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Energy-efficient T8 fluorescent lamps with electronic ballasts are now standard for new fixtures and retrofits in commercial buildings, schools, and many industrial facilities. |
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Lighting systems are responsible for about 35 percent of the electricity costs in a typical commercial building and 10 percent in industrial settings.
In an effort to reduce lighting energy costs, T8 fluorescent lamps with electronic ballast have quickly become the standard for new fixtures and retrofits in commercial office buildings, schools, and a substantial portion of industrial lighting.
Inside the lamp are electrodes and a gas containing argon and mercury vapor. A stream of electrons flows through the ionized gas from one electrode to the other.
These electrons collide with the mercury phosphorand excite them. As the mercury atoms move from the excited state back to the unexcited state, they give off ultraviolet photons. The photons hit the
A typical conversion of T12 fixtures with magnetic ballast to T8 fixtures with electronic ballast costs about $20 per lamp including parts and labor. Energy savings from the conversion can pay back the initial investment in two to three years in an industrial application, and five to seven years in a commercial application.
The table below shows nominal input wattages and efficacy (lighting efficiency in lumens/watt) for T12 and T8 fixtures with comparable light output:
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T12 and T8 Comparison (4 ft. lamps) |
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T12s with magnetic ballast |
T8s with electronic ballast |
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Number |
Input |
Efficacy |
Input |
Efficacy |
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2 |
72 |
73 |
58 |
94 |
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4 |
144 |
73 |
112 |
97 |
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Notes: Input watts are open fixture ANSI watts. |
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Linear fluorescent lamps are specified by diameter size in 1/8ths of an inch. A T8 lamp is a 1-inch diameter lamp and a T12 lamp is a 1.5-inch diameter lamp.
The most common lengths for T8 lamps are four and eight feet, although T8 lamps can also be purchased in two, three, five, and six feet lengths as well.
T8s come in two different shapes, straight or u-tube. U-tube lamps are available in both 6-inch and 3-5/8-inch leg spacing and have an overall length of 22 inches. U-tube T8 fixtures work well for 2' X 2' ceiling panels.
The ballast is the brain of the fluorescent fixture, responsible for maintaining the current through the lamp and providing the initial starting voltage.
Magnetic ballasts, found in a T12 fixture, are less efficient, noisier, and heavier than the electronic ballasts in T8 fixtures. Over the past several years, costs of electronic ballasts have decreased dramatically making them comparable to magnetic ballasts.
Electronic ballasts do have one drawback compared to magnetic ballasts. Unlike magnetic ballasts, which operate at line frequency (60 Hz), electronic ballasts operate at 20,000 to 60,000 Hz and can introduce harmonic distortion or noise into the electric lines within the building, potentially overheating neutral lines, transformers, motors, and interfering with sensitive electronic equipment.
This is normally not a problem except for facilities with heavy lighting loads and a large number of electronic ballasts. In an effort to alleviate this problem most electronic ballast manufacturers have implemented passive filtering within the ballast to keep the total harmonic distortion to less than 20 percent of fundamental 60Hz current.
For facilities with strict requirements on power quality, such as hospitals and some laboratory or testing facilities, electronic ballasts with total harmonic distortion of five percent or less can be found.
Most fluorescent lamps generate less light when operated on a commercial ballast than they do on the laboratory reference ballast used to establish the lamp lumen ratings listed in lamp manufacturers' catalogs.
The ballast factor is the ratio of a lamp's light output for a given ballast and fixture combination, compared to the lamp's rated light output, as measured on a reference ballast at reference conditions. A lower ballast factor means less lumen output.
Electronic ballast factors range from about 0.7 to 1.2 with a ballast factor around 0.9 being the most common. The ballast factor can be multiplied by the lamp rated lumen output to determine actual light output for given ballast/lamp combination.
A low ballast factor does not necessarily mean that the efficiency of a fluorescent fixture is worse because the input watts to a fixture will also decrease with a lower ballast factor.
In a retrofit application, choosing the appropriate ballast factor can help achieve the desired lighting level without resorting to more expensive replacement of the entire fixture.
Another important aspect of fluorescent ballasts is the starting method. The goal of the starter is to provide initial ionization of the gas to allow for an electrical arc between electrodes and begin illumination. T8s are typically equipped with rapid-start or instant-start ballasts.
Rapid-start ballasts heat the electrodes quickly and then apply a 450-550 volt starting voltage to create the arc and begin the illumination process. There is a slight delay before the lamp begins to illuminate of one second or less with rapid start ballasts.
Most rapid-start ballasts continue to heat the electrode even after the lamp has started, which results in a power loss of 1.5-2 Watts per lamp.
Instant-start ballasts allow the lamp to start without delay by applying a high initial voltage, about 600 volts for T8 lamps, and instantly creating an arc across the electrodes.
Instant-start ballasts have the lowest power losses but can decrease the life of the lamp because of degradation of the emissive coating on the electrodes from the high starting voltage.
Lamps operating with instant-start ballasts will withstand 10,000-15,000 switch cycles compared to 15,000-20,000 switch cycles for rapid-start ballast.
The input wattages for T8 fixtures can vary significantly depending on a number of factors including ballast factor and operating ambient temperature.
The table below shows input watts for different 4-foot T8 fixtures with the same ballast factor:
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T8 wattages for 2- and 4-lamp 4-foot fixtures |
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Fixture |
2-Lamp |
4-Lamp |
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Standard Open (ANSI) |
58W |
112W |
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Recessed Troffer Closed |
55W |
111W |
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Recessed Troffer with Heat Extract 1 |
61W |
122W |
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Dedicated dimming ballasts are available for T8 lamps. Dimming ballasts are capable of modulating the light output from 100 to five percent.
