How to estimate how many footcandles a fixture will provide.

[KP2]

Hi All, I have a customer that needs to have 90 - 100 foot candles of light at 5'6" from the floor. the fixtures are 12' from the floor so the distance from the bulb is 6'6" away.

The other items I see I need are as follows:

ballast factor = .87
Number of lamps =1
Total number of fixtures = 152
Square footage is = 4836 sqft
watts per lamp = 28 watts
Mean Lumens = 2515

I have the photometrics table for the fixture, but I'm not sure what to do with it.

Thanks in advance
Kevin

 

[svh19044]

If you want a (free) easy way, download visual basic. I believe the advanced version will have the photometric files for the fixture you are looking for, but contact them for their library first, before you pay for the advanced version. You will need to know a couple more easy to find factors such as light loss factor and the cu value, and can continue with the free version.

With what you state, you are looking at something more along the lines of 70 footcandles, using 152 28w T5 as the example. That's just a ROUGH guess with the information you provided, and using general information of my own (CU of .9 and LLF of .97). For 100fc, you are looking at more towards 220 luminaires, and at 90fc you are at 190 fixtures.

 

[realolman]

there are a couple methods that I have done once or twice... zonal cavity method and something else I can't remember. I think a good bit of it was a guess, reflectivity of the walls and what not, but it ended up with more fixtures that I had anticipated, but the amount of light was just about right. here's a link

http://upupaepops.files.wordpress.com/2008/02/zonal-cavity-method-for-indoor-calculations.pdf

 

[KP2]

SVH, your estimate was perfect. We installed 2 rows of lighting and took readings and the average was almost 70 foot candles on the button, so we doubled the lamps and now we are at 110 foot candles. That should be perfect since the level will drop a littlein 6 months.

Realolman, thanks for the link.

As always, Thanks to all how make this forum so helpful.

Kevin

 

[KP2]

Photometrics Chart

Photometrics Chart

I'm Still having trouble understanding this chart is supposed to fit into the calculation.

Any help would be greatly appreciated.

Thanks

 

[david luchini]

I'm Still having trouble understanding this chart is supposed to fit into the calculation.

Any help would be greatly appreciated.

Thanks View attachment 6373

You would use this chart in conjunction with the Zonal Cavity Method mentioned earlier.

First you would figure a Room Cavity Ratio: RCR=5*(cavity height 6.5')*(length+width)/(length*width.)

You then use the RCR with the ceiling and wall reflectances in the chart to get a Coefficient of Utilization. For instance with 80% ceiling, 50% wall and 20% floor reflectance, and an RCR of 3, your CU would be 0.72 from the chart.

You could then calculate number of required fixtures from:

#Fixt=floor area 4836 * desired footcandles 100/(lamps per fixture 1 * lumens per lamp * CU * ballast factor 0.87 * lamp life depreciation * lamp dirt depreciation.) I think I'd use initial lumens rather than mean lumens since there is a lamp life/dirt depreciation. Or maybe you could use the means lumens and remove the lamp depreciations.

 

[KP2]

OK so this chart uses a floor reflectance of 20%

The following abbreveations are
RC = Refectance of the cieling
RW = Reflectance of the wall

Calculate the other 2 cavities and reference the chart in order.
First use the RC then select the RW to find the CU.

I think I get it.
Thanks
Again

 

[TNBaer]

Also, a lot of manufacturers offer a lighting layout tool right on the site. For instance, Crescent/Stonco offers this when you go to their site and click on any product page. You'll notice toward the bottom is a drop down box for QuickEST. Use that and you can design your room real quick and print out all of the info. Super easy.

 

[KP2]

Also, a lot of manufacturers offer a lighting layout tool right on the site. For instance, Crescent/Stonco offers this when you go to their site and click on any product page. You'll notice toward the bottom is a drop down box for QuickEST. Use that and you can design your room real quick and print out all of the info. Super easy.

Great site, that tool is pretty cool, it's better than the GE one.

Thanks

 

[Strathead]

Hi All, I have a customer that needs to have 90 - 100 foot candles of light at 5'6" from the floor. the fixtures are 12' from the floor so the distance from the bulb is 6'6" away.

The other items I see I need are as follows:

ballast factor = .87
Number of lamps =1
Total number of fixtures = 152
Square footage is = 4836 sqft
watts per lamp = 28 watts
Mean Lumens = 2515

I have the photometrics table for the fixture, but I'm not sure what to do with it.

Thanks in advance
Kevin

I have books and stuff to tell me how to do this, but as and estimator and a person who puts together design build proposals at times, here is how I would calculate out the fixture layout. Also the way that Engineers and Architects often do it too.

Call whichever lighting rep you prefer to work with. If you have no preference, call whichever supply house you prefer to work with. Tell them what the customer wants. Unless they are married to the fixture you have listed above, don't give them that information. More like room dimensions, fixture mounting height, work height desired foot candles, whether direct or indirect, wall and ceiling color if they are dark, and ask them to provide you with layout and price. This means that if you are an honest contractor, you will buy the fixtures from them if you get the job, but that is what you are paying them for.

Tell me what you think of this idea.

 

[TNBaer]

The only problem is if you don't want to be married to a specific distributor or manufacturer.

If anyone has any lighting questions feel free to send me a Private Message or Email. If you want lighting layouts or have questions about energy efficiency I can certainly give you a hand. I can do energy savings in a few minutes. And layouts, depending on the situation and the amount of info you can get me, in an hour or so.

 

[KP2]

I agree with you TN, I have a pretty good grasp now. I have calculated 4 jobs since I posted this question and all 4 have proven to be spot on. The calc was lower than the readings as they should be since loss of life is not part of the equation after the install.

Thanks again for everyone's help.

Kevin