National Electrical Code revisions focus on optical-fiber cable applications

Feb. 1, 1997
This is a continuation of the article published in the January 1997 issue, page 60. This part focuses on cable applications and how the 1996 National Electrical (NEC) has been revised to accommodate technological advances in intrabuilding wiring practices.

Dean Yamasaki

Siecor Corp.

This is a continuation of the article published in the January 1997 issue, page 60. This part focuses on cable applications and how the 1996 National Electrical (NEC) has been revised to accommodate technological advances in intrabuilding wiring practices.

Rather than develop separate performance standards, the NEC adapts existing electrical cabling requirements for optical-fiber communications technology where possible. For example, Section 300-22 incorporates requirements for installing and using electrical wiring and equipment in ducts, plenums, and air-handling areas.

Plenum applications. In addition, Section 300-22 assigns ducts into three categories, each with specific requirements for cable installations. Ducts for dust, loose stack or vapor removal [Section 300-22(a)] cannot be used for any wiring system. Similarly, no wiring system can be installed in any duct used to ventilate commercial cooking equipment.

Ducts or plenums used for environmental air [Section 300-22(a)] and other spaces used for environmental air [Section 300-22(c)] can accommodate Type ofnp cables and listed optical-fiber raceways per Section 770-53(a). Only nonconductive ofnp cables can be installed in a listed ofnp optical-fiber raceway.

Types ofnr, ofng, and OFN cables can be installed in environmental air- handling spaces if Section 300-22 requirements are met. Adherence means that cables be installed in rigid or intermediate metal conduit, electrical metallic tubing, or flexible metallic tubing.

If the primary purpose of habitable rooms and areas is air handling, restrictions of Section 300-22(c) apply whether or not electrical equipment is located in the room. Habitable rooms and areas that are not primarily used for air handling are exempt from this restriction.

Riser applications. Cables installed in vertical runs and traveling over more than one floor or in a shaft must be at least a Type ofnr; a Type ofnp cable is an acceptable substitute. Limited riser optical-fiber raceways using either ofnr or ofnp cables are allowed in vertical runs within a shaft or from floor to floor.

Type ofng and OFN cables can be installed in vertical runs that penetrate more than one floor or in a shaft in two instances: if they are encased in a metal raceway or are located in a fireproof shaft containing firestops on each floor, or if they are installed in one- or two-floor dwellings.

Optical-fiber cables installed in other building areas must be listed as Type ofng or OFN.

Outside plant transition. The 1996 NEC also cites requirements for nonconductive optical-fiber cables that enter from outside a building. Two options are available for using unlisted or unmarked cables:

Cables entering the building without extending more than 50 feet beyond the entrance point and are terminated in a metallic or plastic enclosure do not have to be listed or marked.

Cables entering from outside a building and run in a building raceway installed in accordance with Chapter 3 of the NEC do not have to be listed or marked. Moreover, rigid or intermediate metal conduit that is properly grounded according to Section 800-40(b) can also be used.

Fiber and electrical cables. Nonconductive optical cables can occupy the same cable tray or raceway with conductors for electric light, power, or Class 1 circuits operating at 600V or less. However, these cables cannot be placed in the same enclosure that houses electrical terminations. Some exceptions are referenced in Section 770-52.

In addition, optical-fiber cables can be placed in the same raceway, cable tray, or enclosure with cable-TV, radio distribution, and communications circuits, as well as with power-limited fire-protective signaling, Class 2 and 3 remote-control signaling and power-limited circuits.

Electronic and computer equipment. Article 645 in the NEC contains requirements for wiring used only in a computer room. If dielectric optical-fiber cables extend beyond the computer room or if the cables serve alternate functions, then Section 770 might apply.

Cables must be listed as Type DP when used under raised computer room floors. These cables must, as a minimum, comply with the flame-resistance requirements of UL-1581 or CSA C.22 No. 3-M 1985.

Network designers and planners must also check out local building codes because cables that meet the 1996 NEC requirements might not always satisfy local code requirements.

Dean Yamasaki is an applications engineer at Siecor Corp. (Hickory, NC). This article was adapted from a special report that appeared in the November 1996 issue of Lightwave, an associated publication of PennWell Publishing Co., Nashua, NH.

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