Suspended obstructions

[cb1151]

I have a situation where there is suspended lighting 15" below ceiling. The lights are 1'-0" wide.

By following the chart in NFPA for floor mounted/suspended obstructions I need to be 24 inches up to 30 inches away from the light in order for spray pattern to clear the fixture.

Is there any way around this? Is there a minimum requirement for a width of an obstruction or anything like that? The problem is the lights are 10'-0" apart so going by that rule, there will be a lot of additional heads just to clear the fixture.

If anyone has any experience/advice it would be greatly appreciated.

 

[TravisMack]

I don't think the floor mounted partitions applies in your situation. But, you may be able to use 8.6.5.2.1.3 as help. You will need to be minimum of 24" away from the near edge of the light measured on the diagonal.

 

[cb1151]

The lights are not a solid obstruction. They hang from the ceiling by a very think wire. As I said, they are suspended.

 

[TravisMack]

Well..you have a couple of options then:

1 - you apply the "three times rule" and stay up to 24" away measured diagonally from the near edge.

2 - 8.6.5.2.1.4* For light and ordinary hazard occupancies, structural members only shall be considered when applying the requirements of 8.6.5.2.1.3.

8.6.5.2.1.3 is the "three times rule"

 

[cb1151]

I'm confused. Have I been reading this wrong the entire time?

The chart I refer to is for floor/suspended obstructions which would be the light fixture I am refering to. If the light is hung 15" from the ceiling and a sprinkler is 32" away (on a horizontal plane) according to the book the spray from that head will theoretically land on top of that light fixture. don't think of it as a light fixture. Instead of thinking of it as a light, think of it as a partial height wall, 7'3" below an 8'6" ceiling.

I've never heard or ever used the 3 times rule in any situation like this. To me, it just doesn't seem to apply.
If the situation applied then yes I would agree but it just doesn't seem to fit the senario.

 

[TravisMack]

Based on everything I have seen, and all of the AHJs I have talked to, when you have the scenario you describe, the 3x rule (8.6.5.2.1.3) is what you have to use for the area.

I don't think you have a floor mounted partition with the suspended light. Also, that requirement is typically much more stringent than the 3x rule.

Based on an object being 15" below a sprinkler, and 1' in width, you need to be 19" horizontally from the near side of the object for standard spray sprinklers. If you use Ex Cov, then you need to be 33" horizontally from the near edge of the object.

You may want to re-read the stuff in Chap 8 about obstructions, and use the handbook for clarifications. I find that I have to re-read NFPA 13 every couple of years.