A/V System - Plenum Inquiry

[Ian Peter]

Hi All,

I design A/V Systems for Commercial clients...typically Ceiling mounted Projectors tied together with Audio reinforcement. I have a 3rd party perform the on-site installations.

What I am totally lacking knowledge on is the space above the drop-ceiling and inside a hollow dry wall and what I can / cannot do in these areas.

A: When is it OK to run standard PVC cable vs. Plenum cabling in these areas.

B: Is it OK to locate any electrical equipment (like a 70v sound amplifier for instance) in a space above a drop ceiling. I have situations where clients don't want an amp on a desk or in a cabinet within the room. Self powered speakers could be a realistic alternative in certain situations but I imagine they would have to be 12v or 24v powered. If I could simply sit a small amp in the 2' x 2' metal ceiling tray that is installed for hanging the projector that would be great as I'd have a nicer range of speaker choices, but I have no idea if this is breaking some code.

Any insight for me???

Ian

 

[petersonra]

Re: A/V System - Plenum Inquiry

Not an expert on these things but here is my take.

You can't generally run any kind of cable inside the hollowed out spaces of walls except maybe Romex.

The area above a drop ceiling is not necessarily a plenum.

You can usually run cables of some sort above a drop ceiling but they cannot be laid on the drop ceiling, they have to be supported by the building structure.

I don't know what you can and cannot mount above a drop ceiling. It may make a difference if it is an actual plenum or not.

My take on this is that your 3rd party installer should know these kind of things already.

 

[Ian Peter]

First...thanks so much for your contribution...I really appreciate it!!!

To clarify...as far as the cabling behind dry-wall...it is usually a simple 8 or 4 Ohm type of passive speaker connection...but sometimes it is a 70v speaker connection. I always use a 16awg Plenum Speaker cable throughout a job...it isn't particularly expensive in the 50ft runs I use. I've never considered using Romex for speaker connectivity...maybe I should (?)

The area above the ceiling is a dilemma...I'm not trained to determine a Plenum area or not but I'm reading on other threads about HVAC Returns and Ducting so I may never get it!!!

I agree that my "installer" should know this stuff...but he doesn't ...so maybe I need to shop around for a more knowledgeable parter...any S FL recommendations are welcome.

In the meanwhile, I need to educate myself as best as possible to give a better impression of what a GOOD salesman does (besides sell)...I think I'm in "Technical Sales" if that's a real job title...

 

[fc]

Ian
If most of your work is in commercial buildings the ceiling most of the time will be considered other space used for environmental air you need to ask if that is the case if so your cable needs to be plenum rated or you can have your contractor install all your wire in pipe. Your plenum or non plenum cable can be installed in hollow of walls but you can not bring non plenum cable into the ceiling if it is used for space for environmental air.
When we say space for environmental air we are talking about using the space above for a return air and most of the commercial space today is used that way.
Make sure your projector receptacle is not install above the ceiling. That would be a code violation. We put the box flush with the drop ceiling for our receptacle.

 

[Ian Peter]

Thanks FC,

I'm beginning to get the feeling that, while it's more expensive, it's probably better to just use plenum VGA, Audio, Video and Speaker cables in all situations.

The Ceiling plates I use come from either Peerless or Chief and have A/C receptacle knock-outs. We usually have the client hire their own licensed electrician to bring power to that knock-out so I have to believe that code is being followed.

Any idea if placing a small audio amplifier inside that 2' x 2' ceiling plate would be a code violation in either a plenum or non-plenum space? I can always rig up a mount to secure the amp to the "room side" of the ceiling plate but it sure would be convenient to just sit it up there.

The other thing I'm wondering is if there is some type of plenum jacket that can be added to an A/V cable after the fact rather than having to replace all cables...I think I read about some stuff called "Halar" from TechFlex that would allow me to bundle all my cables inside this flexible mesh sleeve and it was Plenum rated (?)

 

[hbiss]

We do a lot of sound systems in commercial spaces, we don't do video but I know about it and what you are doing.

In walls you can use any listed CL3, CL3R CL2, CL2R,CM or CMR cable as well as any with a "P" for plenum. It will say right on the jacket every few feet as well as on the reel or box what it is. If the cable or cable making up a cable assembly is not one of these types or is unlisted (says nothing on the jacket) then you can't use it within walls or ceilings unless (generally) you install it in raceway (conduit). You cannot use cable designated as flexible cord even in raceway. This would include zip or lamp cord which is commonly misused as speaker wire. Be on the safe side and only use those cables I listed above. It's amazing how most high-end cables like Monster Cable are not even listed. Just goes to show you.

