Plenum data cable in slab

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dema

Senior Member
Location
Indiana
I've been researching the use of plenum rated cable in floor boxes. The NEC states that a slab in direct contact with the earth is a wet location. My research indicates that the main issue of water entering the conduit is not from direct entry but from condensation. The slab is cooler than surrounding air during humid times of the year and it takes a while, but eventually the conduit fills with condensation. I've heard around here that takes about five years. Plenum rated cable is not water proof. Water proof data cable is not plenum rated. BICSI states that it is not NEC code for the plenum rated cable to be used in slab. My personal interpretation is that article 800 gives a choice as to whether or not to follow chapter 3 methods in installing communication cabling and that if you don't choose to follow chapter 3 methods then there is nothing prohibiting the use of plenum rated cable. And whether or not you choose to use chapter 3 methods, the advisability of using wet rated cable that is not plenum rated is questionable.

BICSI - the communication people who certify but are not a law making body - recommend that a splice box be placed at least 50' from the communication rack and then the wet rated cable (presumably in conduit) be run from that location to under slab. The 50' is not optional - if a splice is made in a cable within 50' of the rack, there is reflection that can make the cable unusable. Therefore if the facility is a small facility, there may not be much way of placing a splice 50' from the rack. So from there I guess everything is in enclosures and in conduit.

It appears to me that the NEC committee needs to get with BICSI and figure out a how to that is both legal and works. Currently it appears that we have a choice - make it legal, or make it work. (Or spend an arm and a leg and have a slim chance of doing both.)

Superior Essex has a below grade communication guide with a picture of the recommended installation. You can Google it - I don't know that I should place the link here.

Comments?
 

don_resqcapt19

Moderator
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Location
Illinois
Occupation
retired electrician
Why would a cable in a wet location conduit need to be plenum rated? If a single cable is going to be in both a wet location and a plenum location, you will either have to find a cable with both ratings, or make a couple of splices to change the cable type.

It is my opinion that much of the low voltage data and communication cable installations are covered by Article 725 and 725.3(L) requires the use of cables listed for wet locations were the cables are installed in such a location.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Sure, you can post the links here (but please don't post links without a explanation of what it is, or for pure advertising purposes.)

I assume this is it: http://ce.superioressex.com/uploade...elines/TG58-Copper-TP-Below-Grade-Conduit.pdf

The BISCI committee makes standards to make sure data systems work to a certain standard. The NEC is basically only concerned with safety. I don't believe they will ever get together to make their standards or codes align.

But let me ask this: Are you sure you even have a plenum? Not every space above a dropped ceiling is a plenum space. If the HVAC supply and return are ducted, then its not a plenum space.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
But let me ask this: Are you sure you even have a plenum? Not every space above a dropped ceiling is a plenum space. If the HVAC supply and return are ducted, then its not a plenum space.

Techically, under the NEC, a plenum is something constructed just for the purpose of handling moving air.
Above a dropped ceiling is generally not a plenum, but may well be "other environmental air handling space."

It still requires plenum rated cable if it is. :happyyes:
 

JFletcher

Senior Member
Location
Williamsburg, VA
I'd need more information as to the install. If you do have an air-handling space, and conduit going to a wet location, pull CMP and dont worry about the moisture. Is this a ceiling conduit diving down a column and running under the slab to a free standing thing like a hotel kiosk?

They do make Cat6 splice blocks that do not degrade the performance of the cable if you terminate them correctly. iow, if you have to certify the cable, you arent going to fail for using a splice block. Not sure about 6a or shielded splice blocks, or waterproof, but if you need some combination of those, Im sure they can be had.

Unless your project specs call to follow TIA/EIA as code, you can disregard what they say. Comm cables do not fail if installed correctly (support, pulling tension, no kinks, bend radius) and if you use good cable. Damage from lightning, rodents, accident, sabotage, and the like are not going to be covered under warranty anyway. OSP (out side plant) cable is designed more for UV protection than moisture.

I cant remember the deal with the '50' rule', it's been a while since I thumbed thru my TDMMs, but if you need to create a splice 25' away, simple coil 25' more in a service loop out of the way to get your distance.
 

dema

Senior Member
Location
Indiana
50 feet

50 feet

When you coil the cable, you get other interference issues. I imagine it would be better than the reflection - but have you heard of people who pick up radio stations in their dental work?

The conduit in slab necessarily fills with water within a few years in most climates due to condensation, and the CMP necessarily will fail. I hate to design an installation that I know is only good for 5 years and after five years the conduit itself is useless. Can't even replace the plenum cable - it is the water that destroys it - as I understand it it takes five years for the conduit to fill with water, but then - what? You vacuum it out and install new CMP for five more years?

I'm going to follow the recommendations in that link from now on - generally the installations I am concerned about have that other air space issue going on so there is no clear win-win. Where the installation is small, I will have to work it out case by case. Likely requiring conduit through the other environmental air areas.

There are some interesting posts going back a few years about battles with inspectors over this issue.

