Yes, it’s true that low and high pressure sodium lamps have the highest efficacy among possible choices. But there is not a lot of tolerance for their dreadful color, except perhaps in the most utilitarian of applications. And indoor commercial LED lighting is many years away from being practical. For the majority of white light applications, skinny fluorescent tubes rule. It’s impossible to beat the 100+ mean lumens per watt of “super” T8 and the latest T5 and T5HO lamp and ballast systems.

T8 lamps

T8 lamps and electronic ballasts now are the standard commercial fluorescent lighting system in the United States. Introduced in the early 1980s to replace T12 lamps, T8 lighting systems account for more than 60 percent of all new fluorescent lighting systems.

However, the T8 system has evolved over the past 25 years. For instance, many people don’t remember that the T8 originally was a magnetically ballasted system, sort of like the older T12. It wasn’t until about 1990 that electronic ballasts began to dominate the marketplace. The lamp has changed and been improved, too. In addition to reducing the mercury content to meet federal TCLP standards, high performance lamps offer better color rendering (CRI), 15 to 20 percent more light and 20 to 50 percent longer life than the cheapest lamps.

Table 1 illustrates the range of products from three major lamp manufacturers.

The bottom row is referred to as “high efficiency T8” or “super T8” lamps. The Consortium for Energy Efficiency (CEE), an independent organization that develops and maintains standards for a number of energy-efficient products, has established standards for these lamps. In addition to the three major U.S. lamp companies, other suppliers provide high performance lamp products. For a complete list of high efficiency lamps, visit www.cee1.org/com/com-lt/lamps-ballasts.xls.

Four-foot T8 lamps also are available in reduced-wattage versions. Depending on the circumstances of their use, reduced-wattage lamps can be an efficient alternative. These lamps include the following:

There are drawbacks to using reduced-wattage lamps. According to the CEE, “For new construction and major renovation, higher system efficiency can be obtained by using an optimum combination of 32W high performance lamps and high efficiency electronic ballasts. Most reduced-wattage lamps are designed to be operated on instant start ballasts exclusively. Some will also operate on program start or rapid start ballasts. Compatibility varies; consult manufacturer’s literature before specifying products. Lamps are typically incompatible with dimming ballasts and should not be used in dimming applications unless the lamp and ballast manufacturers have approved a specific application for dimming. Reduced wattage lamps may demonstrate dim light, spiraling, pulsing and other undesirable behavior in cooler temperature rooms and while warming up.”

In other words, unless you have a well-thought-out and specific reason for doing so, the usage of reduced-wattage lamps is less efficient and more problematic than using the proper combination of 32-watt lamp and ballast.

T5 lamps

In the 1990s, T5 lamps started to show up. The modern T5 families include standard output (the 4-foot lamp being an F28T5), high output (F54T5HO), circlines and long twin tubes. T5 lamps were designed from the beginning to operate from electronic ballasts. In general, all of the improvements in high efficiency T8 lamps are built into the T5 already.

Many think that if going from T12 to T8 saves energy, then going from T8 to T5 will be even better. This is generally not true. There are several things you need to remember about T5 technology:

1.T5 lamps are metric lamps, and the 4-foot T5 is about 2 inches shorter than a 4-foot T8.

2.T5 lamps have different bases and use different sockets than T8 lamps.

3.In terms of glare, T5 lamps are 50 percent brighter than T8 lamps.

4.T5 lamps are more sensitive to cold temperatures. They take more time to warm up and are best used in enclosed luminaires. Outdoor use should be carefully evaluated.

Nonetheless, because they are smaller and quite energy efficient, many new lighting systems use T5 and T5HO lamps. See Table 2 for a comparison of lamps.

Note that the lumens per watt of the T5 system goes down if the lamps are measured at 25°C (77°F), just like the T8. But the T8 lumens per watt also goes down if measured at 35°C, like the T5. In other words, the T8 is more efficient in open luminaires, and the T5 is more efficient in closed luminaires. This can be affected also by location where thermal stratification can occur; in high bay fluorescent lighting, the tendency of air temperature to increase means that even in open luminaires, the T5 may be better.

There are some important new developments in T5 lamps. At least one manufacturer has introduced a “super T5” that raises the mean lumens per watt to 98 (at 35°C), making it the highest of all fluorescent systems. Also, there now are amalgam T5HO lamps, usable over a much wider range of temperatures, ranging from outdoor applications to comparatively hot industrial lighting, while maintaining at least 90 percent nominal lumen output. And a new lower wattage T5HO lamp that maintains the current lumen rating was revealed at a recent trade show.

