Design, color and decor may seem, at first glance, to be the primary factors in a room’s usability and comfort. Without proper lighting, however, those factors fade into the darkness. In addition, occupants may not be able to see enough to perform tasks, especially at night. Using the correct amount of light for the room size can avoid these issues.
To determine the number of watts needed by a particular room for general lighting, multiply the width times the length of the space to determine the square footage. For example, a room measuring 10-by-20 feet equals 200 square feet. Then multiply the product by 1.5. In this case, the 200 square-foot room needs 300 watts of lighting. For task lighting in areas such as the kitchen, where stronger light is needed, multiply the square footage by 2.5. In this case, a 200-square-foot kitchen needs 500 watts of lighting.
These guidelines determine the number of standard incandescent watts required for a room. Using halogen incandescent and compact fluorescent bulbs can provide the same amount of light for far less energy. The California Energy Commission states that the amount of lumens, which is a measurement of lighting power, produced by a 100-watt traditional incandescent is 1,600 to 1,700. (The packaging lists the lumens of a light bulb.) A halogen incandescent produces the same amount of light for only 72 watts and a compact fluorescent uses only 23 watts. So a 200-square-foot room that requires 300 watts of standard incandescent lighting needs only 216 watts of halogen incandescents and only 69 watts of compact fluorescents to produce the same 1,600 to 1,700 lumens.
The number of fixtures needed in a room depend on the total wattage required divided by the number and wattage of bulbs used by each fixture. For example, a dining room that requires 300 watts of incandescent lighting can have three recessed lighting fixtures with 100-watt bulbs in each. Alternatively, it can have one chandelier with 10 bulbs at 30 watts each. The height of the fixture can also determine whether more or fewer are needed to illuminate all the floor space. A bulb generally covers four times its mounting height in square feet. For example, in a fixture mounted 10-feet high covers 40 square feet. So a 200-square foot room would need 5 fixtures mounted at 10-feet high for total coverage.
Local and state building codes in the U.S. are typically based on the International Residential Code or IRC, which is maintained by the International Code Council, a nonprofit organization. The IRC states that habitable rooms such as living and dining rooms need at least one wall-switch-controlled lighting outlet. At least one of these types of outlets are also required in hallways, stairways and garages. Storage spaces such as attics, utility rooms, basements and under-floor spaces also need at least one wall-switch-controlled light.
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