Dangers of Overloading Cable Trays
By Safety Management Group
Today’s rapidly growing array of technologies has given companies and individuals access to a tremendous array of powerful tools and information sources. As the technologies have expanded, the need to for adequate infrastructure has grown, too.
Faced with a wide array of cabling and wiring, and recognizing the future technologies will probably require even more cables and wires, building designers have increased their use of cable trays to organize and route those many lines. Cable trays also provide additional protection from physical damage.
Different applications and preferences have given rise to a variety of cable tray types and configurations, from solid-bottom trays, to ventilated troughs, and wire mesh designs. The majority of the trays are manufactured from aluminum, taking advantage of that material’s flexibility and comparatively low cost.
Because cable trays tend to be sturdy, installers and other construction crews give little thought to their capacity. A popular belief seems to suggest that if additional wires or cable will physically fit into a tray, that tray can accommodate them.
In reality, cable trays have something in common with ladders, wheelbarrows, and pickup trucks: if they’re loaded beyond the limits of their capacity, they’ll fail. Besides the potential physical damage to occupants and contents of the space below the cable trays, and beyond the disruption to data and communication that can result, when cable trays carry wires that are delivering electric power, there’s a potential for short circuits and fire, or even arc flash.
Additionally, because many types of wire and cables may emit heat when in use, overloading a cable tray may suppress much-needed airflow, leading to disruptions and premature system failures as insulation breaks down from the excess heat. (And keep in mind that some coatings or insulation could provide toxic fumes if they burn.)
Electrical codes specify the appropriate fill for different types of cable trays and applications. The voltage rating of the cables, the ampacity requirements, and whether the tray itself is ventilated help to determine the parameters. In most cases, cable trays should not be filled to more than 40 or 50 percent of the tray’s physical capacity or weight. In addition, cable trays must be properly grounded and tested before any wire or cables they contain can be energized.
Codes also provide specifics as to the types of wires, cables, and wiring methods that can be used safely in cable trays. In addition to materials, the choices may depend upon who will handle the installation and any maintenance. Some conductive materials may call for qualified persons to perform all work. Certain materials, such as flexible cords, may be inappropriate because their insulation is more likely to become brittle over time, increase the danger of fires and electrical shorts.
It’s generally easy to identify a cable tray that has been loaded beyond safe limits. Obviously, a tray that is showing damage from being overstuffed presents a hazard. But given that that 40 to 50 percent of capacity figure mentioned earlier, any tray that is completely filled or that has wires and cables stacked up beyond its height is clearly overloaded. Steps should be taken to bring the tray back to a safe capacity. Often, some of the cables may have been abandoned when systems were upgraded. Removing the abandoned cables many bring the trays back into compliance with the code.
Finally, when sizing trays, remember that 40 to 50 percent of capacity limit, and consider the fact that future expansion of the facility’s technology may require still more wiring. It may be prudent to install trays that will be only 25 percent filled upon completion. That will leave plenty of space to ensure that future additions of wires or cables do not create safety hazards for the structure or its occupants.