Article 770: Optical Fiber Cables
By Mike Holt for EC&M Magazine
Understand three key concepts behind Article 770 requirements.
Several factors are driving the popularity of optical fiber cabling. Chief among those is the growth of high-tech applications that require the massive bandwidth that fiber optic cabling makes possible.
Further adding to the popularity of optical fiber is its immunity to electromagnetic interference (EMI). Fiber is now showing up in “home runs” for voice, data transfer, process control, and machine control.
Optical fiber isn’t always the best or most economical choice, but it is becoming the best and most economical choice for applications that involve data or voice transmission through physical infrastructure.
Other articles
Only those provisions of Article 300 specifically referenced in Article 770 apply to optical fiber cables and raceways [770.3]. Five of these are of particular interest:
- Article 770 doesn’t reference 300.15, so you don’t need to put splices or terminations of optical fiber cable in boxes. On the other hand, composite (hybrid) optical fiber cables [770.9(C)] that contain conductors for electric light and power must comply with Chapters 1 through 4.
- You must firestop optical fiber cables and raceways if you install them through fire-resistant-rated walls, partitions, floors, or ceilings, to avoid substantially increasing the possible spread of fire or products of combustion. Follow the instructions supplied by the manufacturer for the specific type of cable and construction material (drywall, brick, etc.), as required by 300.21.
- You can install non-plenum rated optical fiber cables beneath a raised floor in an information technology equipment room that meets the requirements of 645.4 [300.22(D) and 645.5(D)(5)(c)].
- You can put plenum-rated cables and plenum-rated optical fiber raceways above a suspended ceiling or below a raised floor used for environmental air movement (Figure 770-5).
- You can install optical fiber cables in ducts or plenums, provided you install them in electrical metallic tubing, intermediate metal conduit, or rigid metal conduit as required by 300.22(B); but only if installing them in these locations is necessary for direct action upon (or sensing of) the contained air. This situation is highly unlikely with optical fiber cables.
Abandoned Cable
The concept of “abandoned cable” confuses many people, primarily on the issues of why and what. Let’s clear those up, right now.
Why: To limit the amount of combustible materials within a building, and to limit the spread of fire.
What: Remove the accessible portion of optical fiber cable that isn’t terminated at equipment, and not identified for future use with a tag [770.2 and 770.3(A)] (Figure 770-4).
You don’t have to remove concealed cables, even if they are abandoned. Per the Article 100 definition of “concealed,” cables in raceways are concealed.
Composite cables
Composite optical fiber cables contain optical fibers and conductors for electric light and power. Thinking about using a composite cable to allow for “future use?” Carefully consider such a move, because:
- You can use composite cables only where the optical fibers and current-carrying conductors are functionally associated [770.133(A)].
- Composite cables add Chapter 3 wiring requirements to the installation.
If composite cable makes sense for achieving a certain design function, then use it. Just keep in mind that doing so brings additional costs in materials and installation.
Raceways for optical fiber cables
You can install listed optical fiber cable in any type of listed raceway permitted by Chapter 3, if you comply with Chapter 3 requirements for it—with certain exceptions.
For example, if your optical fiber cables don’t have current-carrying conductors, you can install them in a raceway without applying the raceway fill tables of Chapter 3 and Chapter 9. But if they do have current-carrying conductors or you install such conductors with them, you must apply the raceway fill tables of Chapter 3 and Chapter 9 [770.12 (A)].
If you install listed optical fiber cables in listed plenum optical fiber raceway, listed riser optical fiber raceway, or listed general-purpose optical fiber raceway [770.182], the cables and raceways must comply with 770.154 and you must follow 362.24 through 362.56 [770.12(B)] (Figure 770-10). The UL General Information for Electrical Equipment Directory (White Book) allows only listed nonconductive optical fiber cable to be installed in a listed optical fiber raceway.
You can install listed optical fiber raceway as innerduct in any Chapter 3 raceway, if you comply with 770.154 [770.12(C)]. If you use unlisted underground or outside plant construction plastic innerduct and it enters the building from the outside, you must terminate and firestop it at the point of entrance [770.12(D)].
