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Cable trays can be a very convenient method of transporting large numbers of signaling, communications and control cables, as well as other cables, from one location in a facility to another. If cable trays are to be used, however, some specific requirements must be followed.
Cable tray, as defined in Section 392.2 of the 2002 National Electrical Code (NEC), is a unit or assembly of units (commonly called sections) and the associated fittings that form a structural system used to securely fasten or support cables and raceways.
Section 392.3 covers uses of cable trays and permits cable trays to be used as a support system for service, feeder and branch circuit conductors. Cable trays can also support communications, control and signaling circuit conductors.
The article covering cable trays and their installation requirements has been moved from Article 318 in the 1999 NEC to new Article 392 in the 2002 NEC. Cable trays are not limited to industrial facilities and can be installed in commercial and residential facilities.
Most control or signaling cables are multiconductor cables containing small conductors, such as No. 18 AWG, 16 AWG, 14 AWG, 12 AWG, etc. Unlike power conductors, these low-voltage or signal cables usually carry small or limited amounts of electrical current; therefore, any spacing between conductors in the cable is not critical. Installing control or signaling cables in a single row in the bottom of the cable tray to allow heat dissipation, as is done for power cables, is usually not necessary.
The critical part of installing low-voltage or signal cables in a cable tray is the calculation for sizing the cable tray without exceeding its maximum allowable cross-sectional area fill. Although other applications are covered in Section 392.9, such as combinations power and control and/or signaling cables, only two sections, Section 392.9(B) for ladder and ventilated trough cable trays and Section 392.9(D) for solid- bottom cable trays, will be discussed here.
Section 392.9(B) covers only multiconductor control and/or signal cables in a ladder or ventilated trough cable tray. The sum of the cross-sectional areas of the cables in a ladder or ventilated trough cable tray having a usable inside depth of 6 inches or less must not exceed 50 percent of the internal cross-sectional area of the cable tray. A depth of 6 inches must be used to compute the allowable interior cross-sectional area of any ladder or ventilated trough cable tray that has a usable inside depth of more than 6 inches. A 6-inch-deep by 12-inch-wide tray would have an internal cross-sectional area of 72 square inches and the cable fill could not exceed 50 percent or 36 square inches. A 7-inch-deep by 12-inch-wide cable tray would also have a maximum tray fill of 36 square inches.
Section 392.9(D) covers multiconductor control or signal cables in a solid-bottom cable tray. With a solid-bottom tray, the sum of the cross-sectional areas of the cables in the cable tray, with a usable inside depth of 6 inches or less, must not exceed 40 percent of the internal cross-sectional area of the cable tray. A depth of 6 inches must be used to compute the allowable interior cross-sectional area of any solid-bottom cable tray that has a usable inside depth of more than 6 inches. A 6-inch-deep by 12-inch-wide tray would have an internal cross-sectional area of 72 square inches and the cable fill could not exceed 40 percent or 28.8 square inches. A 7-inch deep by 12-inch-wide cable tray would also have a maximum tray fill of 28.8 square inches.
For example, let’s assume that there are 100 multiconductor control cables with 30 No. 14 gauge PVC-insulated conductors in each cable installed in a solid-bottom cable tray. The cable manufacturer provides the nominal outside diameter of each cable as 1.035 inches.
The diameter must be converted to a square-inch area. The formula for conversion is the area of a circle (square inches) is
A 6-inch-deep cable tray x 36-inch wide = 216 square inches x 40 percent = 86.4 square inches will provide the necessary-size tray to contain the 100 cables. If a ladder or ventilated trough cable tray was used, then the permitted fill increases to 50 percent and a 6-inch by 30-inch-wide tray could be used.
Once the basic fill calculation has been accomplished, there are many other factors that must be considered, such as corrosion protection, basic construction material for the cable tray, expansion and contraction of the tray, grounding and bonding and the load capacity of the tray. Read Article 392 and apply all of the pertinent sections to your particular installation. EC
ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or via e-mail at [email protected].