Rookie question re: wattage of light fixtures

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dcw6363

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Madison, WI
I'm new to the field and there's something I don't understand about wattage in multi-lamp fixtures (specifically tube fluorescent light fixtures). Well actually there's a lot I don't understand about electrictal systems in general, but let's start here.

I keep seeing charts that list specs of various fluorescent tube fixtures, T8 vs. T12 etc.

example: http://www1.eere.energy.gov/femp/pdfs/fluor_lamps.pdf (second page)

The chart will list a multi-lamp fixture and its input power, but the overall input power for the fixture is less than the power required if you added the power for the mutiple lamps. So in the chart on p. two of the link above, a "T8, 32 watts, electronic ballast, two lamp" fixture runs on 57 watts.

I would have thought a fixture w/ two 32-watt lamps would require 64 watts for those lamps (plus ballast power).

I guess the wattage required for input of the whole fixture doesn't just add the wattage of each component?
 
Dennis Alwon

Dennis Alwon

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When dealing with fluorescents all you do is use the nameplate of the ballast, not the lamps. There is energy lost in the ballast and it has a power factor. The power factor for ballast can vary, so simply using the bulb wattage is no good. The higher the power factor the more energy the ballast saves.
 
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dcw6363

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Dennis Alwon said:
When dealing with fluorescents all you do is use the nameplate of the ballast, not the lamps. There is energy lost in the ballast and it has a power factor. The power factor for ballast can vary, so simply using the bulb wattage is no good. The higher the power factor the more energy the ballast saves.
Click to expand...

So if you know the power factor of the ballast, the wattage of the lamps, and the # of lamps per fixture, that's all you need to determine the input power for the fixture?
 
augie47

augie47

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Dennis, I agree with you, but see confusion in the chart he reference as is shows a two lamp fixture with 32 watt lamps at the best efficiency using 57 watts,
 
Dennis Alwon

Dennis Alwon

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dcw6363 said:
So if you know the power factor of the ballast, the wattage of the lamps, and the # of lamps per fixture, that's all you need to determine the input power for the fixture?
Click to expand...

Yes, I am no expert but watts of the bulb * Power Factor = wattage used. 64 watts * .89 PF = 57 watts. I think that is what it is.
 
jumper

jumper

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220.18(B)

(B) Inductive and LED Lighting Loads. For circuits supplying
lighting units that have ballasts, transformers, autotransformers,
or LED drivers, the calculated load shall be
based on the total ampere ratings of such units and not on
the total watts of the lamps.

PS. Welcome to the forum.
 
Dennis Alwon

Dennis Alwon

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dcw6363

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Location
Madison, WI
dkidd said:
Once again, its ballast factor. Many electronic ballasts have a ballast factor of .88.
Click to expand...

So would the following be correct:

"Fluorescent lights are tested with a standard ballast w/ a certain ballast factor, and that's how the light's wattage rating is determined. So it takes (say) 32W to achieve X lumens.

With a more efficient ballast w/ a better ballast factor, the same amount of lumens can be produced with less power input to the lights, and so each 32W light is running on only (say) 26W."

?

"PS. Welcome to the forum." Thanks
 
dkidd

dkidd

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The lamps data published is based on a ballast with a ballast factor of 1.0.

The efficacy of a given fluorescent tube is pretty nearly constant independant of the ballast factor. The lumens produced and the watts consumed both are dependant on the ballast factor.
 
steve66

steve66

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Engineer
dkidd said:
The lamps data published is based on a ballast with a ballast factor of 1.0.

The efficacy of a given fluorescent tube is pretty nearly constant independant of the ballast factor. The lumens produced and the watts consumed both are dependant on the ballast factor.
Click to expand...


So if a ballast runs a 32 W lamp at 26 watts, you will only get about 80% of the lamps rated lumens.

The wattage listed on the lamp is really just a nomial rating. The ballast really determines how much current is forced into a lamp, and the current determines how many watts the lamp consumes.
 
cadpoint

cadpoint

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Location
Durham, NC
dcw6363 said:
I keep seeing charts that list specs of various fluorescent tube fixtures, T8 vs. T12 etc.

example: http://www1.eere.energy.gov/femp/pdfs/fluor_lamps.pdf (second page)

The chart will list a multi-lamp fixture and its input power, but the overall input power for the fixture is less than the power required if you added the power for the mutiple lamps. So in the chart on p. two of the link above, a "T8, 32 watts, electronic ballast, two lamp" fixture runs on 57 watts.

I would have thought a fixture w/ two 32-watt lamps would require 64 watts for those lamps (plus ballast power).

I guess the wattage required for input of the whole fixture doesn't just add the wattage of each component?
Click to expand...

I really like Jumper's answer the best.

The NEC does state how it wants this aspect of the application applied.

IMO ...

There's some (frankly a lot) fluff build into services and circuits that are in the NEC, but it's still the NEC and it will just tell one what to do at times, and how to apply, what requires, and when...

The problem could be more times than not qualifying the question, then searching for the correct answer!

Enjoy the Forum!
 
dkidd

dkidd

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here
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PE
steve66 said:
So if a ballast runs a 32 W lamp at 26 watts, you will only get about 80% of the lamps rated lumens.

The wattage listed on the lamp is really just a nomial rating. The ballast really determines how much current is forced into a lamp, and the current determines how many watts the lamp consumes.
Click to expand...

That is pretty much the situation. You really need to find a data sheet for the specific ballast with the specific lamp.
 
B

broadgage

Senior Member
Location
London, England
A 32 watt flourescent lamp may use more or less than 32 watts depending on the design of the ballast.
In the past, most flourescent lamps were run at full power, and losses in the ballast would be in addition to the lamp power.
Therefore a fixture with 2 lamps each 32 watts, and ballast losses of say 6 watts, would have used in total 70 watts. 32+32+6=70

Many modern fixtures run the lamps at less than full power, and also have reduced losses in the ballasts.
The 32 watt lamps might be run at 27 watts each, with say 3 watts losses in the ballast, that would be 57 watts in total. 27+27+3=57 watts.

For design purposes, use the input current marked on the ballast, not the lamp wattage.
 
Electric-Light

Electric-Light

Senior Member
In US & Canada, almost all new indoor fluorescent installs use electronic ballast.
Most offer power factor so close to unity that for all practical purposes, they can be treated like a resistive load.

For 277v, best 0.87BF 2 lamp ballasts use about 54W. It's a watt or two more for 120v.

0.85 to 0.89 is the standard for "normal output" ballast. In recent years, we have low output (0.7 to 0.8) and high output (1.1 to 1.2).

The low BF units use less input, but they provide correspondingly less power to lamps.
 
T

TNBaer

Senior Member
Location
Oregon
Your best bet is think of a T8 Ballast as a Permanent dimmer. Depending on the Ballast Factor, it will dim the lights accordingly and therefore affect the wattage of whatever it is controlling.

If you're new to the world of lighting I highly recommend taking Philips' Advance University. It's free and will teach you all the fun basics. Enjoy!: http://www.advance.philips.com/university/
 
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