Mechanicals
Install the equipment and cabling in a neat and workmanlike manner, so you comply with 770.24. Not sure what this means? Consult the BICSI installation standards or see ANSI/NECA/BICSI 568, Standard for Installing Commercial Building Telecommunications Cabling.
Locate all above-ceiling cables in a way that allows service technicians to move the suspended-ceiling panels to gain working access. If your cables make the panels hard to move, you have a violation of 770.21.
Use the structural components of the building to support exposed cables, and use the appropriate fittings. Don’t use the ceiling-support wires or ceiling grid to support optical fiber raceways or cables [300.11] (Figure 770-12). You can support raceways and cables with independent support wires that are attached to the suspended ceiling, per 300.11(A).
Indoor Cables
Optical fiber cables installed within buildings must be listed per 770.154 and 770.179, and marked per Table 770.113. The NEC doesn’t require outside or underground cable to be listed, but the cable must be approved by the authority having jurisdiction as suitable for the application per 90.4, 90.7, and 110.2.
But you don’t have to use listed and marked cable if:
- The length of the cable within the building (from its point of entrance) doesn’t exceed 50 ft and the cable terminates in an enclosure [770.113 Ex 1], or
- You install the cable (entering the building) in intermediate metal conduit, rigid metal conduit, rigid nonmetallic conduit, or electrical metallic tubing [770.113 Ex 2].
Installation
You can use composite cable to also contain electric light, power, Class 1, or nonpower-limited fire alarm circuits, only if the functions of the optical fibers and the electrical conductors are associated [770.133(A)].
You can install nonconductive optical fiber cables in the same cable tray or raceway with conductors for electric light, power, Class 1, or nonpower-limited fire alarm circuits. But conductive optical fiber cables must not occupy the same cable tray or raceway as conductors for electric light, power, Class 1, or nonpower-limited fire alarm circuits.
Nonconductive optical fiber cables cannot occupy a cabinet, outlet box, panel, or similar enclosure housing the electrical terminations of an electric light, power, and Class 1 circuit, unless the cable is functionally associated with the electric light, power, Class 1, or nonpower-limited fire alarm circuit.
You can install optical fiber cables in the same raceway or enclosure as jacketed cables of any of the following:
- Class 2 and Class 3 circuits in accordance with Article 725.
- Power-limited fire alarm circuits in accordance with Article 760.
- Communications circuits in accordance with Article 800.
- Coaxial cables in accordance with Article 820.
Miscellaneous Requirements
Optical fiber cables must comply with the requirements of 770.154(A) through (E), or the applicable requirements of the cables substituted as permitted in 770.154(F). Table 770.154 shows the permitted substitutions for optical fiber cables.
Part IV of Article 770 details the listing requirements. This information is primarily for manufacturers. The requirement for you is that you use cables listed for the application. Give special consideration to cables in areas that move or transport environmental air, so as to reduce the hazards that arise from the burning of cable insulation and jackets. Because listed plenum-rated cables have adequate fire-resistance and low smoke-producing characteristics, you can use them in environmental air space. But you can’t use them in constructed ducts or plenums.
Now that we’ve shed some light on fiber optic installation requirements, how might we sum them up? Three main concepts come into play.
The first concept is that optical fiber cables don’t carry current, so the Chapter 3 requirements designed to protect current carrying conductors from overheating don’t apply. The second concept is that current-carrying conductors mixed into your fiber-optic cancels the first concept.
Finally, if you penetrate any fire barriers with fiberoptics (or anything else for that matter), you must restore the integrity of that barrier. So, pay close attention to firestops and fire rated walls.
Definitions
These three definitions are critical for properly applying Article 770.
- Abandoned Cable. Cable that isn’t terminated at equipment, and not identified for future use with a tag.
- Optical Fiber Raceway. A raceway designed for enclosing and routing nonconductive optical fiber cables.
- Point of Entrance. The point at which the cable emerges from an external wall, from a concrete floor slab, or from a rigid metal conduit or an intermediate metal conduit grounded to an electrode, per 800.100.
