Many electrical professionals believe that cable trays are raceways. Based on the definition, this couldn’t be further from the truth. 

Article 100 of the National Electrical Code defines raceway as “an enclosed channel designed expressly for holding wires, cables, or busbars, with additional functions as permitted in the NEC.” Cable trays are mechanical support systems for wiring methods, as covered in Article 392, and are not considered raceways. They are typically installed as an industrial-type wiring method but not limited to industrial uses. Cable trays may be installed in commercial facilities as a support system for other wiring methods.

Cable trays must be installed as a complete system. The electrical continuity of the cable tray system and support for the cables must be maintained after field bends or modifications. Cable tray systems are permitted to have mechanically discontinuous segments between cable tray runs or between cable tray runs and equipment.

Each cable tray system must be complete before cables are installed and must be exposed and accessible, except where extended transversely through partitions and walls, or vertically through platforms and floors in wet or dry locations. Where installed through walls or floors, cable trays and cables must comply with 300.21, so as not to increase the possible spread of fire or products of combustion. Openings around penetrations into or through fire-­resistant-rated walls, partitions, floors or ceilings must be firestopped using approved methods to maintain the fire resistance rating.

Understanding the support system

Though cable trays are common, many people do not totally understand the basics of these support systems. Section 392.10 provides permission for cable trays to be used “as a support system for wiring methods containing service conductors, feeders, branch circuits, communications circuits, control circuits, and signaling circuits.”

Table 392.10(A) provides a list of all of the wiring methods permitted to be installed in a cable tray system, and, with the exception of CATV cables, Class 2 and Class 3 cables, fire alarm cables, instrumentation tray cable and communication cables, all other wiring methods are raceways and metal cables of some kind. These raceways and metal cables must be installed as described in each respective raceway article and section of NEC Chapter 3.

Single insulated cables and single insulated conductors are also permitted in a cable tray only when installed in industrial establishments in accordance with 392.10(B)(1). The definition of an industrial occupancy is located in 6.1.12.1 of NFPA 101, Life Safety Code. An industrial occupancy is “an occupancy in which products are manufactured or in which processing, assembling, mixing, packaging, finishing, decorating, or repair operations are conducted.” Industrial occupancies may expose occupants to a wide range of processes and materials of varying hazards. Special-purpose industrial occupancies—characterized by large installations of equipment that dominate the space—are addressed separately from general-purpose industrial facilities, which have higher densities of human occupancy.

Industrial wiring methods

The wiring methods in Table 392.10(A) are permitted to be used in any industrial establishment under conditions described in their respective articles. Based on the definition of an industrial occupancy, 392.10(B)(1) states that “in industrial establishments only, where conditions of maintenance and supervision ensure that only qualified persons service the installed cable tray system, any of the cables in 392.10(B)(1) and (B)(2) must be permitted to be installed in ladder, ventilated trough, solid bottom, or ventilated channel cable trays.” Section 392.10(B)(1) permits single-­conductor cables and single insulated conductors to be installed in accordance with 392.10(B)(1)(a) through (B)(1)(c).

Permissive text

The permissive text in 392.10(B)(1)(a) reads: “Single-conductor cables and single insulated conductors must be 1/0 AWG or larger and of a type listed and marked on the surface for use in cable trays. Where 1/0 AWG through 4/0 AWG single-conductor cables and single insulated conductors are installed in ladder cable tray, the maximum allowable rung spacing for the ladder cable tray must be 9 inches.”

The permissive text in 392.10(B)(1)(b) applies to welding cables in compliance with Article 630, Part IV, and the text in 392.10(B)(1)(c) requires single conductors used as equipment grounding conductors to be insulated, covered or bare, and be 4 AWG or larger. Where installing welding cable in a cable tray, 630.42 requires that cable trays, used to support welding cables, be dedicated for welding cable installations only.

Section 392.10(B)(2) requires single- and multiconductor medium-voltage cables, installed in a cable tray, to be Type MV cable. Any single-conductor medium-voltage cables must be installed in accordance with 392.10(B)(1), as noted above.

I have seen a few cable tray installations in commercial installations that do not comply with 392.10(A) and (B). Next month, I will cover ampacity of cables in cable trays and termination to electrical equipment.

colleen beaty