I'm a design engineer with repeat customers. I want to keep them coming back. This is a design issue - if contractor installs CMP, he/she is following standard practice and the letter of the law and I would expect the contractor to be found blameless. However, it would be nice if the installation continued to work after five years.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
When you coil the cable, you get other interference issues. I imagine it would be better than the reflection - but have you heard of people who pick up radio stations in their dental work?

The conduit in slab necessarily fills with water within a few years in most climates due to condensation, and the CMP necessarily will fail. I hate to design an installation that I know is only good for 5 years and after five years the conduit itself is useless. Can't even replace the plenum cable - it is the water that destroys it - as I understand it it takes five years for the conduit to fill with water, but then - what? You vacuum it out and install new CMP for five more years?

I'm going to follow the recommendations in that link from now on - generally the installations I am concerned about have that other air space issue going on so there is no clear win-win. Where the installation is small, I will have to work it out case by case. Likely requiring conduit through the other environmental air areas.

There are some interesting posts going back a few years about battles with inspectors over this issue.

I'm a design engineer with repeat customers. I want to keep them coming back. This is a design issue - if contractor installs CMP, he/she is following standard practice and the letter of the law and I would expect the contractor to be found blameless. However, it would be nice if the installation continued to work after five years.
Depending on the function of the cable, it may not be compliant if the project is under the 2014 NEC.
 

JFletcher

Senior Member
Location
Williamsburg, VA
When you coil the cable, you get other interference issues. I imagine it would be better than the reflection - but have you heard of people who pick up radio stations in their dental work?

The conduit in slab necessarily fills with water within a few years in most climates due to condensation, and the CMP necessarily will fail. I hate to design an installation that I know is only good for 5 years and after five years the conduit itself is useless. Can't even replace the plenum cable - it is the water that destroys it - as I understand it it takes five years for the conduit to fill with water, but then - what? You vacuum it out and install new CMP for five more years?

I'm going to follow the recommendations in that link from now on - generally the installations I am concerned about have that other air space issue going on so there is no clear win-win. Where the installation is small, I will have to work it out case by case. Likely requiring conduit through the other environmental air areas.

There are some interesting posts going back a few years about battles with inspectors over this issue.

I'm a design engineer with repeat customers. I want to keep them coming back. This is a design issue - if contractor installs CMP, he/she is following standard practice and the letter of the law and I would expect the contractor to be found blameless. However, it would be nice if the installation continued to work after five years.

No, you splice OSP to CMP a few feet inside the building, in a dry location. They do make OSP Cat6 (if that's what we're talking about; you didnt specify).

Again, try making an approved Cat 6 splice <50' from the panel and see if it certifies. I havent seen one that didnt. You dont have to coil the cable; an "S" shape or any other configuration than loops of cable next to itself should work fine.

I dunno that CMP would degrade if the jacket was subjected to submersion in water. OSP cable is PE , UV resistant jacketed with gel fill. CMP is teflon for smoke characteristics in a plenum/air handling space. The two are incompatible. iow, you cant have your cake and eat it too.

You can either splice OSP to CMP or run CMP the whole way and deal with the possible degradation. I do remember asking here many years ago if an EMT conduit thru an air handling space was considered part of that space; the answer was 'no'. You could maybe run OSP point to point in a Chapter 3 metal raceway method thru an air handling space but definitely not thru ENT or something like Carlon Plenum-Guard.

Why is the underground conduit toast after 5 years? I agree I would want one of my installs to last significantly longer than that.
 

dema

Senior Member
Location
Indiana
Underground conduit

Underground conduit

The conduit would just be full of water. I don't know if you can effectively suck the water out or not.
 

JFletcher

Senior Member
Location
Williamsburg, VA
The conduit would just be full of water. I don't know if you can effectively suck the water out or not.

A good vacuum pump can suck water up 22' vertically. A decent shop vac can do 2' vertically and quite a distance horizontally. Even if it stayed flooded, a wire lube like Ideal's Yellow 77 doesnt wash off easily; you could use the old to pull in the new and not worry about removing any water in the conduit. :)
 

JFletcher

Senior Member
Location
Williamsburg, VA
Would you pull directly with the old cable or use the cable to pull a rope/string through?

Depends on if there are other cables in the conduit and if they are staying. Usually I pull a string through because there is almost zero tension with a string, and 23/24 ga wire doesnt hold up well when there's any tension on it, even 25lbs or less. I may even run a nylon tape thru the conduit because 2 or more data cables tend to spiral around each other to some degree making repulling harder than it needs to be. Just depends on how easily the old cable comes out. If it's just one cable in the conduit, I would use the old to pull in the new.