A ceiling plenum space is that which is used for environmental air. Generally that would be the return air for the HVAC system. Dead giveaway would be when you see unducted louvers installed in place of tiles every so often that let air up into the ceiling space. You can only use plenum rated cables in these spaces, those with a "P" at the end: CL3P, CL2P, or CMP.

You will want to get yourself a copy of the NEC and read articles 725 as well as 800 on. Not a lot of reading but you will be able to see where these cable listing come from as well as what can be substituted for another. Because a plenum rated cable can always be used in place of "lesser" listings I usually only stock and use plenum versions where cost is not excessive. Just easier.

As far as putting electronics (especially an amp) in the hung ceiling, whether it's plenum or not, code or not (generally it's against code if it plugs in), I always felt that that was unprofessional. Who knows it's there and how do you adjust it?

-Hal

 

[Ian Peter]

Hal...Thanks VERY much!!!

For my speaker cabling I have, in fact, always used West Penn Plenum cabling (it's 16awg listed as CMP).

Most of my installations are in Conference Rooms, Training Rooms, or Board Rooms for B/I clients. School Classrooms, Media Centers and Auditoriums. Some nice City Council Chamber for Government work. But I never really know what the "above drop-ceiling" area rating is and normally nobody at the facility does either...I think you need a "Plenum Diploma". I'll definitely get the NEC and read articles 725 as well as 800 as you've recommended.

The only problem that I run into is the cost complaint of the A/V Cabling when I specify Plenum cables. For instance a standard 50ft VGA cable costs me $20 while the Plenum version costs me $120!!! Video and Audio cables aren't nearly as prohibitive but still a normal 50ft composite video cable costs me $20 and the Plenum runs $40. It adds up quickly especially if I have multiple wall plate feeds.

I also appreciate your feelings regarding having an amp above the ceiling. It isn't something I'd normally do...typically there is additional Video gear (like a DVD or VCR) that requires placement within the room and so I would place an amp with it. In this case, the client wants no equipment in the room...just a wall plate to a projector and some speakers on either side of a manual screen. So, the amp will have no user controls as volume is manipulated through the projectors' wireless remote (i.e. all audio from the wall plate goes into the projector and the amp is fed from the audio output of the projector).

But, if as you say, once the amp is plugged in it would break code then I think my only alternative is to rig up a mount on the "room side" of the ceiling plate. This happens to be a fairly small 70v amp (4" x 8.25" x 11" @ 10lbs) so a metal box shouldn't be too hard to find...although a vented box would be preferred)

 

[hbiss]

For instance a standard 50ft VGA cable costs me $20 while the Plenum version costs me $120!!! Video and Audio cables aren't nearly as prohibitive but still a normal 50ft composite video cable costs me $20 and the Plenum runs $40. It adds up quickly

That's why you need to be able to recognize a plenum space so you can quote a price upfront to cover your costs. 16ga plenum speaker wire is no big deal to always use but with premade cables and coax there is a big difference in cost. The difference in cost between plenum and non-plenum CAT5e is substantial also.

-Hal

 

[Ian Peter]

Hal,

While I understand how you disagree with the idea of having the amp sitting on the ceiling plate...For this application I still want to ask if code could be met by placing the amp inside a metal enclosure like a NEMA1 'wall mount" enclosure. If so, then I could easily fasten it to the "plenum side" of the ceiling plate and have the audio and power cables needed for the amp run directly to the "projector side" through a predrilled hole in the ceiling plate covered by the enclosure.

Ian

 

[hbiss]

That's a tough call. I would think that if the amp was completely enclosed in the plenum and accessable only from the room side. You would be getting into an area of custom manufacture or fabrication here and an AHJ would have to make the call.

I just thought of something that may make your problem a little easier. I have been looking for an application for this and it sounds like what you are doing would suit it perfectly. I assume you have only one input- take a look at this http://www.rdlnet.com/product.php?page=91

-Hal

 

[Ian Peter]

Hal,

Nice find!!! I actually looked at RDL and didn't get past the "stick-ons" so I moved on. This looks like a good solution and the ceiling tray would make for a clean install and some flexibility of adding either a "stick-on" mixer (for supporting a Mic input) or just another 70v amp for a system needing more expansion.

This may solve my dilemma!

Ian

 

[Pierre C Belarge]

The cord for you amp is not permitted to pass through a ceiling...400.8(2)