Lighting Calcs for indiv. branch circuits

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starbright28

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I need some help with a calculation that I am working on at work here. It is a commerical building.

We have fluorscent light fixtures we use for these different rooms. Soemtime rooms are grouped together and put on one circuit. On the panelboard schedule we list the VA and Watts for the fixtures associated with this circuit. The engineer here usually does the calculations, but I want to know how to do them as well, so I can lay out the panelboards as well.

For example: I have 5 rooms connected to one circuits. Each light fixture ranges in the total number of watts associated with the fixture, as well as the room may have more than one fixture in the room. When added up, the total wattage for all the light fixtures in this room I get 1024 watts. This is just for one circuit.

I am merely trying to figure out how the engineer got his number on the panelboard schedule. He has listed for this circuit the VA and watts as 960.

I'm wondering if there is a special calcualtion that I am not aware of, or if he just added up wrong or what.

Can you help me out on what to do, or steps to figure this out? I've tried looking in the code book for examples, and couldn't find none.

It's on a 20 amp circuit.

Thanks in advanced.

Amanda
 
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Its not unusual for 2 people to get different watages on lighting circuits.
So did you use the wattage of the lamps or ballasts to get your total watts?

A ballast that feds (2) 32 watt lamps can have a draw of only 60 watts.

I'm assuming you have a total of 16 2 lamp fixtures ;)

Brilliant!
 
Amanda, it appears as though you are trying to use the lamp wattage for your totals (32 lamps at 32 watts) and if that is the case that would be the problem.

You have to use the ballast input.

Roger
 
I'll explain the wattages of the lamps and how I got the number:

3 - (2) 32 watt t8 lamp fixture = 192 watts
1 - (2) 32 watt t8 lamp fixture = 64 watt
6 - (3) 32 watt t8 lamp fixture = 576 watts
2 - (3) 32 watt t8 lamp fixture = 192 watts

total wattage of this fixture = 1024 watts


yes I am using the lamp fixture wattage for my calculations, and hence maybe why things are not lookig correctly to me?

I never knew how to assign circuits to light fixtures in a building, and wanted to learn so I can do it for up and coming projects at my company.
 
Amanda you will never have a problem doing it that way. It is a conservative calculation. I round all my lighting calculation up anyway and do not load the circuits past 14 amps. If you want to be exact you have to use the ballast wattage, altho that varies with different types, harmonics etc..

However if you are circuiting metal halide fixtures below 175 watts you must refer to the ballast data for amps during restart. I've seen 150 watt metal halide fixtures draw 400 watts if you turn the lights off then back on again, yes I learned this the hard way!

Stay warm!
 
with flourecents lights u mostly go with the ballasts amperage rating. commercial bldg's, the majority lights r 277v. now of days most ballasts only takes .25 amps or a little more, so u can actully put alot of 2x2's or 2x4's on 1 20 amp circuit
 
u use the wattage input for incadesents. u want to use the wattage calc if u r doing a residential projects, what is it, no more that 1440 watts on a 15a circuit and 1920 watts on a 20 amp circuit.
 
cloudymacleod said:
u use the wattage input for incadesents. u want to use the wattage calc if u r doing a residential projects, what is it, no more that 1440 watts on a 15a circuit and 1920 watts on a 20 amp circuit.
Click to expand...


Where can I find the 1440 and 1920 watt limitations?
 
Cloudy, why are you putting the .80 in the calculation for a residential circuit?

Roger
 
okay - since I am home now I can't exactly give you the exact parabolic fixture and other fluorscent fixtures that we used, but I can give you something that we similar use, and see if you can help me figure out how much to put down for VA and Watts on the panelboard schedule.

we usually specify the fixture type from manufactuer's website specs.

for example: Columbia Lighting
2x4 ficture. 4 lamps. 4ft t9 32 watt lamps. usually a electronic t8 ballast.

how do you get the amperage of the ballast to figure, so you can figure out how many to put on a circuit?

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EDIT: For commerical don't you base your loads for 100% demand for lights to stay on all the time in worse case sincerio's?

Please try to have responses so I clearly understand them, some are just making me more confused!!!!
 
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well, is it a lighting load under a continous load so if u don't know the the breaker size u multiply by 125% but if u know the breaker size or circuit size u mutiply by 80%?
 
u have to look at the ballast it self, mostly its listed under the ballast voltage. or if the fixture itself has paperwork, u can look at that
 
We're in an Engineering Office, specfiying these fixtures for a job. We don't have the fixtures on hand.

So how does the Engineer in my office calculate how many he can put on a circuit?

That's all I want to know, because I want to learn how to do it as well.
 
Cloudy, a residential lighting circuit would almost never be continuous, and if even if a fixture (or a few) on this circuit were left on for more than 179 min, it certainly wouldn't be enough load to consider the circuit as continuous.

Amanda, using the ballast info you could / would use Cloudy's formula in calculating the circuits for your commercial installation.

Roger
 
Amanda

The Columbia catalog lists the 4 lamp T8 electronic ballast as 114 input watts. You would multiply 114 watts by the number of fixtures on a circuit, say 15, and get a total wattage on the circuit. In this case it would be 1710 watts. Then divide the wattage by the voltage, in my example it would be 1710/120 or 14 amps.

If you believe the lighting is going to be on for 3 hours or more at a time, then you must multiply by 1.25 for continuous load.

Hope that helps.

Jim T
 
Jim

More than liekly our lighting for commerical projects is going to be on for more than 3 hours.

When do you apply the multiplication of the 125% factor into the equation that you gave me?

Thanks

EDIT:
This is the calculation I used for the project I am trying to figure out (And cross reference with the engineer).
3 Lamp, 32 watt t8 lamp. input watts: 86. # of light fixtures on this circuit: 2

So this is what I did.

86*2 = 178 watts

178 watts / 120 = 1.483333333333333

1.48333333333 x 1.25 = 1.85 amps

Now he put numbers under VA and watts, and remember it was up in the 900's. Is he basically taking the input watts and multiplying it with the # of fixtures on the circuit and then adding everything ogether (there is more than one type of light on this circuit).
 
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How to figure out Input Watts??

How to figure out Input Watts??

I foudn out from our engineer in the office on how he figured out the wattage for the lights. He really does worse case scenerio by just using the wattage of the lamps. That is fine.

He did say the formula that I was given is correct. However I pose a big big question.

How do you figure out the input watts for the ballast?? If you look at lighting specification sheets, they give you a input number, however that doesn't pertain to every # of lamps. What if you only have 2 lamps, or 3 lamps, or 4 lamps - how do you figure out the input wattage then?

This is a big question that I would like help on. Otherwise I totally understand how to sort my lighting into circuits now. I know how many watts can be on a 20 Amp circuit, using 120 voltage.
 
First of all, don't add up watts. Add volt-amps (VA). They're almost the same but as long as we're getting technical... :) Get the amps from a ballast mfr catalog such as Sylvania or Advance, where the exact ballast/lamp combo will be listed and will give you the amps drawn by the ballast. Multiply the amps by the voltage to get the VA for the panelboard schedule. An example of a ballast catalog is http://www.advancetransformer.com/products/fluorescent-electronic.jsp

It's better to use the ballast info taking the exact lamp into account because ballasts are available to drive the lamp anywhere from about 70 to 110 percent of its rated wattage. (That's known as ballast factor.) And if you're stuck with a tight energy code you don't want to be adding up lamp watts. You'll get an artificially high number and the codes require ballast input to be used anyway.
 
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