What to use

Whenever possible, use 4-foot straight lamps. T8 lamps are also available in 2-, 3-, 5- and 8-foot lengths as well as U-bent lamps. Remember that all of the advances that resulted in the high performance T8 are pretty much restricted to 4-foot straight tubes and some 2-foot U-tubes. High performance U-tubes emit only 3 percent less light per watt than high performance straight tubes, making them a good choice for two-foot fixtures. As a rule, steer clear of designs that use short lamps.

T5 lamps are available in 2-, 3- and 5-foot lengths as well as circline and twin tubes. It’s tempting to use the circline lamp, as it is only 12 inches in diameter. However, it is 24 percent less efficient than a straight lamp. A better choice is the 22.5-inch-long, 55-watt twin tube lamp—it’s only 11 percent less efficient than the straight 4-foot HO lamp. But remember, these T5HO lamps all are less than 80 mean lumens per watt.

Best ballasts

While there are minor differences between T8 and T5 ballasts, most of the issues are the same. As a general rule, use the latest “efficient” electronic ballasts. These efficient ballasts are about 10 percent more efficient than ordinary electronic ballasts. For T8 lamps, a list of efficient ballasts is available on the CEE Web site. Note that efficient ballasts are not yet available for T5 lamps.

In order to select the right ballast, there are a lot more considerations now than ever, and for most of us, way too many choices. The chart below shows how to choose.

For many T8 applications, use of the various light output levels is a big bonus. Take, for example, an ordinary parabolic troffer installation in an office. Many class “A” office buildings standardize on three-lamp, 18-cell fixtures spaced on 8-foot-by-10-foot centers. With F34T12 lamps and magnetic ballasts, each fixture uses about 108 watts. Change to standard T8 lamps and generic instant-start ballasts, and each fixture uses 90 watts and produces about the same light level. Switch to high efficiency T8 lamps and ballasts, and you can use the low light-level ballast and still maintain the same light level using only 72 watts.

When it comes to fluorescent ballasts, it pays (literally) to be careful and specific. For example, the difference between a standard and an efficient T8 ballast is about 3 watts per lamp. With commercial lighting operating between 3,000 and 4,000 hours per year, the regular ballast will consume 9 to 12 kilowatt hours (kWh) per lamp more energy than the efficient ballast. At 10 cents per kWh, choosing the wrong three-lamp ballast will cost the building operator almost $4 per ballast per year in wasted energy.

With regard to dimming ballasts, the wrong choice will end up costing you, the installer, quite a bit. For the most part, the various types of dimming ballasts are not at all compatible, which means you’ll be accountable when the dimming system does not work. Carelessness in specifying ballasts will result in costly changes in the field.

Competing alternatives

Skinny tubes can be challenged by ceramic metal halide in a number of applications. The latest ceramic metal halide lamps can achieve almost 90 mean lumens per watt, with CRI 85 or higher. In high bay applications where lighting systems are turned on and left on, such as retail or transit, the optical efficiency of the HID source can easily make it compete and, in many cases, beat the fluorescent high bay. But only the 250-, 320-, 360- and 400-watt ceramic metal halide lamps have this level of performance. Lower wattage metal halide lamps with electronic ballasts can compete with standard T8, and sometimes T5HO and twin tube lamps, but not with the super T8 and T5 lamp systems.

Induction lamps are a form of fluorescent lamp that operate without electrodes, using instead an electromagnetic field to excite the gases within the bulbs. Practical induction technologies operating at up to 80 mean lumens per watt are available, making them competitive with ceramic metal halide and T5HO lamps for high bay spaces.

Compact fluorescent lamp systems still are a good choice for many applications, but like low-wattage MH lamps, they really can’t compete with the T5 and T8 systems. For example, the 32-watt triple lamp with electronic ballast produces about 60 mean lumens per watt; good screw-in CFL lamps produce 50–55 mean lumens per watt, but they can be unscrewed and replaced with an incandescent lamp. Very low wattage compact fluorescent lamps produce less than 50 mean lumens per watt, making them half as efficient as T5 and T8 skinny tubes.

Then there are LED lighting systems. The very latest and best products suitable for commercial lighting are less than 60 mean lumens per watt, expensive and not yet a proven technology. There is a lot of excitement about the prospect of LED lighting, but don’t expect to employ this technology on real projects for at least the next few years.

Picking lighting systems

For the foreseeable future, T5 and T8 systems should be used for most commercial and institutional lighting. Picking which will be harder:

To put it simply, pick the best lighting system for the application. In open applications, the T8 will always have an efficiency advantage, but the cost advantages of T8 will diminish as the lower cost of making and shipping T5 lamps takes over. Likewise, smaller luminaires using the T5 eventually will cost less than bulkier and heavier T8 systems. In short, expect the T5 to take a very strong position next to the T8 but neither above nor below. It will be quite a few years, it appears, before any other lighting technology rivals either of these systems in both energy and cost efficiency. EC