What is a Plenum?

During some investigation for our new office space, cabling quickly became a subject of interest.  It seems the previous tenent had been there for some time (nearly 15 years) and underwent numerous updates, most of which ended with cat5 cabling.  Being the bandwidth-hungry company we are, we opted for category 6 cable, giving us the increased throughout and support for future bandwidth intensive applications.  In the past, we had to use plenum cable, as we had plenum space (above ceiling tiles) whereas our new space has higher ceilings direct through to the rafters, so I needed some clarification to justify the extra “plenum rated” expense – which I found below.  Now I’m just waiting for cable vendors input..

According to the National Electric Code (NEC) a plenum is a “compartment or chamber to which one or more air ducts are connected and [which] forms part of the air distribution system.” To qualify as a plenum, the space above an acoustic tile ceiling would have to extend above other rooms in the same building or be open to ducts connecting it to other parts of the building. The concern is that during a fire, if there is burning material in a plenum air space, smoke and fumes can travel through air ducts to the whole building. For this reason, there are codes to restrict the types of materials (such as wiring) that can be placed in the plenum.

It’s quite common to have an acoustic tile ceiling without having a plenum. If your room-dividing walls extend above the dropped ceiling and seal off the space above, you do not have a plenum air space and so may not require plenum-rated wires. (See illustration.) You can lift up an acoustical tile in your room and peek in to see if your room has a plenum.

So, what is the code?
According to the National Electric Code (NEC), in plenum air spaces you must use plenum rated cables, also called Communications Plenum Cable (CMP). Plenum cable is only required when cable is installed in a plenum air space. Materials kept below the ceiling — including speaker wire, computer cables, telephone cords, etc. — do not need to be plenum rated according to the NEC.

Remember that even though the National Electric Code may allow non-plenum cable, the final decision is up to your local Fire Marshall.  Most cities adopt the national codes as their own without revision, but some cities modify or expand them and require plenum-rated cable in all situations.  Regardless of the code or its interpretation, your Fire Marshall makes the final decision. We recommend that you contact your Fire Marshall if you have questions.

Why is the regulation for plenum air spaces but not for other wiring such as speaker wire?
It’s dangerous to inhale fumes from any burning material. Speaker wire is no more dangerous than any other plastic item you would find below the ceiling in a typical classroom — computers, carpet, power cords, etc. Therefore, requiring the use of plenum wires within the classroom itself would have little impact. The regulation covers the area where it’s most critical.

How is plenum wire different from other speaker wire?
Plenum rated coating on wire burns at a much higher temperature and emits fewer fumes.

Who sets the guidelines?
The National Electrical Code (NEC) is a set of guidelines recommending procedures to reduce the risk of fires, electric shock and other hazards associated with electrical installations. The code is advisory in nature, but most state and local building departments across the country use the NEC as the basis for their own electrical codes. Some local codes may be more restrictive, so please check with your local Fire Marshall if you’re unsure

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