Cable Tray Applications in R&D Laboratories

Article 318 of the National Electrical Code (NEC) governs the installation and use of cable trays for electrical power distribution. While the Code does not apply directly to cable trays containing only instrumentation cables (e.g. no power cables, including extension cords), it is a "best practice" to follow the same standards.

The NEC definition of a cable tray system (Article 318-2) is a unit or assembly of units or sections and associated fittings, forming a rigid structural system used to support cables. This includes ladders, troughs, channels, solid bottom trays, and other similar structures. The Code specifies that cable trays are not considered raceways. The NEC defines a raceway (Article 100-A) as an enclosed channel designed expressly for holding wires, cables, or busbars. Raceways may be metal or insulating material and includes all types of conduit, tubing, underfloor raceways, cellular concrete/metal floor raceways, surface raceways, wireways, and busways. All conductive cable tray systems and raceways must be grounded per NEC Article 250.

All cables in a tray system where power distribution cables are present must be insulted to the level of the highest voltage in the tray. Otherwise, a solid fixed barrier of a material compatible with the tray (e.g. aluminum to aluminum, steel to steel, fiberglass to fiberglass) must be provided to separate cables of different voltage levels. In addition, only cable tray-rated cables are allowed in a tray used for power distribution. Since there are few (if any) cable tray-rated instrumentation cables, laying power distribution cables and instrumentation cables in the same tray is not allowed unless either the power cables (including extension cords) or the instrumentation cables are placed in a raceway (e.g. conduit), which can be in the same tray. The use of a cable tray barrier between power cables and non-cable tray rated instrumentation cables is not allowed.

If electrical outlets are required in a conduit run associated with a cable tray, the conduit must be run on the outside of the tray. Tray manufacturers have verified that this is a standard, acceptable industry practice. CAUTION must be used in mounting to assure that the fastening devices (e.g. bolts if used) cannot damage cable pulled through the tray. In addition, extension cords and equipment cords may be run in a tray, but they must be continuous and be rated for cable tray use (Article 318-3). Outlets from extension cords must be suspended below the tray and be secured with a strain relief device, such as a Kellum grip, to reduce strain on cord and connectors. The following cable types shall be permitted in trays under the conditions described in their respective articles:

Temporary Utility and Raceway Attachment to Cable Trays

The NEC requires that electrical power distribution and non-electrical systems be physically separated. Article 300-8 states that raceways or cable trays containing electrical conductors (power distribution) shall not contain any pipes, tubes or equal for steam, water, air, gas, drainage, or any service other than electrical. Permanent installation of facility utilities to a tray support system (not the tray itself) is allowed if the support is a permanent part of the facility and the load rating of the support is not exceeded (see Figure 1). Water systems, properly installed, represent a reasonable risk if the power cables in the tray are approved for wet locations and cords are approved for damp locations. Attaching utilities directly to the bottom of the tray or tray support rods is not acceptable (see Figure 2).

TEMPORARY installation of utilities required to support a project, test, or experiment may be made to the cable tray system by mounting them to a standoff device (Unistrut or equivalent recommended) that provides a minimum of 1-5/8 inch clearance between the utilities and the bottom of the tray (see Figure 3) if the following requirements are met:

1. Liquid temperatures are not cryogenic or do not exceed the temperature limits of domestic hot water;



2. Ambient operating temperature rating of the cables will not be exceeded by the heat transfer from the other utilities;



3. The weight load, including cables, does not exceed the rating of the tray or its support system;



4. The cables are not exposed to accidental contamination of corrosive liquids or gases;



5. Adequate ventilation is maintained for the cables.