Mark A. DeSorbo
Is conduit simply conduit, and duct just duct, whether it be for inside or outside plant? Two manufacturers say yes, but a chairperson for a National Electrical Manufacturers Association (NEMA--Rosslyn, VA) committee that deals with metallic conduit and duct says there is a difference.
"The shapes and dimensions are the same, and most [non-metallic] conduit and duct are made of polyvinyl chloride [pvc]," says David Kendall, manager of technical services for Carlon Telecom Systems (Cleveland, OH), a manufacturer of raceways used for cable television, fiber, copper, and coaxial-cable installations. "Some are made with polyethylene to be flame-retardant, and metal is often used inside and outside," he adds.
Echoing Kendall, Jim McGregor, vice president of engineering at Cantex Inc. (Mineral Wells, TX), says the pvc manufacturer does not distinguish between inside- and outside-plant products. "They are essentially made of the same material," he says, adding that the company manufactures pvc products used in electrical, communications, hardware, plumbing, irrigation, and sewer markets. "We don`t really make a distinction between outside or inside. In our case, it`s the same material."
But Pat Horton, nema`s chairperson for Section 5 RN Steel rigid Conduit and Electrical Metallic Tubing, says metal and pvc conduit and duct have separate and particular standards. Furthermore, she adds that the term "duct" is generally referred to as a product that is used for outside-plant projects. "I have not heard it called ducts for interior use," says Horton, who also is the director of industry affairs for Allied Tube & Conduit Corp. (Harvey, IL). "When you use it inside, it`s called conduit or tubing. When you`re outside, you have duct banks." Article 310-15d of the 1996 National Electrical Code (NEC) defines electrical duct as, "Electrical ducts as used in Article 310 shall include any of the electrical conduits recognized in Chapter 3 as suitable for use underground, and other raceways round in cross section, listed for underground use, embedded in earth or concrete."
Some underground duct provides only a route for cable and offers little or no protection; conduit provides a route and physical protection. As far as pvc goes, Horton says the lightweight material is best-suited for outside routing rather than physical protection. "Some outside-plant duct products do not have an Underwriters Laboratories (UL) listing for indoor use because they are too flammable for indoor use," she adds.
An example is the UL standard on rigid nonmetallic schedule 40 and schedule 80 pvc conduit. According to the standard, rigid nonmetallic conduit is listed in sizes from a 1/2 inch to 6 inches. It is available in straight, elbow, and bend shapes and is "suitable for use in aboveground and underground and for direct burial without encasement in concrete," according to the UL Listing Directory. The listing also notes that schedule 40 conduit is suitable for exposure work where it will not be prone to physical damage, while schedule 80, a heavier conduit, is suited for exposure work.
Unless it is marked for higher temperatures, nonmetallic conduit is intended for use with wires rated at 75oC or less, "including where it is encased in concrete within buildings and where ambient temperature is 50oC or less." nema tubular steel conduit advertisements highlight how the product neither burns nor contributes to smoke volume or to the spread of flames.
"Steel conduit is noncombustible," Horton says. "It provides the greatest physical protection, and it also provides a shield for electromagnetic fields, which is an aid to preventing electromagnetic interference. It also has no temperature limitations and no use limitations."
In addition to the UL standard, the NEC is stringent about the use of rigid nonmetallic conduit. Section 347-3 indicates that it cannot be used:
- in classified hazardous locations;
- to support fixtures or other equipment, but it can support bodies of nonmetallic conduit that contain no devices or support fixtures;
- where it will be subject to physical damage, unless identified for that use;
- where it is subject to exceeding ambient temperatures;
- for conductors whose insulation temperature limitations exceed those of the conduit`s;
- in theaters and similar locations. Exceptions are spelled out in Sections 518 and 520 of the NEC.