National Electrical Code revisions focus on optical-fiber cables

Changes in the 1996 version of the National Electrical Code affect optical-fiber communications cables used in nonmetallic plenum raceways, hazardous locations and network infrastructures

dean yamasaki

The National Electrical Code (NEC) has been revised this year to accommodate technological advances in intrabuilding wiring practices. Specifically, the 1996 version of the code includes modifications and clarifications to sections that affect optical-fiber communications cables. For example, Article 770 covers the requirements for optical-fiber technology used in place of, or in conjunction with, electrical conductors for communications, signaling and control circuits. It also references Article 300 in regard to application rules.

As users recognize the advantages of installing optical-fiber cables, optical-fiber structured systems are becoming more common within buildings. Fiber is the only medium that provides the high-capacity bandwidth needed to meet present and future multimedia communications needs. In addition, optical-fiber cable furnishes a communications infrastructure that can endure a substantial portion of a building`s life-cycle.

In fact, all-dielectric optical-fiber cables are preferred over metallic conductor cables because of their high-performance capabilities and immunity to electrical interference and noise, among other advantages.

Adds, changes and clarifications

The following changes have been made in the 1996 NEC that reflect clarifications to existing standards as well as the inclusion of new applications:

Reference eia/tia standards. A definition of the acceptable execution of optical-fiber installation work has been added, as referenced in standards established by the Electronic Industries Association/Telecommunications Industry Association (eia/tia) of Arlington, VA. The goal is to make installation easier for network contractors, designers and planners so they can develop standard communications infrastructures. Section 770-8 of the 1996 NEC directly references eia/tia-568, "Commercial Building Telecommunications Wiring Standard," and eia/tia-569, "Commercial Building Standard for Telecommunications Pathways and Spaces."

Clarification on cables in raceways. Section 770-53(a) of the 1996 NEC now specifies that only Type ofnp (optical-fiber nonconductive plenum) cable can be placed in listed ofnp raceways. In the previous NEC, the use of cables other than ofnp cables was not definitely prohibited in listed ofnp raceways.

Hazardous locations added. Section 770-53(d) of the 1996 NEC now includes hazardous (classified) locations in the intrabuilding standards. These hazardous locations are areas where fire or explosion hazards exist and are typically found in industrial applications where solvents and other chemicals might be stored or transported through an area. The same listing and application requirements of Article 770 now apply to these classified areas.

Listing requirements

The 1996 NEC requires that optical-fiber cable used within buildings be listed for its application, installed properly and marked correctly for its intended use (see Table 1). The appropriate markings are listed in descending order of fire-resistance ratings with the applicable American National Standards Institute (ansi), Underwriters Laboratories (UL) and Canadian Standards Association (CSA) tests.

Type ofnp cable. Optical-fiber cables installed in ducts, plenums and other building spaces used for environmental air must be listed Type ofnp and possess adequate fire-resistant and low-smoke producing characteristics. These optical-fiber cables must pass the National Fire Protection Agency 262-1994 test, which is identical to the standard ansi/ul-910 test, "Test Method for Fire and Smoke Characteristics of Electrical and Optical-fiber Cables Used in Air-Handling Spaces." The ofnp listing is equivalent to the CSA FT 6 designation.

Type ofnr (optical-fiber nonconductive riser) cable. Optical-fiber cables used in vertical shafts or in runs from one floor to another must possess fire-resistant characteristics capable of preventing the spread of fire from floor to floor. These cables are tested to the ansi/ul 1666-1991 test, "Standard Test for Flame Propagation Height of Electrical and Optical-fiber Cable Installed Vertically in Shafts."

Type ofng (optical-fiber nonconductive general-purpose) cable. This cable listing was first adopted in the 1993 NEC. Type ofng cable is suitable for general-purpose use, but cannot be used in riser or plenum applications. The code requires that cables used in these applications be resistant to the spread of fire. One method used to define this requirement is the "Vertical Flame Test--Cables in Cable Trays," as described in the Canadian Standards Association (CSA) C22.2 No. 0.3-M 1985 standard. The ofng listing is equivalent to the CSA FT 4 designation, which is used in Canada as specified by the Canadian Electrical Code and local Canadian province ordinances.

Type ofn (optical-fiber nonconductive general-purpose) cable. This classification covers cable suitable for general-purpose use, excluding plenum and riser applications, but is less stringent than the ofng classification. Ofn cables must also be fire-resistant. The vertical tray flame test described in ansi/ul-1581-1991, "Reference Standard for Electrical Wires, Cables and Flexible Cords," offers one test method for this classification.

Note that Type ofng and ofn cables are different listings even though both are identified as "general-purpose" cables. Type ofng cables must pass the FT 4 (CSA) flame test. Type ofn cables must pass a less demanding flame test and do not have to meet the FT 4 (CSA) requirements. However, the CSA C22.2 No. 0.3-M 1985 test can be used as an alternative test for ofn cables because it is more restrictive than the ansi/ul-1581-1991 test.

An alternative evaluation method that applies to the ofn category addresses both flame resistance and smoke characteristics. Test Procedure UL-1685, "Vertical-Tray Fire-Propagation and Smoke-Release Test for Electrical and Optical-Fire Cables," includes a flame test equivalent to the UL-1581 vertical-tray flame test, as well as measurement and pass/fail criteria for smoke density. Cables meeting this requirement are designated Type ofn-ls (UL); the "LS" indicates that the cable generates an acceptable low-smoke density when flame-tested.

Cable substitutions

Substitution of one cable type for another is allowed by the NEC. The general acceptance rule is that a cable with a higher designation can be substituted for a cable with a lower designation. For example, a Type ofnp cable can be used in riser applications because it meets the more stringent requirements of plenum use (see Table 2).

Cable applications

Rather than develop separate performance standards, the NEC readily adapts existing electrical cabling requirements for optical-fiber communications technology whenever possible. For example, requirements in Section 770-2 state that electrical circuits and equipment must comply with requirements in Sections 300-21 and 300-22. Section 300-21 states that electrical installations in hollow spaces, vertical shafts, and ventilation and air-handling ducts must be constructed in a manner that does not substantially increase the spread of fire. 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 or shaft where the duct is used to remove vapor or to ventilate commercial-type 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). Note that only nonconductive ofnp cables can be installed in a listed ofnp optical-fiber raceway.

Types ofnr, ofng and ofn cables can be installed in ducts, plenums and other environmental air-handling spaces if Section 300-22 requirements are met. Typically, 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

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. This relief permits the reasonable conversion from outdoor, non-fire-resistant cable to indoor, fire-resistant cable at a convenient location.

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. Both Type ofn and ofng cables satisfy the Type DP requirements. These interconnecting cables can be enclosed in conduit or raceways.

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. u

Dean Yamasaki is an applications engineer at Siecor Corp. in Hickory, NC.

The History of the National Electrical Code

The first version of the National Electrical Code (NEC) was published in 1897 to establish standards for electrical wire construction. It resulted from the combined efforts of various electrical, insurance, architectural and related building interest groups. The National Fire Protection Agency in Quincy, MA, began supporting work to update the NEC in 1911 and still continues to sponsor the code.

Various committees, which include representatives from many industry sectors, review and update the code every three years. They play an active role in providing technical input to incorporate widely accepted standards for electrical construction. Although the NEC requirements are advisory only, they are referenced in many local building codes and used as industry standards by many building inspectors and contractors.

Optical-fiber communications cables have been documented in the code for several years. The increasing use of optical-fiber cables in riser (floor-to-floor) and horizontal (including fiber-to-the-desktop) building applications has identified a growing need to develop standards for installation and safety considerations.