NotAnExpert
Member
- Location
- California
- Occupation
- Technician
Can the experienced forum members comment on some conflicting advice I am getting regarding article 770 (fiber optic cable)?
This is concerning armor-clad Corning OSP (outside plant) fiber that is not indoor rated.
The big question is about what constitutes the “entrance” of a building. Some of my planned destinations are greater than 50’, so many installers will terminate at the point of entry and either patch or splice to indoor-rated fiber and continue to the destination. But each patch or splice degrades the signal. From a purely network perspective, it is more ideal to find a way to have an unbroken fiber cable to the destination data closet as long as the solution is within code.
A writer named Tim Kuhlman wrote an article for an on-line magazine called: “Consulting – Specifying engineer.” In the article, Mr. Kuhlman says that the entry can be extended by completely enclosing the fiber in IMC or RMC.
Unlisted cables and the NEC | Consulting - Specifying Engineer (csemag.com)
Here is his exact quote:
“To extend into a building beyond 50 ft, the conduit entering the wall or concrete floor slab needs to be extended with IMC or RMC conduit. This allows the entrance point to move from the wall or concrete slab. The 50-ft limit starts when the cable exits the IMC or RMC conduit. Extending the entrance point with IMC or RMC is a useful provision in applications when it is not practical to have the entrance facility on a ground floor or adjacent to the exterior of the building.”
Mr. Kuhlman works for perhaps the one of the most reputable MEP Engineering firms in the world: Jacobs. If you are building a hyper-scale data center, then his employer is going to be on your short list of firms to engage. So, I know that he is consulting on many major datacenter builds each year and has to pass inspection in a variety of locations.
On the other hand, I have gotten a reply from a technician at Corning, the maker of the fiber, saying that nothing can be done to install unrated cable beyond 50’ in a way that meets code. Here is what the technician said:
“You cannot bring the OSP (non-flame rated) cable more than 50' into the building. It doesn't matter if you put it into conduit or pour concrete around the cable, it's against the code. You can purchase I/O cable and avoid terminating in the room that is within 50' of the entry point.”
I don’t know the background of the technician. I am not sure if he has an electrical engineering background or not. But his advice on the I/O rated cable is something I plan to use moving forward. Just spend the extra money and get a cable type that can be used anywhere. I’ll even go one step further and get the dielectric I/O version that doesn’t require grounding.
Mike Holt wrote about Article 770 and in it he mentioned three exceptions for non-listed cables, but it seems to only list the first exception.
Article 770 - Optical Fiber Cables and Raceways (mikeholt.com)
Is Mr. Kuhlman’s advice correct? Can the entrance be extended? Is the Corning technician correct? It doesn’t seem that both can be correct. I am not hoping to pit the two against each other. But they both work for reputable companies, and I want to have some additional opinions before taking this to the local inspector for comment.
This is concerning armor-clad Corning OSP (outside plant) fiber that is not indoor rated.
The big question is about what constitutes the “entrance” of a building. Some of my planned destinations are greater than 50’, so many installers will terminate at the point of entry and either patch or splice to indoor-rated fiber and continue to the destination. But each patch or splice degrades the signal. From a purely network perspective, it is more ideal to find a way to have an unbroken fiber cable to the destination data closet as long as the solution is within code.
A writer named Tim Kuhlman wrote an article for an on-line magazine called: “Consulting – Specifying engineer.” In the article, Mr. Kuhlman says that the entry can be extended by completely enclosing the fiber in IMC or RMC.
Unlisted cables and the NEC | Consulting - Specifying Engineer (csemag.com)
Here is his exact quote:
“To extend into a building beyond 50 ft, the conduit entering the wall or concrete floor slab needs to be extended with IMC or RMC conduit. This allows the entrance point to move from the wall or concrete slab. The 50-ft limit starts when the cable exits the IMC or RMC conduit. Extending the entrance point with IMC or RMC is a useful provision in applications when it is not practical to have the entrance facility on a ground floor or adjacent to the exterior of the building.”
Mr. Kuhlman works for perhaps the one of the most reputable MEP Engineering firms in the world: Jacobs. If you are building a hyper-scale data center, then his employer is going to be on your short list of firms to engage. So, I know that he is consulting on many major datacenter builds each year and has to pass inspection in a variety of locations.
On the other hand, I have gotten a reply from a technician at Corning, the maker of the fiber, saying that nothing can be done to install unrated cable beyond 50’ in a way that meets code. Here is what the technician said:
“You cannot bring the OSP (non-flame rated) cable more than 50' into the building. It doesn't matter if you put it into conduit or pour concrete around the cable, it's against the code. You can purchase I/O cable and avoid terminating in the room that is within 50' of the entry point.”
I don’t know the background of the technician. I am not sure if he has an electrical engineering background or not. But his advice on the I/O rated cable is something I plan to use moving forward. Just spend the extra money and get a cable type that can be used anywhere. I’ll even go one step further and get the dielectric I/O version that doesn’t require grounding.
Mike Holt wrote about Article 770 and in it he mentioned three exceptions for non-listed cables, but it seems to only list the first exception.
Article 770 - Optical Fiber Cables and Raceways (mikeholt.com)
Is Mr. Kuhlman’s advice correct? Can the entrance be extended? Is the Corning technician correct? It doesn’t seem that both can be correct. I am not hoping to pit the two against each other. But they both work for reputable companies, and I want to have some additional opinions before taking this to the local inspector for comment.
Last edited:
