A Quick Look at Cable Tray
Cable Trays Support all Types of Wiring
Industrial and commercial constructions look for quick, flexible, and economical solutions for their cabling requirements. Cable trays, capable of supporting various types of wiring provide an adequate solution. They carry optical cables, telecommunication wiring, sensitive control wiring, power distribution cables, and high voltage power lines with equal ease.
Guidelines and Standards
Cable tray systems mostly follow Articles 250, 318, and 800 of the National Electrical Code publications, which define the standards for all types of electrical applications. In addition, major cable tray manufacturers also follow industrial documents such as VE1, FG1, and VE2 from the National Electrical Manufacturers Association (NEMA). The NEMA website offers free downloads for these documents.
While the NEMA VE2 offers guidelines for cable tray installation covering general installation practices, receiving, uploading, and storage of trays, NEMA VE1 covers the definition of cable trays in general. NEMA VE1 also provides manufacturing standards for cable trays, including performance standards, test standards, and application information. NEMA FG1 handles the standards that cable tray systems made of fiberglass must meet.
In addition, major cable tray manufacturers of the industry have formed a trade association, the Cable Tray Institute (CTI). They educate installers, designers, and specifiers on the advantages of using cable tray systems over using other types of products such as ladder racks, conduits, and similar.
The 2014 National Electrical Code lays down the requirements for cable tray systems including provision of design solutions from practical experience.
An Example of Ladder-Type Tray
Functioning of Cable Trays
Cable trays provide a safe passage for transport of wires and cables through a building just as bridges allow traffic to cross open spans safely. Cable trays form a structural component of the electrical system in a building.
Types of Cable Trays
Cable trays are available in six basic types. These are the Single Rail, Wire Mesh, Channel, Trough, Solid Bottom, and Ladder Type of cable trays.
Low voltage and power cable installations requiring speed to install, side fill, and maximum cable freedom typically use the Single Rail Cable Tray system. These offer the fastest installations, along with the maximum freedom for cables to enter and exit. Single rail cable systems can be wall mounted or single hung in multiple or single tiers.
Fiber optic and telecommunication applications operating at low voltages prefer to use the Wire Mesh Cable Tray system, as these can be installed on short support spans of 4 to 8 feet lengths. Wire mesh cable trays are typically made of zinc plated steel wire mesh.
Installations requiring branch cable runs and cable drops from the backbone cable tray system prefer the Channel Cable Tray Systems. Used where conduits are unsuitable, metallic and non-metallic channel cable trays carry a limited number of cables.
Trough Cable Tray Systems are typically used with moderate heat generating applications, as these systems offer short to intermediate support spans of 5 to 12 feet lengths. The bottom configuration in trough cable tray systems provide added cable support every 4 inches, while providing moderate ventilation.
With short to intermediate support spans of 5 to 12 feet lengths, Solid Bottom Cable Tray Systems are best suited for minimal heat generating telecommunication and electrical applications. Solid bottom cable trays with steel covers offer excellent EMI/RFI shielding protection for sensitive circuits. However, because of their closed nature, moisture can easily build up inside the tray. Although drilling drain holes does overcome this disadvantage, it is degrades the EMI/RFI shielding offered.
Applications that require intermediate to long support spans of 12 to 30 feet generally use the Ladder Cable Tray System. With several desirable features, the ladder cable trays are predominantly used for over 75% of wiring system installations. Used without covers, ladder cable trays permit free flow of air across the cables they carry—effectively dissipating heat produced in the cable conductors, thereby preventing premature aging of cable insulation.
Materials Used for Cable Trays
The nature of the application of cable trays requires them to be corrosion resistant and sturdy. Therefore, manufacturers fabricate most cable tray systems from aluminum alloy, stainless steel, or low-carbon steel, as these are metals highly resistant to corrosion. A corrosion-resistant finish of epoxy or zinc also helps. Electrical considerations, corrosion-resistance requirements, and costs are the major guiding factors that decide the choice of material for any specific installation.
Thermal expansion and contraction may affect cable tray systems made of metal, and designers must consider this before installation.
Use of composite materials, engineering plastics, and reinforced fiberglass for cable tray systems are also common. However, these are application specific, useful in highly corrosive environments.