Wiring system design: Cable tray vs. conduit
To be useful, electrical wiring must get from one place to another. Distribution is a necessary phase of system wiring design in order to get power or impulse signals to their final destinations. Historically, wires and cables have been pulled through conduit.
To be useful, electrical wiring must get from one place to another. Distribution is a necessary phase of system wiring design in order to get power or impulse signals to their final destinations.
Historically, wires and cables have been pulled through conduit. Plant environments are characterized by patterns of plentiful parallel conduit runs (Fig. 1). Conduit continues to be the mainstay of electrical power distribution.
However, cable trays are making inroads into industrial plants. In Canada, about 85% of industrial plants use cable trays instead of conduit. That figure is somewhat smaller in the U.S., accounting for less than 50% of industrial plants using cable trays. But the trend in the U.S. is moving toward cable tray use. To date, the U.S. data and communication cable market has readily accepted wire cable tray, while power system engineers and contractors are yet to fully embrace this trend. Wire cable tray is still a relative newcomer to the electrical power segment of the market.
According to the National Electrical Code, a cable tray system is "a unit or assembly of units or sections and associated fittings forming a rigid structural system used to securely fasten or support cables and raceways."
Cable tray advantages include wiring system design flexibility, simplicity, and lower installation cost. In plants where equipment is added, taken away, or is moved, cable trays provide a flexible advantage (Fig. 2). Cable trays can typically adapt to complex configurations with a simple set of tools. The cost of material procurement for cable tray systems is not necessarily lower than that of conduit systems in all cases. However, compared to labor cost of conduit installation, cable trays present significant savings.
There are six basic cable tray types:
Ladder — provides solid side rail protection, system strength, smooth radius fittings, and a wide selection of materials and finishes. Ladder cable tray is generally used in applications with intermediate to long support spans
Solid bottom — provides nonventilated continuous support for delicate cables with added cable protection available in metallic and fiberglass. Also available are solid bottom metallic trays with solid metal covers for nonplenum-rated cable in environmental air areas. Solid Bottom cable tray is generally used for minimal heat-generating electrical or telecommunication applications with short to intermediate support spans.
Trough — provides moderate ventilation and added cable support frequency, with the bottom configuration providing cable support every 4 in. Available in metal and nonmetallic materials, trough cable tray is generally used for moderate heat generating applications with short to intermediate support spans.
Channel — provides an economical support for cable drops and branch cable runs from the backbone cable tray system. Channel cable tray is used for installations with limited numbers of tray cable when conduit is undesirable.
Wire mesh — provides job site or field-adaptable support systems primarily for low-voltage wiring. Wire mesh tray generally is used for telecommunication and fiber optic applications. Wire mesh tray systems are typically zinc plated steel wire mesh.
Single rail — provides the quickest system installation and the most freedom for cables to enter and exit the tray system. Typically, single-rail cable tray is used for low-voltage and power cable installations where maximum cable freedom, side fill, and installation speed are factors. These aluminum systems may be single-hung or wall-mounted systems in single or multiple tiers.
Competitive life-cycle costs
Physical protection of conductors
Proven equipment grounding conductor
Chemically compatible with concrete
Coefficient of expansion compatible with common building materials
High tensile strength.
Rigid metal conduit (RMC) has the thickest wall, making it the heaviest steel conduit. Inside and outside are zinc-coated to provide corrosion resistance. RMC can be used indoors, outdoors, underground, and in concealed or exposed applications
Intermediate metal conduit (IMC) has a thinner wall and weighs less than RMC. A zinc-based coating is used on the outside; an organic corrosion-resistant coating is used on the inside. IMC can be used for the same applications as galvanized rigid metal conduit
Electrical metallic tubing (EMT) is the lightest weight steel conduit manufactured. EMT is made of galvanized steel and is unthreaded. It is joined by setscrew, indentation, or compression-type connectors and couplings. This joining method makes EMT easy to alter, reuse, or redirect. Even though EMT is made of lighter-walled steel, it provides substantial physical protection and can be used in most exposed locations except where severe physical damage is possible.
RMC, IMC, and EMT are permitted as an equipment grounding conductor in accordance with NEC 250.118. A supplementary equipment grounding conductor sized in accordance with NEC 250.122 may be added as well. If a supplementary equipment grounding conductor is used, it is still important to comply with NEC 300.10 and 300.12, since approximately 90-95% of the ground current flows on the conduit and not in a supplementary conductor.
