Published In January 2001
One of the nicest benefits of a cable tray system is its flexibility. Since a cable tray system is an open one, you don’t have to de-rate cables because of airflow. (The open tray allows the heat to dissipate.) However, in a closed system, such as with power cables in conduit, you have to allow for heat buildup and therefore have to de-rate the cables. Yet many specifiers continue to specify covers for cable tray when they don’t need them. This “conduit syndrome” defeats a key advantage of cable tray—the easy access to the cables. Cables that go into cable tray have to be tray-rated. They have shell protection built in, and the cable jackets are treated for UV so there is no need for covering. Some specifications, however, do require that cables coming up from a floor vertically be covered for 6 feet to prevent people from exploring them. (According to Article 318 of the National Electrical Code (NEC), 6 feet is, in fact, the maximum length of cover that can be placed on the cable tray without de-rating the cables.) It also makes sense to cover cable trays that run under cross-walkways or other high-traffic areas to prevent people from dropping things into the trays and then climbing or reaching into the trays to recover dropped items. You should also consider cutting down, altogether, on the structural strength of the whole system of cable tray for a particular installation. Sometimes, manufacturers have noted, specifiers act like the published weight capacity of a tray (which already has a built-in safety factor of 1.5) is for the total load, not per foot. All that overspecifying costs excess money in materials and labor that could throw a bid off the table. If you specify correctly, cable tray is lighter and easier to install and more competitive in price, to boot. Many manufacturers’ catalogs provide the different load capacities of tray for different spans. (The relationship of span to load is exponential, a comparison of the lengths squared as opposed to the weight.) Very heavy trays are suitable for very long spans such as 20 to 30 feet, as might be found in the petrochemical industry. However, most installations don’t require the heft and might of the strongest tray available from a manufacturer that some specifiers almost automatically spec. For example, Jim Sargent, director of product management, Cooper B-Line, has seen specifications that call for 200, 240, or even 280-pounds-per-foot capacity on a 36-inch-wide tray. Yet, he points out, according to the Code, regardless of type of cables, “You can’t even get past 140 pounds per foot no matter how you load it on, even with the burliest, knarliest steel-jacketed power cables.” Care should be taken, however, to include environmental considerations such as wind, snow, and ice loads if the tray is to be installed outdoors. The first factor in specifying cable tray is to find out the number and size of the cables that will occupy the tray. Once you know how many and what kind of cables you are running, then you can determine the width and strength of the tray system. Remember to factor in how long the span can be, including consideration of a set of splice plates in a span. Some splice plates are not rated for the full load capacity of a particular tray. “You start with the cables because that is the purpose of the tray,” noted Richard Buschart, PE and technical director of the Cable Tray Institute. “Once you have determined the width of the tray and weight per foot, you can then select the proper class of tray and determine the necessary support distances. The support distances can also control the size of the tray.” Another factor is how the tray will be supported—from the ceiling, from the wall, or on a pipe rack. “You should always take the time to select the proper tray for the job,” Sargent said. “Choosing a tray that was designed and tested for use on 20-foot spans and then supporting it every 10 feet is like shooting yourself in the foot. You’re just throwing away money.” In some installations of cable tray, if you have the room, you may be able to specify individual trays to separate instrument and signal systems from power systems (to accommodate code requirements and/or to prevent possible signal interference). If space is tight, you can specify cable tray that uses dividers or barriers to separate the cables. Buschart said, “Sometimes you can’t mix these cables for code or noise reasons.” For more information, in detail, on noise and other topics, go to the Technical Assis-tance section at www.cabletrays.com. The FELDMANS provide Web content for companies and write for magazines, trade associations, building product manufacturers, and other companies on a broad range of topics. They can be reached at firstname.lastname@example.org or ( 914) 238-6272.