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FAQ: T8 or T5?
Numerous discussions surround the decision to use T8 or T5 HO when replacing HID. As a manufacturer that utilizes both lamp types, we would like to offer some important points for consideration. If you ask the right questions, you should get the right answers. For clarification, we are addressing the use of T8 and T5 HO lamps in industrial applications, specifically high and low-bay lighting. Indirect lighting for office space is a separate topic.
We feel T8 lamps provide better quality light with fewer glares in application heights below 18 feet. T5 HO delivers a brighter light source within a smaller diameter, allowing us to design fixture reflectors with better optic precision...in turn, this provides increased light output and better fixture efficiency for higher applications. There is an overlap between 18 and 25 feet where either T8 or T5 HO may be used successfully. This is were you really need to pay attention to what your asking for and what you may get.
Lamp and Ballast Choices
If you look in a current catalog there are over 50 types of 4-foot T8 lamps available as replacements (standard CRI, Standard CRI w/ extended performance, high CRI, high CRI w/ extended performance, high-lumen extended performance, energy saver, long-life energy saver) as well as the whole selection of colors. At this point, there are only two types of T5 HO lamps: 28-watt T5, and for our purpose, the 54-watt T5 HO. Both lamp types are 45.8".
To get high output performance out of a T8 fixture, T8 800/XPS series lamps (also known as high-lumen or high performance) need to be combined with a T8 high ballast factor ballasts. This combination will safely overdrive the lamp's output by an additional 12-15% and provide the highest lumen per watt ratio currently available on the market today.
The older 700 series T8 lamps are as common today as T12 lamps were yesterday. Price is very low and they can be found in most hardware and DIY stores. High performance T8 lamps (800/XPS series), and for that matter, all T5 HO lamps, are typically only stocked by lighting and electrical distributors. Keep in mind; well-stocked distributor may carry complete lines of all lamp types mentioned above, so be specific about your needs.
Re-Lamping and Attention to Detail
When it's time to re-lamp, it's crucial to remember that if you want the same or equal performance as the original installation, you must re-install the same type of lamp. If you previously used the high-lumen / high-performance T8 lamp there will be somewhere between 8-10% lumen depreciation at end of life (not counting lamp outages). If you re-lamp with the less expensive 700 series T8 lamp, you'll end up with lower lighting levels prior to re-lamping.
Remember, a standard 700 series T8 starts out with 15% less lumens compared to a 800 series T8. This means the chances are very good you may end up with a 5% decrease in lighting levels and still not have maintain any energy savings.
What T8 lamps are currently in use elsewhere is your facility?
We've heard several customers and specification technicians suggest standardizing on lamps for individual facilities. T8 lamps are chosen over T5 HO solely on the basis that other T8 lamps are presently being used in office areas or elsewhere in the facility.
Frequently we find the less expensive T8 lamps are being used primarily because the purchasing department has a pre-arranged buying agreement or there is no apparent need for the additional lumen output (as well as the cost involved) from high performance lamps.
Keeping the purchasing routine of replacement lamps simple should be a priority. We suggest checking with the buyers who purchase replacement lamps as well the contactors responsible for the installation. If they are set on buying what is currently under contract, or what is available, there are three choices for action:
1.Change the lamp specification of the new fixture to correspond with future purchases (this may change the light output of your fixture selection).
2. Try to educate the purchasing and maintenance departments on the value of new lamp types and have replacement stock added into inventory.
3. Offer fewer T8 lamp options (along with the opportunities to install the wrong lamp) and go with T5 HO.
Controls and Frequency of Switching
Because fluorescent lamps have instant start capability, it is often recommended that occupancy sensors be installed to maximize energy savings. Occupancy, or motion sensors, allow the fixture to be automatically turned on when a space is occupied, and off when unoccupied.
The most commonly used T8 electronic ballasts used to overdrive T8 lamps utilize an instant start technology. These ballasts rely on an immense amount of electrical current being applied to the cathode to start the lamp, possibly contributing to lamp degradation (shorter lamp life), especially with frequent on/off cycles.
The T5 HO electronic ballast uses programmed start technology resulting in a gentler lamp start. This will provide 100,000 starting cycles without any lamp degradation. If sensors are used on fixtures supplied with T8 lamps and instant start ballasts, you can expect to re-lamp sooner than a comparable T5HO system.
Although a T8 lamp is less expensive than T5 HO, any potential savings may be negated by the cost of having to re-lamp sooner than expected. Keep in mind; If the on cycles are longer, or the lamps are on constantly, then T8 lamps may actually outlast T5 HO.
Beware! Not all fixtures are created equal.
Now that you've made the decision, you need to concern yourself with fixture performance. Any fixture that is originally designed for T12 lamps and converted to operate T8 or T5 HO will probably not achieve expected results. In order to get the most out of any lamp, the optics of a fixture must be designed specifically for it's lamp type.
Another consideration may be the actual size of the fixture. T8 lamps are larger in diameter than T5 HO. Accordingly; a well-designed T8 fixture (with the same lamp configuration) should be wider than a T5 HO.
A properly designed fixture may cost more initially, but with better performance you may end up using fewer fixtures resulting in lower installation costs. You may also find a well designed fixture will provide satisfactory lighting levels and energy savings while using fewer lamps.
Wrapping It Up!
Lastly, the number of lamps required to achieve desired lighting levels is an important factor when upgrading. As a general rule, a 4-lamp T5 HO fixture will replace a standard 400-watt HID fixture on a one for one basis while improving light levels.
It takes 6 T8 800/XP series lamps with high ballast factor ballasts to achieve the same results. Certainly, the T8 lamp with it's various ballast types and ballast factors offer more options to fine tune desired lighting levels and cost savings.
When the time comes to make your decision, ask around; see what's worked for other applications. If there is still any doubt, ask for a demonstration or sample fixtures to install.
Even with exhaustive research and calculation, ultimately, there is nothing more convincing than actually seeing the installed performance of a fixture.
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