Article 770 - Optical Fiber Cables and Raceways
by 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 (see Figure 770-2 un770-02 770-03A 01.cdr). 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 dont 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-its 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 (see Figure 770-1 un770-01 770-03 01.cdr).
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 (see Figure 770-3 un770-03 770-04.cdr). The NEC recognizes three types:
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 (see Figure 770-6 un770-06 770-06y.cdr).
In an optical raceway, you can use only nonconductive optical fiber cables. This is a manufacturers 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 dont 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 wont damage the cable. You must secure a cable in a manner that doesnt 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 dont 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.
Heres a quick summary of the listing requirements for optical fiber cables and raceways, as laid out in 770.51:
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 (see Figure 770-10 un770-10 770-52A 02.cdr).
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:
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 (see Figure 770-11 un770-11 770-52B.cdr):
In 770.53, the various requirements for plenums, and risers can seem like so much alphabet soup. Even so, youll 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 isnt even in that article: Article 90s statement that the NEC is not a design guide or installation manual. Article 770 doesnt 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 doesnt tell you how to install and test cable safely-for example, it doesnt caution you to not to look into a cable even if you cant 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.
Copyright © 2002 Mike Holt Enterprises,Inc.