overdriven fluorescents - SL15 revisted
My neighbor, who has been lurking on this board for some time, asked me about overdriving lamps with Sunpark SL15 ballasts.
There are better options.
The SL15s are very inefficient and not particularly well built.
Efficiency: The SL15 ballasts have a 50-60% power factor. This means that 40-50% of the power consumed goes to heating up the ballast not to lighting the lamp -- raising the electric bill and heating up the fixture. Decent electronic ballasts have a .98 or better power factor.
Durability: I hung 20+ SL15 based shoplights, for what was supposed to temporary lighting a few years ago. After 16 months I had to start replacing ballasts at the rate of 2-3/yr in a not demanding environment (Price: 2 SL15 shoplights now run about $16 all told at my local HD. Shopping online Tech Consumer Products Inc ballasts run about $14-$15 ea, GE UltraMax run about $16-$17ea.
If IÂm not overdriving a two lamp fixture, for $7 more I can get a really good ballast that uses one-half the power, paying for itself in about 1000hrs.
If I want to overdrive, for the price of two SL15s I can get one really good 4 lamp ballast and use it to overdrive 2 lamps.
Ballast factor:
ÂNormal ballast factor lamps (including the SL15s) have a ballast factor of .88 -- they run a lamp at about 88% output compared with a lab standard and are tuned to maximum efficiency at that point. High ballast factor lamps typically have a ballast factor of 1.15  they generate about a third more light than a Ânormal ballast and are tuned to maximum efficiency at that point.
To moderately overdrive your fixture use a name brand high ballast factor 2 lamp ballast. YouÂll get a third more light and only use 75% as much power compared to using a single SL15. YouÂll also be using a ballast specifically designed for the job.
Using a Ânormal bf 4 lamp ballast to drive two lamps youÂll get about 40% more light while still using 20% less power than a single SL15.
Using a Âhigh bf 4 lamp ballast to drive two lamps youÂll get almost double the light while using only about 30% more power than a single SL15. (My spec sheet graphs extrapolate to twice the lumens, but you are operating outside the design envelope so I would expect 180% to be a more reasonable guess).
By comparision, using a pair of SL15s gives you about half again as much light, using 1.7 times the power of a single SL15.
For design flexibilty TCPI offers a 4-3-2 ballast you can wire as a normal two-lamp, normal four-lamp, or overdriven two-lamp fixture. (I donÂt think it comes in a high ballast factor model).
Finally, a note on lamp starting:
Rapid start ballasts (such as the SL15) constantly heat the lamp filaments, even after the lamp is lit. This means that about 5% of the power going to the lamp is just converted to waste heat, not lighting the lamp,
Instant start ballasts just heat the filament at starting, hence are more power efficient, at the expense of reduced lamp life (in terms...
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