If the old cable jacket is shredded/disintegrated, or there's mud, rust, cigarette butts, drywall chips, etc. partially blocking the raceway, you can always blow out the conduit with air or water before repulling new wire. Makes a hell of a mess at the other end if you arent prepared. :D
 

GoldDigger

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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
If the old cable jacket is shredded/disintegrated, or there's mud, rust, cigarette butts, drywall chips, etc. partially blocking the raceway, you can always blow out the conduit with air or water before repulling new wire. Makes a hell of a mess at the other end if you arent prepared. :D
To apprentice: "Watch this end of the conduit and tell me if anything happens...." ?:angel:
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
Techically, under the NEC, a plenum is something constructed just for the purpose of handling moving air.
Above a dropped ceiling is generally not a plenum, but may well be "other environmental air handling space."

It still requires plenum rated cable if it is. :happyyes:

Since when is the "interior" of ANY slab a plenum space? Why is no one asking the question of how the heck someone thinks the interior of a conduit running in a slab is somehow a plenum? :? Run TC and be done with it.
 

don_resqcapt19

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Location
Illinois
Occupation
retired electrician
Since when is the "interior" of ANY slab a plenum space? Why is no one asking the question of how the heck someone thinks the interior of a conduit running in a slab is somehow a plenum? :? Run TC and be done with it.
I believe that part of the run is in the wet location conduit and part of it is in a 300.22(C) space.
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
The 50' is not optional - if a splice is made in a cable within 50' of the rack, there is reflection that can make the cable unusable.

No, that's ridiculous. The 50 foot limit comes from 800.48

800.48 Unlisted Cables Entering Buildings
Unlisted outside plant communications cables shall be permitted
to be installed in building spaces other than risers,
ducts used for environmental air, plenums used for environmental
air, and other spaces used for environmental air,
where the length of the cable within the building, measured
from its point of entrance, does not exceed 15 m (50 ft)...


So if the ceiling is indeed an environmental air handling space you can't even extend your unlisted wet location cable beyond the top of the partition. Rather than splicing I would consider EMT or a plenum listed innerduct. If you must transition to a plenum listed cable you could just do it in the partition. Two gang ring and blank cover for access, install a jack on the end of each cable and a short patch cord to connect the two.

You are overthinking this.

-Hal
 

GoldDigger

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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
No, that's ridiculous. The 50 foot limit comes from 800.48

800.48 Unlisted Cables Entering Buildings
Unlisted outside plant communications cables shall be permitted
to be installed in building spaces other than risers,
ducts used for environmental air, plenums used for environmental
air, and other spaces used for environmental air,
where the length of the cable within the building, measured
from its point of entrance, does not exceed 15 m (50 ft)...


So if the ceiling is indeed an environmental air handling space you can't even extend your unlisted wet location cable beyond the top of the partition. Rather than splicing I would consider EMT or a plenum listed innerduct. If you must transition to a plenum listed cable you could just do it in the partition. Two gang ring and blank cover for access, install a jack on the end of each cable and a short patch cord to connect the two.

You are overthinking this.

-Hal

I think you are underthinking it. The OP did not say that the 50' limit came from the NEC.

BICSI - the communication people who certify but are not a law making body - recommend that a splice box be placed at least 50' from the communication rack
This is a performance issue, although the importance of it is affected by just how good the splice is.
The idea, I believe, is that there is enough attenuation in 50' of cable that very small reflections caused by the impedance mismatch at the splice will not be noticeable back at the source/destination at the rack.

There have been other threads that discuss the impact of small impedance mismatches on transmission lines (CAT or coax) and the fact that the effect is greatest when the physical size of the mismatch is small compared to the relevant wavelengths.
GigaHerz and higher network transmission speeds may be involved and 1/4 wavelength in typical transmission line of 1GHz is less than 7 cm.
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
I'm familiar with reflections and short coax jumpers in CATV and I have no doubt that it can happen with UTP. However I have no doubt that data cabling in 99% of installations will never be affected by such a situation because it will never see the bandwidth necessary to produce it. There are often many jacks/plugs/patch cable combos between a premises run and its final destination, most from a few inches to several feet in length. How come BICSI isn't complaining about those?

BICSI - the communication people who certify but are not a law making body...

There's a reason why their requirements don't carry any weight outside the IT community.

-Hal
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
I believe that part of the run is in the wet location conduit and part of it is in a 300.22(C) space.

The OP used "plenum" as a modifier for "cable". Otherwise it's "floor boxes", "slab", "direct contact with earth" and "condensation". There is absolutely nothing in his description of where the cable is being installed to remotely suggest it is, in fact, a plenum. And it's clear from his description that he thinks the conduit is a plenum, or why worry about trying to find wet location rated plenum data cable. He needs to clarify why he thinks cable pulled through conduit buried in a slab in contact with the ground needs to be plenum rated.
 

iwire

Moderator
Staff member
Location
Massachusetts
The OP used "plenum" as a modifier for "cable". Otherwise it's "floor boxes", "slab", "direct contact with earth" and "condensation". There is absolutely nothing in his description of where the cable is being installed to remotely suggest it is, in fact, a plenum.

Not a plenum but 'other air space' so it is treated as a plenum.

- generally the installations I am concerned about have that other air space issue going on so there is no clear win-win. Where the installation is small, I will have to work it out case by case. Likely requiring conduit through the other environmental air areas.
 
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