9th grade math or whats watt?

[Bruce Seidner]

It is really frustrating.

I read the summary article on fluorescent lights that is the executive summary and fine feature article article on the Creek. It read really well until I had to do the math.

The formula:

"The footcandle was previously defined as lumens of light per square foot. To provide 100 footcandles we need 100 lumens per square foot times 1500 square feet (in the article example), or 150,000 lumens at the work surface. Since only half of the lumens on the ceiling make it down to the work surface we need twice as many on the ceiling, or 300,000 lm. Assuming a 32w T8 lamp rated at 2800 lm we need 300,000 lumens divided by 2800 lumens = 107 lamps. Each 8’ fixture contains 4 lamps so we need 27 fixtures. In math terms this takes the form:

The needs in a 1500 sq ft shop would be = (100 fc) x (1500 sq. ft.) x (2) / (2800 lumens) x (4) = 27 fixtures

# fixtures = (desired fc) x (shop area) x (2) / (lumens per lamp) x (# lamps per fixture)


I am more than 2x 25yrs old so I am going with the 100 foot candles.

But I have spent an hour getting this everything but sensible,

The effective work area of my shop is 15 x 25. This is 375 sq ft. This much I get.

So this has to be doubled because of loss and that gives me 750 sq ft.

Ok, so 100fc x 375 sq ft is 37500 x2 is 75000/2800 lumens is about 26 units?

Do I have this right? I need to arrange 26 4' T8 bulbs in a 15'x25' shop space?

Have I a bright idea or am I mathematically incontinent?

[John Hays]

I think you'd be just fine with 10 to 12 four foot T8 fixtures in a 15x25 space.

For example, my main work area is about 10x20 and I only have eight T8 fixtures, which is more than enough light for my 46+ eyes (with the aid of bi-focals)... it's like a mini sun in there! Then again, my ceiling is roughly 8' high, so that might make a difference.

I too tried to do all the math to make the correct decision on how much lighting I needed, but sometimes you just gotta' go with your gut and what logic dictates.

[Ken Fitzgerald]

Bruce,

Your math is correct

In my later reply to Jacks SMC article, I attached an Excel spreadsheet that calculates the number. It shows using your shop size that you need 27 bulbs or 7 - 4 bulb fixtures.

You could go with 6 fixtures I'm sure. I would use 6 T-8 8' fixtures that use 4- 4' bulbs. I would place them length-wise 3 connected end-to-end in 2 rows. The rows would be located 3 1/2 feet from the middle of the room and 7' apart. Thus they would be 4' from the walls. By placing the fixtures end-to-end, you can run the interconnecting wiring inside the fixtures.

You will have a lot of guys who wing it but I can tell you that I used Jack's original Fine WoodWorking article to light my shop 7 years ago and everybody who comes into it remarks how well the lighting is and how there are no shadows.

In my 30x24 shop, I installed 11 8' 4-bulb fixtures. 3 rows of 3 in series and 1 row of two. I only put 2 in the one area because I intend to wall in my DC some day and I left room in the ceiling by not placing a fixture in that area. I placed each row on it's own switch and I can turn on what I need or turn off a row of 3 fixtures to replace bulbs or work on one of the fixtures if necessary.

Here's Jack's calculator: lightingcalculatorwithtableVer.5.xls


Good luck with your lighting.

[Bruce Seidner]

Bruce,

Your math is correct..

Good luck with your lighting.

I have more than luck going for me. It really is an incredible blessing to have the help of Y'all.