Ladder Cable Tray
Ladder cable tray is used for about 75 percent of the cable tray wiring system installations.
It is the predominate cable tray type due to its many desirable features:
- A ladder cable tray without covers permits the maximum free flow of air across the cables.
This allows the heat produced in the cable’s conductors to effectively dissipate.
Under such conditions, the conductor insulation in the cables of a properly designed cable tray wiring system will not exceed its maximum operating temperature.
The cables will not prematurely age due to excessive operating temperatures.
- The rungs of the ladder cable trays provide convenient anchors for tying down the cables in the non-horizontal cable tray runs or where the positions of the cables must be maintained in the horizontal cable tray runs.
This capability is a must for single conductor cable installations.
Under fault conditions (short circuit), the magnetic forces produced by the fault current will force the single conductor cables from the cable tray if they are not securely anchored to the cable tray.
- Cables may exit or enter the ladder cable trays through the top or the bottom of the cable tray. Where the cables enter or exit conduit, the conduit to cable tray clamps may be installed upright or inverted to terminate conduits on the top or bottom of the cable tray side rail.
- Moisture can’t accumulate in ladder cable trays.
- If cable trays are being installed where working space is a problem, hand access through the cable tray bottom may help to facilitate the installation of small diameter cables: control instrumentation, signal, etc.
- The most common rung spacings for ladder cable tray is 9 inches(=228.6mm). This spacing may be used to support all sizes of cables.
This spacing is desirable for the small diameter Type PLTC and TC cables as the support distance is such that there is no visible drooping of the small cables between rungs.
12 or 18 inch(=304.8mm or 457.2mm) rung spacing provides adequate cable support but the slight amount of small diameter cable drooping between rungs may be aesthetically objectionable for some installations.
The maximum allowable distance between supports for 1/0 through 4/0 AWG single conductor cables is 9 inches (=228.6mm) [1993 NEC Section 318-3(b) (1)].