[TowerTalk] Using Plenum Plenum Cable

Jim Lux jimlux at earthlink.net
Wed Jun 21 15:34:00 EDT 2006

At 10:55 AM 6/21/2006, K8RI on Tower talk wrote:
>Couple of things I need clarifying.
>What is plenum rated cable? I worked in industry for years, but have been
>retired for over 7, and never heard the term used.  We used thhn and other
>cables in cable trays and conduit.

Plenum rated cables are ones that have insulation that has limited fire 
spreading ability.  In many buildings, the space above a suspended ceiling 
is used as a HVAC return (i.e. a plenum), and, so, cables that lie in that 
space must be plenum rated.

There's also what's called "riser rated", which is suitable for long 
vertical runs in a riser, because it won't turn into a natural draft flame 
thrower or Rijke tube.

I think this ONLY applies to communications cables, and not to power wires 
(which have to be contained in something like conduit or another jacket of 
insulation). [An interesting question.. if anyone knows, I'd be interested 
to know if there is such a thing as Plenum Rated "romex"]

Interestingly, even if you have ducted returns and feeds for your HVAC, the 
airspace above the ceiling (or below a raised floor) might still be 
considered a plenum. In code speak, you need to consult your AHJ (Authority 
having Jurisdiction).

The MGM Grand fire is what triggered a lot of the changes in rules, because 
the spread of that fire was greatly facilitated by the suspended ceiling.

I also find it amusing that "plenum rated" is used as a selling point for 
short (3-6 ft) audio and video interconnecting cables for stereos and 
TVs.  Plenum rating is irrelevant for consumer equipment interconnects, and 
certainly doesn't have anything to do with the electrical properties.  If 
your stereo catches on fire, I suspect you have bigger problems than 
worrying about whether the cable from the CD player is going to make things 

[note, if you're stringing cables inside your walls, as for speakers, etc., 
then plenum rating has (possibly) some worth...)

> >I don't have any experience with plenum rated coax but I do have experience
> > with plenum rated 10baseT cabling and that stuff won't last but a couple
> > of
>Nor have I heard of any base T cabling. I use Cat5e used for a gigabit
>netork and lots of it. Like many cables it has to meet a spec for handleing
>signals.The mechanical quality of Cat5e cable varies widely.  I have one
>piece that has been just laying on the ground now for a couple of yerars
>with no noticable deteoriation.
> > months exposed to the sun.   For 10baseT they make a special outdoor
> > version
> > with a black UV resistant covering that's also plenum.
>I have red, gray, and blue in the 1000' auto feed boxes. They all seem to do
>well, but like most Cat5e cables they do not like short radius bends which
>can make it a bit difficult to get into conduit.
>Roger Halstead (K8RI and ARRL 40 year Life Member)
>N833R - World's oldest Debonair CD-2
> >
> > Matt
> > ----- Original Message -----
> > From: "Doug Rehman" <rehman at surveil.com>
> > To: <towertalk at contesting.com>
> > Sent: Wednesday, June 21, 2006 10:36 AM
> > Subject: [TowerTalk] Using Plenum Plenum Cable
> >
> >
> >> Does anyone have any experience with plenum rated cable being used
> >> outdoors?
> >> I'm curious if the white outer sheath will stand up to the outdoor
> >> elements,
> >> especially UV, or if it will harden and crack.
> >>
> >> Tnx
> >> Doug
> >> K4DDR
> >>
> >>
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