The nomenclature of T12 and T8 fluorescent lights describes the diameter of the tube. A T8 fluorescent tube is eight-eighths of an inch, or 1 inch, in diameter. A T12 fluorescent tube is twelve-eighths of an inch, or 1 1/2 inches, in diameter. Both T12 and T8 fluorescent tubes come in a variety of lengths.
T8 fluorescent tubes consume significantly less energy than T12 tubes. A four-lamp F34T12 fixture with a standard magnetic ballast consumes 148 watts per year, while a four-lamp F32T8 fixture with a high efficiency ballast consumes only 96 watts per year, according to the Universal Lighting Technologies website. Converting T12 fixtures to T8 fixtures can result in an energy savings of 52 watts per year per fixture.
T8 fluorescent tubes may produce a greater or lesser light level than comparable T12 fluorescent tubes, depending on the fixture and the tube used. A four-lamp F32T8 fixture with a high efficiency ballast can result in an increased light level of 3 to 12 percent, depending on the tube, according to the Universal Lighting Technologies website. A four-lamp F28T8 fixture with high efficiency ballast may result in a decrease in light levels of up to 3 percent, but may improve the perceived quality of color. The Philips F32T8/TL735/ALTO tube produces 2,800 lumens, compared to the Philips F34T12 CW RS EW ALTO tube which produces 2,300 lumens.
T8 fluorescent light fixtures typically cost less to operate than comparable T12 fluorescent light fixtures. Universal Lighting Technologies estimates an energy cost savings of $30.72 per year for a four-lamp T8 fixture when compared to a T12 fixture based on 4,000 annual operating hours at a utility rate of 12 cents per kilowatt hour. The Food Service Warehouse website notes that the cost to transition from T12 fixtures and tubes to T8 fixtures and tubes was approximately $20 per lamp as of 2011.