Galvanized rigid steel and aluminum are the traditional materials for electrical conduit, but since their introduction in the 1960s, plastic and competitive pipe have been rapidly gaining market share. In fact, according to 2008 industry research by Cleveland-based Freedonia Group, U.S. demand for plastic and competitive pipe will reach 11.1 billion ft in 2013. Plastic pipe will grow at the fastest pace, states the report, “Plastic & Competitive Pipe to 2013,” with polyvinyl chloride (PVC) remaining the dominant resin (click here to see Fig. 1), followed by high-density polyethylene or HDPE (Resistance to Change below), acrylonitrile butadiene styrene (ABS), and reinforced thermoset. About 50% of total PVC use in the United States and Canada can be ascribed to demand for plastic pipe.
Almost two-thirds of global demand for PVC comes from the building and construction industry, notes Rapra Technology's market report, “PVC World Markets and Prospects,” authored by Professor Geoff Pritchard, technical consultant, Hackwell Group, Tunbridge Wells, England. This is mostly for rigid nonmetallic conduit (RNC) — flexible PVC only accounts for 30% of all PVC sold worldwide. The construction industry sectors with the largest demand for rigid PVC pipe, tubing, and fittings include potable water and wastewater, agricultural irrigation and drainage, and electrical and tel/data conduit and ducting (click here to see Fig. 2).
Notes from underground
In electrical installations, the properties of PVC pipe make it an excellent replacement for metal pipe in underground installations in concrete and anywhere else (excluding areas of extreme temperatures) there may be corrosive or wet conditions for which the material is specifically approved by the authority having jurisdiction (AHJ). “It's best for burial applications,” says Greg Winchester, program manager in the Technical Services Department of the National Electrical Manufacturers Association (NEMA), Rosslyn, Va. “With plastic conduit, you don't have any kind of corrosion issues so you can put it underground. You don't have to worry about the water seepage rusting things out. To me, that is what separates the plastic from the metal conduit.”
According to Art. 352 of the NEC (see 2008 NEC Outlaws Some Nonmetallic Materials below), PVC conduit can be concealed within walls, floors, or ceilings; directly buried; or embedded in concrete in buildings of any height. “Rigid PVC conduit … provides many of the advantages of rigid metal conduit, while allowing installation in areas that are wet or corrosive,” says Mike Holt, NEC expert, proprietor of MikeHolt.com, and Code consultant to EC&M. “This type of conduit is commonly used as an underground raceway because of its low cost, ease of installation, and resistance to corrosion and decay.”
For instance, at Fort Irwin, Calif., Bergelectric Corp., Los Angeles, replaced rigid steel pipe with PVC pipe embedded in concrete, eliminating any exposed conduit and reducing future maintenance.
“It's an industry standard for underground,” says Douglas Vice, foreman at Butterfield Electric, Woodland, Calif., who is currently working on a project that contains a whopping 55% PVC conduit (typical projects contain only 15% PVC). Because of the amount of utilities involved, the firm has installed approximately 4 mi of 4-in. PVC for a 2,400kV power primary, as well as around 2 mi of PVC piping for telecommunications, and a single duct for fiber optics, all of which will be contained in a concrete vault. Within the building, 126 call locations — each with a receptacle — will be connected with ¾-in. PVC enclosed within concrete. “We're saving a lot of money and a lot of time just by using PVC,” Vice explains. “We use it for pretty much anything underground that has to travel any kind of distance or anything that's concealed or in concrete. You're not going to want to leave it out exposed to the sun.”
Indecent exposure
Despite proponents' claims of resistance to direct sunlight, heat, and all weather conditions, some electrical contractors argue that PVC pipe contracts when exposed to the sun or extreme temperatures. In addition, in extreme cold, PVC conduit can become brittle and is more susceptible to physical damage. “It gets brittle and comes apart,” says Vice.
