below freezing. LED light fixtures are not impacted by cold weather and use 128 input W, resulting in an energy cost savings of approximately 33% compared to 150 W metal halide or high-pressure sodium fixtures. The unit cost of LED fixtures is three to four times the cost of a high-intensity discharge fixture, which generally precludes their widespread use. Retrofitting parking structure lighting with automatic controls and more energy-efficient light sources not only reduces parking facility operational costs, but it also minimizes the need to expand power plants that contribute to global warming. Monahan is vice president at Walker Parking Consultants. He has 31 years of experience in parking facility design with specialized expertise regarding lighting, safety, and security of multilevel parking structures. He is past chairman of the Off-Roadway Lighting subcommittee of the Illuminating Engineering Society, Chairman of the Parking Facility Lighting task force, and was a contributing author of IES publication RP-20-98, Lighting for Parking Facilities. 48 Consulting-Specifying Engineer • DECEMBER 2008

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Consulting-Specifying Engineer - December 2008

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