Plenum Air Space?

GerryB

Senior Member
Is a hung ceiling in a commercial building always a plenum? I need to run two Cat 5's for a customer, I have Cat 5e CMR in the garage but I think I need CMP, unless it is not considered a plenum. There is duct work up there that I saw.
 

GrayHair

Senior Member
Location
Nashville, TN
Plenum or Non-plenum?

Plenum or Non-plenum?

Is the space an open return (no return ducts or partial return ducts)? Many years ago I could just shine a flashlight through the ceiling return grill; if I saw no sheet metal above the grill, the above-ceiling space was a plenum. If I saw sheet metal, returns were fully ducted back to the air handler.

Then they started using what I called a "directing duct" (a 90 directing the air flow toward the air handler); so much for the flashlight. :cry: It became "up the ladder" and if there was even one return not fully ducted, the space was a plenum. I'm sure my use of "worst case" may have cost me some jobs, but it always kept me in the good graces of inspectors.

Regards!
 

GoldDigger

Moderator
Staff member
Is the space an open return (no return ducts or partial return ducts)? Many years ago I could just shine a flashlight through the ceiling return grill; if I saw no sheet metal above the grill, the above-ceiling space was a plenum. If I saw sheet metal, returns were fully ducted back to the air handler.

Then they started using what I called a "directing duct" (a 90 directing the air flow toward the air handler); so much for the flashlight. :cry: It became "up the ladder" and if there was even one return not fully ducted, the space was a plenum. I'm sure my use of "worst case" may have cost me some jobs, but it always kept me in the good graces of inspectors.

Regards!
Keep in mind that although plenum rated wiring methods may be required, the space above a dropped ceiling is not really a plenum. It is at most "other environmental air" space.
If it were actually s plenum the wiring restrictions would be much more restrictive (nothing but air related circuits, and even those strictly limited.)
 

iwire

Moderator
Staff member
Location
Massachusetts
Keep in mind that although plenum rated wiring methods may be required, the space above a dropped ceiling is not really a plenum. It is at most "other environmental air" space.
If it were actually s plenum the wiring restrictions would be much more restrictive (nothing but air related circuits, and even those strictly limited.)
Exactly. :)
 

ron

Senior Member
It is a tough question (for me).

NFPA 13 defines a concealed space differently than NFPA 70 (NEC), and according to 13, the space above a suspended ceiling is concealed space. Thus either requiring sprinkler coverage or no combustibles to be in that space.

Many AHJ's consider plenum wiring to be "non-combustible", although of course it is combustible and maybe just low smoke producing.

Because of this, I interpret the code to require plenum rated cabling above a suspended ceiling, because we generally do not put fire suppression heads above a suspended ceiling.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
It is a tough question (for me).

NFPA 13 defines a concealed space differently than NFPA 70 (NEC), and according to 13, the space above a suspended ceiling is concealed space. Thus either requiring sprinkler coverage or no combustibles to be in that space.

Many AHJ's consider plenum wiring to be "non-combustible", although of course it is combustible and maybe just low smoke producing.

Because of this, I interpret the code to require plenum rated cabling above a suspended ceiling, because we generally do not put fire suppression heads above a suspended ceiling.
We do this as well. In fact, we just run plenum rated no matter where it's going. Keeps everything simple.
 

GerryB

Senior Member
We do this as well. In fact, we just run plenum rated no matter where it's going. Keeps everything simple.
I will be running plenum rated also, it's just the fact I already have the riser rated (cmr) in the garage. (you would think a riser if I'm interpreting correctly would need plenum also)
 
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