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Joined: Apr 2003
Posts: 362
I must be getting old!! I connot seem to remember how to find the wattage of a fluorescent fixture. 2 f40's with a magnetic ballast? I think .51 for the ballast.


Choose your customers, don't let them choose you.
Joined: Jul 2007
Posts: 1,335
It is based on tha ballast rating. If the fixture info is in front of me, I go to manufacters website and find a similar ballast and pad my numbers to be on the safe side

"Live Awesome!" - Kevin Carosa
Joined: Jul 2004
Posts: 9,917
Likes: 29
I think there is some safety margin built into that "40w".
When I put my clamp on a shop lite fixture with to F40CWs in it I actually get less than 80w, ballast and all.

Someone point out it might just be the meter is not handling the wave form properly tho.

Greg Fretwell
Joined: Jul 2007
Posts: 1,335
According to the GE Lighting Institute, the watt rating of the bulbs has nothing to do with the draw of the fixture. Hence, use the ballast rating

Last edited by sparkyinak; 12/10/12 04:11 AM.

"Live Awesome!" - Kevin Carosa
Joined: Apr 2003
Posts: 362
This is why I am confused. I thought that you add the ballast to the lamps and thats your total. Then I googled just to see and found so many different ways and none ended with the same result.

Choose your customers, don't let them choose you.
Joined: Dec 2012
Posts: 2
New Member
I agree with SparkinAK. When determining the size of the connected load or branch circuit for lighting, the ballast input in watts is used.

Joined: Apr 2002
Posts: 7,380
Likes: 7
Ballast data is used for actual lighting connected load calcs. Using the bulb wattage (2x40, 4x32,) can be used for a 'ballpark' figurre, which will be higher most of the time, than actual.

The same applies for HID lighting, use ballast data, not bulb wattage.

Joined: Feb 2003
Posts: 939
Normally I use the ballast data to sized the circuit and I don't bother to read the lamp wattage but useally resvered for ballpark figures plus add about 20% or so so you should land right on the spot.


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and
I agree with the previously posts - use the Ballast Data.

BTW, A Magnetic Rapid Start Ballast Load for Driving (2) F40T12 Lamps would be:

Line Input - 120 VAC:
Line Amperes = 0.69A
ANSI Watts = 82W
Average Ballast Factor = 0.84
BEF (Eff. Factor) = 1.02

Line Input - 277 VAC:
Line Amperes = 0.33A
ANSI Watts = 90W
Average Ballast Factor = 0.87
BEF (Eff. Factor) = 1.04

BTW, these figures are for F40T12 Lamps driven with the Cathodes being constantly Heated - AKA Rapid Start / Hot Cathode Operation.
Values will be lower for Instant Start Ballasting.
For Programmed Start Ballasting, the initial or Maximum Values will be similar.

Lamps will almost always draw less Wattage than they are rated at - especially when operated in extremely Cold or Warm environments.

--Scott (EE)

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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