Q: I have a reception area that has a sprinkler head in the middle of a 2’x2′ ceiling tile and less than twelve inches from this sprinkler head the interior designer has placed a circle light that is flush to the ceiling tile. Doesn’t this violate the 18″ rule and the 24″ circumference rule?
A: No, what you described does not appear to be a problem. The 18 inch rule that you refer to is measured down (or vertically) from the sprinkler head deflector to a horizontal plane that extends from wall-to-wall in the space or room. If the light fixture that you described is truly flush mounted, and does not extend below the sprinkler head deflector, then I do not see a problem. I am not familiar with the 24 inch circumference rule that you are referring to. You do have the correct idea concerning ceiling mounted obstructions, though. The 2000 Life Safety Code refers to NFPA 13 (1999 edition) for compliance involving sprinkler installations, and Table 5-184.108.40.206 provides the minimum distances that a ceiling mounted obstruction is permitted to a sprinkler head, depending on the distance the obstruction projects below the sprinkler head. So, in your example, the Table does not permit any object that is ceiling mounted to project below the sprinkler head within the first 12 inches of the sprinkler head. Then for each additional 6 inches, the allowable distance that a ceiling mounted obstruction may extend below the sprinkler head increases. So, you have the right idea concerning ceiling mounted obstructions, you just didn’t have all the details.