Understanding cable trays

It is increasingly common to see a large amount of data and communication cabling with bandwidth and throughput demands squeezed into a limited area. All of this may require robust air conditioning to control the I²R temperature rise, not to mention clean electric heat and ventilation where precise temperature control is a factor.

The cable tray is a wonderfully efficient tool used to manage all these wires. With a grounded metal barrier along the centerline to separate power wiring and data/communication cabling, a single, large cable tray installation is capable of routing a large amount of wiring. It heads off the possibility of a disorganized mass of conductors that are difficult to trace when changes must be made or faults located.

A point to note is that there is a difference between cable trays and raceways. The NEC says a cable tray system is a unit or assembly of units or sections and associated fittings forming a structural system used to securely fasten or support cables and raceways. That said, raceways generally mount on a wall in a room where power and communications cables are terminated. Cable trays are usually overhead or under raised floors but carry open wiring. They are used to distribute cables from the source to the point of use.

If a facility resides in a single room, it is common practice to install the cable tray so it follows the perimeter of the workspace. (Right angle fittings are readily available.) It is generally mounted high enough so workers can pass underneath without stooping but not so high that it is close enough to the ceiling to limit access. This presupposes a sufficiently high-ceiling construction in the first place.

As the cable tray follows the perimeter of the room, power and data conductors can enter and exit at various locations, thence traveling in the raceway to the points of usage. For raceways terminating at a tray, a listed cable tray clamp or adapter fastens the raceway to the cable tray system.

Cable trays are available in a wide variety of sizes, styles (including ladder, ventilated trough, ventilated channel, solid bottom and similar structures) and materials, metallic or non-metallic. If made of conductive material, the cable tray is one of the permitted types of equipment grounding conductors. Where sections bolt together, paint is supposed to be scraped off and jumper wires installed to ensure ground continuity.

Ladder trays generally get used where there are larger bundles or heavier cables. The ladder cable tray has two side rails connected by cross members, or rungs. The rungs provide convenient anchors for tying down the cables. A trough cable tray is a prefabricated structure consisting of a ventilated bottom with side rails. The ventilated trough cable tray supports cables better than the ladder type, but the additional support is not significant.

Fiber-optic cable installations frequently go with solid-bottom cable trays because the drooping of fiber-optic cables may degrade system performance. However, the main argument for selecting solid-bottom trays is to reduce electromagnetic/radio-frequency interference or for sensitive circuitry.

When the equipment room serves both computer and telecommunications equipment, under-floor cable-tray systems frequently get installed. Single unjacketed conductors and splices are permitted in cable trays. And the NEC says cable tray is permitted for service conductors, feeders, branch circuits, communications circuits, control circuits and signaling circuits. These requirements as well as Uses Permitted and Uses Not Permitted as well as other specifications and construction and installation details are included in NEC Article 392.