Mar 10 2014

Plenum Rated Cable

Category: Ceilings,Questions and Answers,VentilationBKeyes @ 6:00 am
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Q: We have decorative wood panels in our cafeteria ceiling which do not provide a seal to limit the passage of smoke. In fact, there are wide gaps between the wood panels. HVAC supply ductwork is mounted above the wood panels and return air is drawn through the open space above the wood panels as well. Are we required to use plenum rated cabling above the wood panels?

A:  The Life Safety Code, 2000 edition, section 3.3.150 defines a plenum as a compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.Given that the above-ceiling space is used as a chamber for the return air of the HVAC system, the space appears to be a plenum and I would conclude that plenum-rated cable should be used. NFPA 70 (1999 edition), section 300-22(b) specifies that Type MI or Type MC cable must be installed in ventilating air plenums. This standard does allow limited lengths of flexible metal conduit and liquid-tight flexible conduit to devices permitted to be in the plenum.


Jan 20 2014

Sprinkler Heads and Ceiling Mounted Obstructions

Category: Ceilings,Questions and Answers,SprinklersBKeyes @ 6:00 am
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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 5-6.5.1.2 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.