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#56356 - 09/19/05 04:47 PM fluorescent watts
kd Offline
Registered: 12/14/04
Posts: 52
Does the 32 watt rating of a fluorescent tube include the electricity used by the ballust?
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#56357 - 09/19/05 04:50 PM Re: fluorescent watts
Active 1 Offline
Registered: 02/22/03
Posts: 687
Loc: Grayslake IL, USA
#56358 - 09/19/05 06:01 PM Re: fluorescent watts
renosteinke Offline
Cat Servant
Registered: 01/22/05
Posts: 5316
Loc: Blue Collar Country
More to the point, the watt rating of the bulb is nominal, not actual. The actual electricity used by the complete fixture is indicated by the current marking on the ballest itself.
#56359 - 09/19/05 07:18 PM Re: fluorescent watts
ShockMe77 Offline
Registered: 06/11/05
Posts: 821
Loc: Rahway, New Jersey
This topic is something I did not know for the longest time. It's very important for calculating loads for several fluorescent light on the same circuit. And until rcently, I would always mispell FLUORESCENT.
#56360 - 12/23/05 06:47 PM Re: fluorescent watts
Registered: 12/08/05
Posts: 854
Loc: Titirangi, Akld, New Zealand
Add at least 30% for choke on top of wattage rating of tube.
The product of rotation, excitation and flux produces electricty.
#56361 - 12/24/05 01:38 AM Re: fluorescent watts
Scott35 Offline

Broom Pusher and
Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA
Fluorescent Lamps will almost always consume less wattage than the listed rating shows.

For example, an F32T8 Lamp draws a nominal 29 watts, and the most it will ever draw will be 31 watts.

Lamps using Rapid Start (Hot Cathode) operation will have wattages drawn by both the heating of the cathodes, and the transduction of Electron emission / inert material bombardment, into visible light output.

The True Power drawn for heating of the cathodes is "parasitic", as viewed to overall output light vs. input wattage. It does not directly transduce into visible light output, only helps make the emission of Electrons more efficient.

Ballast load consumption of True Power is also "parasitic", as it does not produce any visible light at the lamp.

Whatever heat is felt at the Ballast is the resultant of the True Power it has consumed from the Generating Source.

Typicals for a 2 lamp Magnetic Ballast, driving two F40T12 Lamps, were like 4 watts consumed by the ballast - and around 77 watts by the lamps - with an additional 2 watts
drawn to heat the Lamps' cathodes.


Do not confuse input (Line) Volt-Amperes with input wattage

Linear Reactor Core Ballasts will normally draw around 1.25 times the rated lamp wattage, as input VA.
This would equal what would be seen on an ammeter measuring the input to the Ballast -
the input system voltage -
so there is an obvious Power Factor Scenario here

For the "Non-Magnetic" flavors of Ballastry currently being used

Depending on the designed functions of Electronic & Hybrid type Ballastry, some may drive lamps at high power levels, some at low power levels (a lot of the ones I deal with drive lamps at the lower power levels), and others will drive lamps at the "best for the current conditions" power level.

Also, these Ballasts may have a Leading Power Factor, Lagging Power Factor, or the mind boggling
"Almost as close to unity Power Factor as a pure Resistance Load would be - yet still be a Reactive Load"

Again, whatever heat is felt at the ballast, will represent a large portion of the wattage drawn from the generating source by it (drawn by the ballast).

BTW: The "Generating Source" might be:
  • Steam Power Prime Mover, using Coal as Fuel,
  • Steam Power Prime Mover, using Oil as Fuel,
  • Steam Power Prime Mover, using Uranium 235 as Fuel (via reactor core and moderator),
  • Steam Power Prime Mover, using Geothermic Energy (or the Earth as Fuel),
  • Internal Combustion Engine Prime Mover, using Gasoline or Diesel as Fuel,
  • Gas Turbine Prime Mover, using Natural Gas as Fuel,
  • Pedal Powered / Human Being Prime Mover, using Carbohydrates (food) as Fuel.


edited to courrekt spel-lieenng miss steaks

[This message has been edited by Scott35 (edited 12-24-2005).]
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#56362 - 12/24/05 03:30 AM Re: fluorescent watts
iwire Offline
Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
Code wise there is this.

210.4(B) Inductive Lighting Loads.

For circuits supplying lighting units that have ballasts, transformers, or autotransformers, the computed load shall be based on the total ampere ratings of such units and not on the total watts of the lamps.
Bob Badger
Construction & Maintenance Electrician
#56363 - 12/24/05 08:24 AM Re: fluorescent watts
pauluk Offline
Registered: 08/11/01
Posts: 7520
Loc: Norfolk, England
Over here, there was a long-standing rule that in the absence of more detailed information and specific p.f. correction the VA rating for calculating current demand must be taken as the lamp rating in watts multiplied by at least 1.8.

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