# Thread: how do you figure lighting coverage

1. Junior Member
Join Date
May 2003
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1

## how do you figure lighting coverage

i am trying to figure out how to figure out howmany can lights i need to light a given size room

2. ## Re: how do you figure lighting coverage

The code says you need as little as a switched outlet receptacle (exept for halls, stairways, kitchens, bathrooms, garages and basements).

anything beyond that is up to you, or the owner.

210.70 Lighting outlets required

Four works well for most simple applications.

Some manufacturers have lighting charts that help "design" a lighting arrangement.

[ May 17, 2003, 01:25 PM: Message edited by: physis ]

3. Senior Member
Join Date
Feb 2003
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1,257

## Re: how do you figure lighting coverage

A lighting lesson:
First of all, what type of room are you trying to light? Common tasks in different rooms require different amounts of light(measured in footcandles).
Typical light levels;
Dining room: 10-20
Food Prep: 50-100
Hobbies/sewing: 100 or more.
Typical spacing for recessed downlights in a 8-9ft. ceiling would be approximately 6' on center and no more than 3' from a wall for even light distribution.
If they are going to be installed in a high ceiling, you might want to concider using halogen lamps to throw the light where it will be needed.
A standard 60 watt incandescent bulb is rated at 890 lumens.
A 40 watt fluorescent bulb is rated at 3200 lumens. (this is usually why a fluorescent fixture is used in the center of a kitchen).
Also (for your math) a footcandle is a measure of how many lumens fall onto a surface. One footcandle of illumination is the equivalent of One lumen falling on One square foot.
Hope that helps, Todd

4. Senior Member
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## Re: how do you figure lighting coverage

What you need to do is a simple calculation. Well, simple to some anyway. The basic calculation to determine the number of light fixtures needed in a room is called the Zonal Cavity Method. You need the following information:

Room Width - W, Room Length - L, Room Area - A, Ceiling Height - CH, Room Cavity - RC, Room Cavity Ratio - RCR, Lumens per Lamp- Lu, Coefficient of Utilization - CU, Lamps per Fixture - LPF, Maintenance Factor - MF.

Calculate the Room Cavity First. It is the difference between the height the fixture is mounted in the room and Horizontal plane in the room at which you want to calculate your footcandles. Usually we want to calculate the number of footcandles at a point 2.5' above the finished floor. That is where we do most of our work at desks, tables, counter tops, etc.,. So, if your fixture is mounted directly on the ceiling or recessed in the ceiling just subtract 2.5' from the ceiling hieght to get RC. If you have a fixture that is suspened 12" below the ceiling then you subtract 12" from this number. In other words instead of subtracting 2.5' from the ceiling height you decrease the ceiling height by 12" and then subtract 2.5'.

RCR = (5 x RC x L x W)/(L x W)

With this done, you now need a spec sheet of the fixture that gives you what is called Coefficients of Utilization. This may be a little confusing but light fixtures have a chart on their spec sheets for CU's. The RCR is listed in a column on the far left of the chart. Choose a number close to the one you calculated above and come across and find your CU. Now, you will notice at the top of the chart that it is divided into sections that depend on what Ceiling Reflectance you are using. I would always use 70% to be conservative so come all the way across to the section of the chart that is for 70% ceiling reflectance. You will then also notice that each Ceiling Reflectance section is divided according Wall Reflectance. Use the 50% column for that. Now you have your CU.

Finally, here is the formula for how many fixtures you need in the room:

No. of Fixtures = (A x FC)/(LPF x Lu x CU x .8)

FC is the footcandle level you desire to have in the room. The last value that I used (.8) is a maintenance factor. What this does is put in a factor that makes allowance for fixtures getting dirty, lamp output deteriorating over time and other factors. You can make it smaller but .8 is good enough for our purposses.

If you search around you can probably find some free software that will do most of the work for you. Maybe others can suggest a source.

5. jtb
Senior Member
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Feb 2003
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Pennsylvania
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## Re: how do you figure lighting coverage

Try Lithonia Lighting, see if you can get a copy of Lightware 7.0.

6. Junior Member
Join Date
May 2003
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2

## Re: how do you figure lighting coverage

Take a look at this link you can download a program that will calculate this info for you. Also many other helpful calculations hope this helps.

7. Junior Member
Join Date
May 2003
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## Re: how do you figure lighting coverage

Take a look at this link you can download a program that will calculate this info for you. Also many other helpful calculations hope this helps.

sorry

8. Senior Member
Join Date
May 2003
Posts
156

## Re: how do you figure lighting coverage

Call holophane and ask for a sales rep, email him the drawings. That simple. I won't give u my reps number cuz then he'd be to busy to finish my projects

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