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Service Maintenance Company, Inc. Electrical Contractors |
| NEC Article 770 - Optical Fiber Cables and Raceways | 6/9/2004 |
| Mike Holt for EC&M Magazine The code requirements for optical fiber vary with the type of cable. Fiber-optic cable has many advantages over competing technologies. These include more information capacity (by orders of magnitude), reduced ancillary equipment requirements in key applications, immense scalability and expandability with the same infrastructure, and electromagnetic noise immunity. For these reasons and others, fiber-optic cable installations are increasingly common. Article 770 covers the installation of optical fiber cables used to transmit light for control, signaling and communication. Further, it contains the installation requirements for optical raceways, which contain and support the optical fiber cables. It also contains the requirements for composite cables (often called "hybrid"), that combine optical fibers with current-carrying metallic conductors. Usually when we think of wiring methods, we think of Article 300. However, you need to use only Article 770 methods for fiber-optic cables except where Article 770 makes specific references to Article 300. The first such reference is to 300.21, which addresses requirements for stopping the spread of combustion. That reference is in 770.3, which also requires the removal of the accessible parts of abandoned fiber-optic cables. Article 770 does not refer to 300.15, so you don't have to use boxes for splices or termination of optical fiber cable. The FPN in 770.50 states that splice cases and terminal boxes are typically used as enclosures for splicing, or terminating and splicing optical fiber cables-it's just a good practice. But, an FPN is not an enforceable code requirement [90.5(C)]. On the other hand, composite optical fiber cables [770.5(C)] must comply with the appropriate requirements of Chapters 1 through 4 of the NEC. Another Chapter 3 reference is in regard to ducts, plenums and other air-handling spaces. When installing optical fiber cables and optical fiber raceways in such spaces, you must do so per 300.22. Optical Fiber Cables Optical fiber cables transmit light for control, signaling and communications through an optical fiber. The NEC recognizes three types: Nonconductive. Contain nothing that can conduct electricity, so will not accidentally energize or be energized even when closely associated with electrical conductors. Conductive. Contain noncurrent-carrying conductive members such as metallic strength members, metallic vapor barriers and metallic armor or sheath. Composite. Contain optical fibers and current-carrying electrical conductors. Classify these as electrical cables in accordance with the type of electrical conductors. [770.52(A)]. Use composite cables only where the optical fibers and current-carrying electrical conductors are functionally associated. You must meet Chapter 3 raceway requirements, but there are exceptions. For example, when your raceway contains only nonconductive cables, you can ignore the Chapter 3 and Chapter 9 wire fill requirements. Otherwise, those requirements apply. In an optical raceway, you can use only nonconductive optical fiber cables. This is a manufacturer's listing requirement (see the UL General Information for Electrical Equipment Directory) and not found in the NEC. However, the NEC does require you to use listed equipment. Another thing to remember about fiber-optic cable requirements is you must route those cables so they don't prevent the removal of suspended ceiling panels. That goes hand in hand with 770.8, which requires installing equipment and cabling in a neat and workmanlike manner. One aspect of that pertains to cables exposed on the surface of ceilings and sidewalls. You must support these by the structural components of the building in such a manner that normal use won't damage the cable. You must secure a cable in a manner that doesn't damage it (see Figure 770-7 un770-07 770-08.cdr). If you install cables next to framing members, you must protect them from physical damage via penetration by screws or nails. This requires a 1-1/4 in. separation from the framing member or a suitable metal plate per 300.4(D). Listing and Marking You must use optical fiber cables that are listed as being suitable for the application and they must be marked per Table 770.50. However, there are three exceptions. The first one: you don't need to use listed cables if the length of the cable within the building-measured from its point of entrance-does not exceed 50 ft and the cable terminates in an enclosure. Here's a quick summary of the listing requirements for optical fiber cables and raceways, as laid out in 770.51: Types OFNP and OFCP (Plenum): Suitable for use in ducts, plenums and other space used for environmental air. Types OFNR and OFCR (Riser): Suitable for use in a vertical run in a shaft or from floor to floor. Types OFNG and OFCG (General-Purpose): Suitable for general-purpose use, with the exception of risers and plenums. Types OFN and OFC (General-Purpose): Suitable for general-purpose use, with the exception of risers, plenums and other space used for environmental air. Plenum Optical Fiber Raceway: Have adequate fire-resistant and low smoke-producing characteristics. Riser Optical Fiber Raceway: Have fire-resistant characteristics capable of preventing the carrying of fire from floor to floor. General-Purpose Optical Fiber Cable Raceway: Resistant to the spread of fire. Mixing fiber and electrical If you are mixing optical fiber with other kinds of cabling, pay close attention to 770.52. The requirements can get tricky, but here are the highlights: You can put optical fibers within the same composite cable for electric light, power and Class 1 circuits operating at 600V or less where the functions of the optical fibers and the electrical conductors are associated. This part of 770.52(A) gives permission for a factory-made composite optical fiber cable (hybrid cable). These composite cables have both optical fibers and current-carrying electrical conductors [770.5(C)]. You can use this type of optical fiber cable only where the optical fibers are functionally associated with the electrical conductors. Nonconductive optical fiber cables, but not conductive optical fiber cables, can occupy the same cable tray or raceway as conductors for electric light, power and Class 1 circuits operating at 600V or less. Composite optical fiber cables containing only current-carrying conductors for electric light, power and Class 1 circuits rated 600V or less can occupy the same cabinet, cable tray, outlet box, panel, raceway or other termination enclosure as conductors for electric light, power and Class 1 circuits operating at 600V or less. Nonconductive optical fiber cables cannot occupy the same cabinet, outlet box, panel or similar enclosure housing the electrical terminations of an electric light, power and Class 1 circuit. But, there are four exceptions to this: Exception No. 1: They can occupy the same cabinet, outlet box, panel or similar enclosure where nonconductive optical fiber cable is functionally associated with the electric light, power and circuit. Exception No. 2: They can occupy the same cabinet, outlet box, panel or similar enclosure where nonconductive optical fiber cables are installed in factory- or field-assembled control centers. Exception No. 3: In industrial establishments only, where conditions of maintenance and supervision ensure that only qualified persons will service the installation, you can run nonconductive optical fiber cables with circuits exceeding 600V. Exception No. 4: This is the same as No.3, but for composite cables. Optical fibers can be in the same cable-and conductive and nonconductive optical fiber cables can be in the same cable tray, enclosure-as any of the following: Class 2 and Class 3 circuits in compliance with Article 725. Power-limited fire alarm circuits in compliance with Article 760. Communications circuits in compliance with Article 800. CATV cables in compliance with Article 820. Applications In 770.53, the various requirements for plenums, and risers can seem like so much alphabet soup. Even so, you'll need to match the cable type to the application. Figure 77.53 is helpful in doing this. You must also consider the type of raceway you are using, whether there are floor penetrations, and whether the installation is in a single-family home, duplex, or other type of building. Perhaps the most important thing to remember about Article 770 is something that isn't even in that article: Article 90's statement that the NEC is not a design guide or installation manual. Article 770 doesn't tell you how to ensure your system will meet, and test out to, the performance requirements you need or your contract specifies-for example, it makes no mention of maximum bend radii. It doesn't tell you how to install and test cable safely-for example, it doesn't caution you to not to look into a cable even if you can't see any light coming through it (the light might not be visible, but it can still damage your eye). Use Article 770 in concert with other standards, and your fiber optic work will shine. |
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