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Tractor Talk Discussion Forum

flourescent light

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Gettin OLD

09-14-2006 18:37:48




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how many 8 ft lights can i use safely on a 15 amp breaker.




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MarkB_MI

09-14-2006 19:57:51




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 Re: flourescent light in reply to Gettin OLD, 09-14-2006 18:37:48  
As with everything, it depends. The newer fixtures (T8?) with 4 ft. bulbs are more efficient than the older fixtures with 8 ft bulbs. As John said, just figure out how much each fixture needs, round up and divide into 1800. If you figure 200 watts per fixture, then you should be able to put up to 8 fixtures on a 15 amp circuit and have room to spare.

My shop has six of the new style fixtures on a 15 amp circuit.

I suggest that you do not put all your lights on one breaker; that way if you blow a breaker you won't be in the dark.

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IaGary

09-15-2006 05:45:25




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 Re: flourescent light in reply to MarkB_MI, 09-14-2006 19:57:51  
Mark

At 200 watts per fixture I think 8 lights is 1 too many to stay under the 80% of capacity.

Gary



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Ken Crisman

09-14-2006 19:00:00




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 Re: flourescent light in reply to Gettin OLD, 09-14-2006 18:37:48  
When I was in my own business of residential wiring ; I usually put (3) 8' fixtures on a breaker . Another thing you want to watch is that when the ends of the tubes turn dark , don't wait till they go out . The longer you run a dark tube, the hotter that ballast gets which melts the insulation . Fires have occured due to people being too cheap or lazy to change them . Hope this helps . God bless . Ken

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John T

09-14-2006 18:50:45




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 Re: flourescent light in reply to Gettin OLD, 09-14-2006 18:37:48  
You have to know the watts of each fixture to determine how many total you can put on a 15 amp (14 gauge wire) branch circuit. At 120 volts a 15 amp branch circuit can deliver 1800 watts, sooooo oo you can draw a maximum continuous load of 80% of that which is 1440 total watts. Sooooo if the light fixtures were say 80 watts each, then 80 goes into 1440 18 times i.e. 18 lights = 1440 watts = 80% of the 15 amp 120 volt branch circuit continuous capacity.

Now all you gotta do is to know the total watts per fixture (thats the bulb(s) PLUS the Ballast) and see how many it takes to add up to under 1440 and thats how many fixtures MAX you can place on a 15 amp branch circuit.

Clear as mud?? Its late I didnt check my math lol

John T

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Gerald J.

09-15-2006 09:38:45




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 Re: flourescent light in reply to John T, 09-14-2006 18:50:45  
You can't figure fluorescent ballast line currents strictly from watts divided by voltage. They tend to have an obnoxious current wave form with a lot of peak and a small average. Called poor power factor. Doesn't matter if they are old magnetic ballasts or new electronic ballasts the peak current can be 5 times the RMS current and out of phase with the voltage making the watts lower than the voltamps.

Unless the ballasts have been bought at a premium price to be high power factor which isn't a common home center item.

That said, 3 or 4 two lamp fixtures (4' 2x32 or 2x40 lamps), seems a reasonable load on a 15 amp 120 volt breaker.

Gerald J. (electrical engineer).

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1 Denny

09-15-2006 05:25:28




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 Re: flourescent light in reply to John T, 09-14-2006 18:50:45  
Getting old, listen to what John T. has to say. I have been an IBEW state cert. electrician for 27 years and what John has to say is correct. What a lot of people miss when computing the load on a circut is the 80% NEC rule, which tells us if the load (lights) are going to be on for over a period of three hours the circut amps must be derated to 80% of the maximum wattage. On a 15 amp circut (15 x 120 = 1800 watts) 1800 watts is your maximum load but in this case you need to multiply by .8 which will give you your total LEGAL load on the circut which will be 1440 watts.This will make the NEC (nationl electrical code) and the local inspector and your insurance company real happy.

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John T

09-15-2006 07:19:27




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 Re: flourescent light in reply to 1 Denny, 09-15-2006 05:25:28  
Denny, Thanks for clarifying the 80% rule time requirements, I wasnt sure what it took for a load to be NEC considered as "continuous". I was an electrical design enginner but have been retired for some time and am rustyyyyy yy on the NEC, but was sure of the 80% load rules for "continuous" loads just unsure of the time lol

Its actually a good rule if one considers the use of Thermal Magnetic breakers and their design. The thermal portion trips after a long sustained curent draw near its rated load while the magnetic portion is for clearing a sudden high current shorting type fault. When I built my home in 1979 the arc fault circuit breakers werent around but I may install them now

I still didnt design right up to the limit and seldom used the maximum NEC allowable current draw even considering the 80% rule, 8 to 10 lights is all I liked on a single branch circuit as switchign and zoning are other practical factors that play a role in lighting design.

Take care now

John T

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