Environmental considerations for conduit
The coefficient of expansion for steel conduit/EMT is 6.5x10-6in./in./deg F. This is significant as it relates to whether or not expansion fittings would be required in a particular application. Expansion fittings are installed where significant temperature differentials are anticipated. These temperature shifts cause materials to expand and contract and could result in the conduit being pulled apart at the joints. Expansion fittings are not normally required with steel conduit/tubing because their coefficient of expansion is similar to that of other common building materials. However, when steel conduit is installed on bridges, rooftops, or as an outdoor raceway span between buildings, expansion fittings may be required. In these types of installations, there is a probability that expansion and contraction would occur, resulting from the direct heat of the sun coupled with significant temperature drops at night.
Couplings that accommodate thermal expansion while maintaining grounding and bonding integrity are now available. Such a coupling uses an internal bonding jumper to maintain electrical continuity (Fig. 3). An internal, keyed, sliding bushing allows conduit movement. Installation is simple, requiring no disassembly. These couplings are installed by sliding the fitting onto the moving conduit until it stops at the internal slide bushing, then tightening. The next step is to tighten the gland nut with a wrench to compress the packing, creating a weather-resistant seal around the moving conduit. The final step is to thread the next length of conduit (stationary) into the other end of the fitting.
PLANT ENGINEERING magazine extends its appreciation to Cablofil, Inc., Cable Tray Institute, Square D/Schneider Electric, Steel Tube Institute of North America, and Thomas & Betts Corp. for the use of their materials in the preparation of this article.
Cable tray selection checklist
When selecting cable trays, cable tray configurations, and support methods, seek the answers to the following questions:
Where will the cable trays be used?
Job site and installation considerations include:
Support locations available affect the length and strength of the system.
Industrial installations may require a 200 lb concentrated load.
Office installation may make system appearance, system weight, and space available important factors.
Environmental air handling areas may affect cable types, cable tray material, or cable tray type, as well as the potential need for covers.
Classified hazardous locations affect the acceptable cable types.
Available supports affect length and strength requirements.
Environmental requirements include loads, ice, wind, snow, and possibly seismic situations.
Corrosion requirements affect materials and finishes.
Classified hazardous locations affect acceptable cable types.
What types of cables will be supported, and how many?
NEC cable fill requirements dictate size, width, and depth of cable tray.
Cable support requirement may necessitate bottom type.
Largest bending radius of cable controls fitting radius.
Total cable weight determines load to support.
What are the future requirements of your system?
Cable entry/exit freedom may change.
Designing a partially full or an expandable system may produce big savings later.
Support type should allow for expansion needs.
Conduit installation tip
• Conduit having factory-cut threads are supplied with corrosion protection applied.
• Field cut threads are required to be coated "with an approved electrically conductive, corrosion-resistant compound where corrosion protection is necessary," according to NEC 2002 300.6 (A). Field-cut threads should be protected from corrosion if they will be installed in wet or outdoor locations. Protect the thread surface with conductive rust resistant coating such as zinc-rich paint. Other conductive coatings are appropriate as well.
• Field threads should be cut one thread short. This ensures a good connection and allows the entire thread surface to be inside the coupling.
There are two primary reasons to use steel conduit. According to the Steel Tube Institute of North America, steel conduit is the best possible protection of your electrical conductor and wiring systems, and it facilitates the insertion and extraction of conductors and wiring. Steel conduit is used in more than 50% of U.S. manufacturing and other industrial facilities in a variety of indoor, outdoor, and underground applications, including those where corrosive and hazardous conditions exist.
The three basic types of steel conduit and their applications are:
Cable tray configurations
Straight sections are available to route cables in a horizontal or vertical plane. Fittings route cables in various directions in either the horizontal or vertical planes. Typical fittings include elbows, tees, crosses, and risers. These fittings are available in various radii and bend angles.
Support methods include trapeze (single or multitier), hanger rod clamps, "J" hangers, center hung support, wall support, underfloor support, and pipe stanchions. Trapeze supports are recommended in applications where cables will be pulled through the cable tray. Center-hung supports typically are used when cables will be installed from the side of the cable tray. Also, center-hung supports are especially useful when future cable additions are necessary.
Wall and underfloor supports are useful when ceiling structure is not available or undesired. Outdoor installations are controlled by the structures available to support the cable tray.
The primary benefit of conduit systems is the ability to ground and bond. Grounding and bonding play a significant role in minimizing electromagnetic interference (EMI). Steel conduit reduces electromagnetic fields by up to 95%, effectively shielding computers and sensitive electronic equipment from the electromagnetic interference (EMI) caused by power distribution systems.
Benefits of conduit include:
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