Therefore, some contractors prefer not to use PVC for applications in commercial cold storage areas and outside in certain climates. “I've seen PVC conduit and plastic fixtures used in these environments, and they just don't withstand the extreme colds,” says Steven Glorieux, CPMM, facilities manager for the University of Virginia Health Services Foundation, Charlottesville, Va., “They're easy to break. You try to open up a fixture or get into a PVC junction box, and it ends up snapping on you. So you end up having to either find another cover or replace the whole thing. Sometimes you just walk away and get back to it later.”
Article 352 of the Code identifies PVC Schedule 80 conduit for use in exposed areas subject to physical damage and also identifies PVC conduit Schedule 40 for use in exposed areas not subject to physical damage where the raceway is identified for the application. Yet, some electrical installers resist those applications. “You need to install to the Code and how the Code is written,” says Glorieux. “But even Schedule 80 isn't all that strong and is open to abuse. Rigid metal conduit and regular electrical metallic tubing (EMT) will withstand a lot more physical abuse than PVC. The thought that metal conduit is always there as your last defense for ground faults is nice as well.”
Although John Schmuck, electrical reliability engineer for Lyondellbasell Industries, Corpus Christi, Texas, is not an electrical contractor, his professional background has made him aware of the hazards associated with improperly exposed PVC conduit. “Here in South Texas, PVC that is exposed to the sun gets brittle, and it is not uncommon to see broken PVC,” he says. “I see it most often on stub-ups for signs or lighting where it is subject to being hit by lawn mowers or vehicles. The broken conduit exposes the conductors and presents an electrocution hazard, so I don't believe it is a suitable material for this service.”
Schmuck has witnessed similar situations in boat docks as well. “I've seen situations at the City Marina where they try to combat corrosion from the saltwater with PVC, but it gets brittle and breaks, and then it starts falling down and the conduit can get broken. I just feel like they should use a metal conduit to stub-up above-grade, because it gives you a much stronger, more substantial, longer lasting installation.”
In place of PVC, Schmuck recommends aluminum conduit. “Of course, it's substantially more expensive. I understand the reason people use PVC — it's cheap. I just feel like hazards are being created by using PVC in those places.”
Fast and cheap
The perception of plastic pipe is that it saves money on materials. “PVC is so much more affordable,” says Vice. But recent reports, highlighted in “The Economics of Phasing Out PVC,” published by the Global Development and Environment Institute at Tufts University, Somerville, Mass., say this isn't necessarily true.
According to the reports, life-cycle costs often favor alternatives. A strict comparison of purchase prices, or even installed costs, of PVC and alternatives may give a misleading impression that PVC is more affordable. “Some of the alternatives have higher initial purchase prices than PVC products, but are actually less expensive over the useful life of the product,” reads the study.
Cost may not be the only determining factor in pipe selection. Material preference and installation methods also play a large part. Consumers select products that contain PVC even when lower cost substitutes are available, according to the Washington, D.C.-based building materials watchdog group Healthy Building Network's “PVC-Free Pipe Purchasers' Report.” For instance, contractors who have never used HDPE may be resistant to working with an unfamiliar material. Yet, trenchless installations could dramatically reduce both cost and environmental impact of some underground pipe projects.
PVC is cheaper simply because it's easier to put together, maintains Glorieux. However, he warns that the number of contractors who do not have a clue how to install it properly should be enough for it to carry a warning label. “There is definitely a use for it, but we all need to be better educated on how to install it and where to best use it,” he says.
Vice agrees. “It's fast, and it's cheap,” he says. “With metal, you have either a set screw mechanical-type connector or a compression-type mechanical connector. Either way, somebody's breaking out tools, setting it all together, and setting it all in place.”
Vice also notes that the equipment costs more if you have it directly buried in concrete.
Article 352 of the NEC provides some guidelines for installation. Section 352.28 advises that the cut ends of PVC conduit must be trimmed (inside and out) to remove burrs and rough edges. “Follow the manufacturer's instructions for the raceway, fittings, and glue,” advises Holt. “Some glue requires the raceway surface to be cleaned with a solvent before application. After applying glue to both surfaces, a quarter turn of the fitting is required.” Holt adds, trimming PVC conduit is easy; most of the burrs rub off with fingers, and a knife will smooth the rough edges.
