Checking lighting math please

[ManCave]

I have a pole building that is 39 x 73. That is 2,847 square feet.

I need 100 lumens per square foot. So 2,847 x 100 is 284,700 lumens that are needed.

I'm going with 8 foot T8 lights with four 4 foot tubes. Each tube is rated at 2,850 lumens. So 2,850 lumens times 4 tubes equal 11,400 lumens per 8 foot light strip.

284,700 lumens needed divided by 11,400 lumens per 8 foot strip equals 25 eight foot light strips required for my building.

Is that correct?

I have 3 garage bays with all doors on the same wall. How would you space these 25 lights? Distance from the walls, distance from each other, etc?

 

[Charles (in GA)]

Using Visual

http://www.visual-3d.com/tools/interior/default.aspx?id=15170

I used 39x73 12 ft high (you didn't say) using a tandem 4 bulb T8 striplight with four 32 watt bulbs, default on reflectance of ceiling and walls. 100 fc, It says 32 fixtures.

Charles

 

[Justanoldguy]

25 eight foot light strips required for the building.
correct based on your lumen requirement calcs.

 

[ManCave]

Using Visual

http://www.visual-3d.com/tools/interior/default.aspx?id=15170

I used 39x73 12 ft high (you didn't say) using a tandem 4 bulb T8 striplight with four 32 watt bulbs, default on reflectance of ceiling and walls. 100 fc, It says 32 fixtures.

Charles

Hi Charles,

That tool is pretty interesting. I assume the blue lines represent the lights? There are only 10 of them. Is that meant to be representative rather than an accurate depiction of light placement since you said the tool calculated 32 fixtures and not 10?

 

[Charles (in GA)]

It probably won't work on a phone or tablet. Should be showing 8 rows of four fixtures, and show 32 for a quantity on the calculation results on the right side. That is a live page, you can change the variables to whatever you want. It assumes a work height of 2½ ft but that can be changed also.

This is based on the photometrics Lithonia provides. You go to the Lithonia web site, select the light fixture, select photometrics, from the list pick the exact configuration of bulb type and quantity for that design fixture, and click on the icon with the squiggle colored lines, at the next screeen, you select interior and it brings you to the page I have linked, with the light fixture specs input, you just play with the building size, reflectance, etc. The web version is quite limited in what it can do.

Charles

 

[zmaxmotorsports]

You guys are making my head durt!

 

[Kevin C]

25 eight foot light strips required for the building.
correct based on your lumen requirement calcs.

My take:

Those calcs assume that 100% of the light from the fixtures reaches the work surface. Reality is that for each 2850 Lumen bulb, 50 to 80% ends up where you need it.

Using the numbers from the fixtures will get you a much more realistic result. BTW, I have about 70 Lumen per sq foot and its pretty bright. So that number of fixtures would probably work pretty well, but not hit the number the OP was looking for.

 

[Justanoldguy]

He never said whether his lumen requirements were from the unit or on the floor.
So assuming works both ways.

 

[sands35]

I gotta, say that 100 lumens is *really* bright. You only need 100 lumens when you are doing detailed layout work. Think machine or wood shop stuff. Milling machine or dovetail work. Not wrenching on on cars / lawn equipment stuff.

With a 39 x 73, you aren't going to be working in the whole place all at once. I'd put in ~4-5 different zones. Maybe another zone with just a few edison bulbs as ghost lights so you can walk through without bumping into anything (or get blinded when all you need is a screwdriver and it's after dark outside). Then some task lights above fixed work benches and a retractable trouble light for working under the car hood or in the wheel well.

I know the calculators say you need 25 or 32 fixtures, but really, you only need 1/2 or 2/3rds of that with some task specific lights. Just space them evenly around the garage. Put the tubes around the cars, not over them. No sense in putting light onto the roof.

I have a calculated ~50 lumens in my car/wood shop garage (25 100-Watt equivalent CFLs, 832 square feet.). Just about right with my 41 year old eyes with some under-cabinet tube lights and a trouble light. (white ceiling and light gray walls.)

Then do your electrical so you can put in more lights where you want if you need more later. Main run conduit to a 4-11 J box and then either plug in or armored flex / conduit to the lights. Easy to add more later. (assuming you aren't going to drywall the ceiling as it is pole construction.)

 

[Kevin C]

He never said whether his lumen requirements were from the unit or on the floor.
So assuming works both ways.

Its a common mistake, most of those lumens that are lost never make it our of the fixture. Basicly, using the lumen output of the lamp always overestimates the delivered light at the work surface and from the light.


Another oversight, people take the output of the tube and compare that to the output of a LED fixture. The LED fixture is evaluated as a unit and then gets compared to the output of the tube, not the actual fixture. That's as useful as taking the output of the raw LED chips to calculate how much light your goin to get in your work area.

Thats why Charles figures are different and the prefered method.

 

[ManCave]

One end of my shop is where my mill and lathe and welding equipment is located. My walls and ceiling are white pole building steel. All wiring is going to be in conduit. I plan to have 3 zones for the lights - one for each garage bay. That way I can turn on the lights in the area in which I'm working and leave the rest of the lights off.

25 - 30 light fixtures seems like overkill to me but its what the calculation seems to suggest.

 

[sands35]

It is overkill. Since you are doing conduit, I'd suggest putting whatever lays out symmetrically and then add if you need more later. Hang them from chains or screw them into the joists. Easy to move around.

You said post construction, what is the joist spacing? How many joists? Unless you are putting up wood on the bottom side of the joists, that will govern the number of rows of lights, then you just need to figure out how many per joist you can / want to afford.

It doesn't have to be a science project.

 

[ManCave]

Hi Sand,

Earlier I mentioned that I have pole building steel on my ceilings. I don't want to drill holes into the metal and then move the lights. Once they go up they are there to stay!

Also, I have too many things on my plate to have to come back later and add more lights. I'll get the light requirement figured out ahead of time and then do it once and be done with it.

 

[ManCave]

The vendor ran a model and came up with 28 of the 8 foot lights which is very close to what Charles came up with as well. I've ordered them and they should be here next week! If I need to add a couple of task lights here or there its not a big deal.

Thanks for all of the help!

 

[sands35]

Hi Sand,

Earlier I mentioned that I have pole building steel on my ceilings. I don't want to drill holes into the metal and then move the lights. Once they go up they are there to stay!

Also, I have too many things on my plate to have to come back later and add more lights. I'll get the light requirement figured out ahead of time and then do it once and be done with it.

You can get small clamps for the lights, no need to drill. Costs a little more though.

Sorry, when people say "pole" I assume wood. I missed that.

100 lumens is still *really* bright.