One of my customers is an industrial plant that was informed it needed to upgrade its cable tray systems and bring them up to the minimum codes and standards. They called me with questions pertaining to cable tray installations. Since these questions are common, I decided to address them in the magazine in the order they were asked.
Is a cable tray considered a raceway, and if not, what section of the Code describes or defines cable tray?
Section 392.2 of the 2008 National Electrical Code (NEC) defines a cable tray as a unit or assembly or sections and associated fittings that form a structural system used to securely fasten or support cables and raceways.
The fine print note (FPN) for Section 300.17 contains a list of wiring methods that is considered part of raceways and these, of course, must comply with more than one section of the Code. A raceway is defined as an enclosed channel of metal or nonmetallic materials designed expressly for holding wires, cables or busbars, with additional functions as outlined in the NEC.
These definitions make it clear that cable trays are meant to be used as a supporting means, while raceways are designed so conductors or cables (where permitted) can be pulled through and enclosed so as to protect from physical damage.
Are Class 1, 2 or 3 circuits permitted to be mixed with power conductors when installed in a cable tray or pulled though raceway?
Without the use of special cables, the general rule does not permit Class 2 or 3 circuits to be installed in the same raceway or cable tray system with power conductors per 725.136(A). When the installation calls for these circuits to be mixed with other conductors, Sections 725.136(B) through (I) and 725.139 through 725.141 must be reviewed and verified to ensure the type of conductors mixed is in compliance.
Class 1 circuits and power circuits are permitted to be mixed together when they are electrically associated, functional and are supplying circuitry to a motor system. Sections 300.3(C)(1) and 725.48(B)(1) address this type of installation in raceways or cable trays.
Does the NEC allow an elevation or direction change without installing a cable tray fitting to complete the transition?
Section 392.6(A) (2008) of the cable tray article permits, under certain conditions of use, to install mechanically discontinuous segments between cable tray runs. Therefore, existing cable tray systems meeting the requirements of 392.6(A) through (J) that have elevation or directional changes comply with the Code and can remain installed.
How should the grounding and bonding of the cable tray be accomplished when they are not connected together with fittings suitable for the purpose of connecting together discontinuous segments of cable tray systems?
Section 392.7(A) and (B) covers the requirements for grounding and bonding together cable tray systems. For example, if a metal cable tray has a minimum cross-sectional area of metal of 1.00 as shown in Table 392.7(B), the metal of the tray during a ground-fault condition has the capacity to trip open a 400-amp overcurrent--protective device. When dealing with discontinuous segments of cable trays, the requirements of 392.7 pertaining to cable tray grounding must be followed.
Note that the requirements for aluminum and nonmetallic cable trays are different than those for metal cable trays.
How should conductors and cables be derated when they are installed in cable tray?
Section 392.11(A)(1) governs the derating requirements for multiconductor cables. The section requires that derating be applied based on the number of current-carrying conductors within the cable and not on the number of multiconductor cables installed in the tray. Single conductors must be derated by the requirements outlined in 392.11(B). Each single conductor installed in cable tray that is considered current-carrying must be derated according to the requirements listed in 392.11(B)(1) and (2). For the allowable ampacities of combinations of multiconductor and single conductor cables, see NEC Section 392.11(C)(1) and (2).
Can cable trays be installed and used in other spaces used for environmental air?
Section 392.4 does not prohibit such use of cable trays where the installation and use are in accordance with 300.22(C) and (D) that deals with wiring methods and equipment installed in such space.
Can conductors and cables be stacked on top of each other in cable trays?
Not necessarily. Before installing conductors and cables on top of each other, the requirements in 392.9 and 392.10 must be reviewed and applied according to these sections.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.