Time and a place
Nevertheless, PVC installation can be complicated, particularly with support and joints to allow for expansion. “First, a lot of people don't know all the ins and outs and what you're supposed to be doing with it, such as putting the expansion joints in,” complains Glorieux, who, in his experience with electrical maintenance, has had to fix unattractive and improperly installed PVC conduit. “There can be a problem with it simply because someone didn't get all the straps in perfect alignment. You need to allow the conduit to slide, and then, on top of that, they now have different types of the straps that are made for that sliding purpose as well.”
According to Glorieux, without proper expansion joints, the PVC starts to create a wave. “It just looks really ugly, so for aesthetic reasons I have to remove the PVC and put EMT up when applicable,” he explains. “I've never had an instance where the wires have pulled apart, but when they start expanding, that is a possibility. I'm sure there must be pressures put on the wiring. You're supposed to leave a little extra wire, but not everybody always does.”
In his work with wastewater plants, Glorieux has experienced the benefits of using PVC when he's pulled out corroded rigid metal conduit and replaced it with PVC. “The metal just couldn't withstand the amount of the gases that are given off,” he explains. “They corrode the metal, so having the PVC in there just took care of the problem. So I see a lot of advantages of using it in those highly corrosive areas.”
Sidebar: Resistance to Change
Due to health and environmental concerns over the manufacturing, fire rating, and disposal of PVC, some government institutions, health care organizations, and design firms are opting to use PVC-free alternatives, such as high-density polyethylene (HDPE), according to Washington, D.C.-based building materials watchdog group Healthy Building Network's “PVC-Free Pipe Purchasers' Report.” Although PVC pipe maintains its top position for market share within the worldwide nonmetallic conduit arena, the report continues, HDPE has experienced the most growth in the conduit sector over the last 10 years. Still, by 2006, sales of PE pipe reached only about 1-billion lb annually, compared to 6.5-billion lb of PVC pipe, according to “The Economics of Phasing Out PVC,” published by the Global Development and Environment Institute at Tufts University, Somerville, Mass.
Familiarity with PVC seems to be the greatest obstacle hindering the widespread use of HDPE. Although HDPE has a longer history of use in pipe than PVC, its initial markets were limited to industrial settings, such as chemical companies and mining, while PVC made major inroads in municipal design, according to a recent report from Quebec-based Environment Canada, the Canadian government's conservation department. “As a result, municipal design engineers and contractors are more familiar working with PVC pipe, and seldom specify or design HDPE systems,” reads the report.
Sidebar: 2008 NEC Outlaws Some Nonmetallic Materials
In the 2008 National Electrical Code (NEC) revision cycle, Art. 352, formerly titled “Rigid Nonmetallic Conduit” (RNC), was changed to address only rigid polyvinyl chloride (PVC) conduit, while high-density polyethylene (HDPE) remained in Art. 353, and the fiberglass type, also known as reinforced thermosetting resin conduit (RTRC), was awarded its own article, Art. 355. These changes marked a significant milepost in the Code in that other types of nonmetallic conduit are no longer deemed Code compliant, according to New York-based McGraw-Hill's “National Electrical Code 2008 Handbook.” Materials effectively banned from use by the 2008 NEC (through application of Section 110.8) include styrene, fiber, tile, asbestos cement, soapstone, and others that once were used for underground applications.
Traditionally, RTRC was permitted only for underground applications. Recent advances in its fire resistance now allow it to be installed in above-grade installations where concealed in walls, floors, and ceilings and also in exposed areas, if specified for the application. The new article governing its use, Art. 355, is similar to Art. 352 with two exceptions: no mention of foam-core product and its own thermal expansion table, because it has a 45% lower coefficient of thermal expansion than PVC.
