OSP cable install in buildings

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 Question by Paul Hruby posted 28 Feb 2001
 OSP cable install in buildings
Helpdesk;

On many of the projects that I am managing in NYC, customers are seeking to use existing cable OSP cable stock in reaching colocators within the building, due to it's ready availability/high fiber count/ and lower cost. Many of these cases allow for the use of EMT in routing the cable to sites, however in some cases the proposed route does not allow for metal conduit.

Will UL-listed Plenum Innerduct meet NEC Code in protecting OSP cable? Does this meet building code as well? I have not found a clear definition on this matter in Article 770.

Thank you

Paul Hruby
MasTec North America
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 Answer by Roman Kitaev posted 01 Mar 2001
Hello, Paul!

You are asking a very interesting question. I made some investigation on it and here are the results I hope you will find useful.

The NEC (1999 edition) in the Exception N1 to Section 770-50 states the following:

"Optical fiber cables shall not be required to be listed and marked where the length of the cable within the building, measured from the point of entrance, does not exceed 50 ft (15.2 m) and the cable enters the building from the outside and is terminated in an enclosure".

That means if your cable length is less than 50 ft within the building, you do not have to worry.

If you have more than 50 ft to cover, then Exception N 2 and Exception N 3 provide the needed guidance.

If you have conductive fiber cable (i.e. with metallic parts), you have to use Exception N 2:

"Conductive optical fiber cable shall not be required to be listed and marked where the cable enters the building from the outside and is run in rigid metal conduit or intermediate metal conduit and such conduits are grounded to an electrode in accordance with Section 800-40(b)".

So, it's clear that if the cable is conductive, there are only two options: rigid or intermediate metal conduits and Plenum Innerduct may not be used.


If you have a non-conductive fiber cable, Exception N 3 shall be used:

"Nonconductive optical fiber cables shall not be required to be listed and marked where the cable enters the building from the outside and is run in raceway installed in compliance with Chapter 3".

So, let's see what raceways are described in Chapter 3:

- Section 331 "Electrical Nonmetallic Tubing"
- Section 343 "Nonmetallic Underground Conduit"
- Section 345 "Intermediate Metal Conduit"
- Section 346 "Rigid Metal Conduit"
- Section 347 "Rigid Nonmetallic Conduit"
- Section 348 "Electrical Metallic Tubing"
- Section 349 "Flexible Metal Conduit"
- Section 351 "Liquidtight Flexible Metal Conduit and Liquidtight Flexible Nonmetallic Conduit"

As you can see, UL-listed Plenum Innerduct is not covered by the Chapter 3 and, therefore, may not be used for holding nonconductive optical fiber cables in accordance with the Exception N 3.


As the last resort, contact your fire marshall for her/his advice.

Best regards,
Roman Kitaev, RCDD


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