Electronic dimming ballasts have become dramatically less expensive recently and can be found for around $40 per ballast, compared to $15 for standard electronic ballasts.
Dimming ballasts can be used in conjunction with daylighting to provide an effective energy savings measure for classrooms or offices.
The housing is where the all the electrical wiring and ballasts are mounted. The housing also plays a role in how heat is transferred out of the fixture.
An enclosed housing holds heat in the fixture, increases the ambient temperature surrounding the bulbs, and may decrease the input wattage to a point.
The reflector directs the emitted light from the lamps towards the surface or area where the illumination is intended. A measure known as reflectivity is the percent of incident light that is reflected by the reflector.
Most standard reflectors in fluorescent fixtures are painted white reflectors that produce diffuse, scattered, distribution of incident light with a reflectivity around 70 to 80 percent.
Advances in materials science have resulted in several key new materials capable of improving the reflective effectiveness. These new reflector materials have a mirror like finish that permits precise redirection of incident light rays.
Some of the newer reflector materials include:
Fluorescent fixtures can be retrofitted with new reflectors, often resulting in increased useable light, enabling building owners to remove lamps and saving energy while still maintaining acceptable lighting levels.
Careful consideration should be taken when retrofitting reflectors, because the change or redistribution of the light may not be desirable and depends on the application or space.
Louvers, shields, lenses, filters, or baffles are used to prevent the direct viewing of lamps within the fixture at normal viewing angles and also to diffuse the light coming out of the fixture.
Lenses and shields, like housings, can also trap heat and change the input power of the fixture.
Energy efficiency is important in properly implementing a lighting solution. However, the ultimate success of a lighting system depends on meeting the functional requirements and perceived lighting effect for the people working in the space.
Effective lighting design can be very involved and technical. Often, facility owners enroll the services of lighting engineers or professionals to design a functional and efficient lighting solution.
Retrofits of older T12 fixtures to T8 fixtures with electronic ballasts are simple and can be done without the involvement of a professional lighting engineer or contractor.
The older T12 magnetic ballast is swapped out with new electronic ballast, rewired, and the lamps are replaced. The two-pin bases are the same between T8s and T12s.
This is also a good time to clean any dust and debris from the reflectors. Retrofit costs are around $20 per lamp including parts and labor.
T8s have also become effective replacements in some high-bay applications where metal halide or high-pressure sodium fixtures are often used.
Careful consideration of the distribution and amount of light in the space is necessary because of the dramatically different styles of fixtures and reflectors.
Beyond just being more efficient from a lumens-per-watt standpoint, T8 fixtures can be a more effective solution for rack spaces because of the “line source” of light and more effective distribution of the light between the racks when compared to the “point source” of light for metal halide and high-pressure sodium fixtures.
Another factor when comparing T8 lamps and metal halide lamps is light depreciation. After 10,000 hours of operation, the light output in lumens for a metal halide lamp will depreciate about 20 percent of the initial lumens where T8 lamps will only depreciate about 10 percent.
T8 fixtures can be controlled by occupancy sensors or manually switched off when not needed, unlike metal halide and high-pressure sodium fixtures, which must cool off before being re-lit.
One must be aware of the color rendering requirements of the application.
The correlated color temperature (CCT) is a measure of the warmth of a lamps appearance measured in Kelvins (K). T8 lamps can be found in the 2,700-6,000K range. Select higher CCT-rated lamps when looking for a cool quality to the light and lower for warmer lighting characteristics.
Standard T8 lamps for offices are around 4,100K. This is more towards the cool side and creates a cool feeling, which is typically suggested for work environments.
Warm color temperatures of 3,000K or less create a desirable lighting environment for residences, restaurants, and other “hospitality” spaces.
The color rendering index (CRI) is used to quantify the color quality of a light source. The more accurately a light source renders a sample of standard colors relative to a reference source, the greater the CRI.
CRI is measured on a scale of 0-100. A standard T8 will have a CRI around 75. T8 lamps can also be found containing rare earth phosphors that dramatically increase the CRI for a reasonable cost. Rare earth phosphors can increase the CRI of a T8 lamp to the 80-90 range.
Most ballast manufacturers put passive filters on the ballasts to limit the amount of harmonic distortion.
Typical electronic ballasts have a total harmonic distortion less than 20 percent of the current of the fundamental frequency. If a facility has strict power quality requirements like a hospital or laboratory, ballasts with five percent or less harmonic distortion can be purchased.
Harmonic canceling transformers can also be implemented to filter out undesired harmonic distortion.
With the dramatic improvement in reflectors, the use of T8s in industrial warehousing is becoming more popular.
T8 lamps have less lumen depreciation when compared to metal halide, and can provide a better “line source” of light for certain rack-style warehouses when compared to the “point source” of metal halide fixtures.
Dimming ballasts are becoming less expensive and now, in combination with facilities that are being designed for day lighting or indirect lighting, can be an effective approach to reducing energy consumption in a facility.
Lighting controls, similar to computer networks, are also becoming more popular. Digital addressable lighting interface (DALI) systems allow dimming control or scene control of each light source within a lighting system, as well as the ability to detect faults within a light source and trigger alarms to a central management console.
These controls can be effective for spaces with varying lighting requirements like meeting or conference rooms.
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