AN IN-DEPTH LOOK AT 2002 NEC®
ARTICLE 392 - CABLE TRAY
(The following code explanations are to be used with a copy of the 2002 NEC®.)
To obtain a copy of the NEC® contact: National Fire Protection Association® 1 Batterymarch Park • P.O. Box 9101 Quincy, Massachusetts
392.1. Scope.
Standard Aluminum Ladder
Of the types of cable trays listed in this section, ladder cable tray is the most widely used type of cable tray due to several very desirable features.
•The rungs provide a convenient anchor for tying down cables in vertical runs or where the positions of the cables must be maintained in horizontal runs.
•Cables may exit or enter through the top or the bottom of the tray.
•A ladder cable tray without covers provides for the maximum free flow of air, dissipating heat produced in current carrying conductors.
•Moisture cannot accumulate in ladder cable trays and be piped into electrical equipment as happens in conduit systems.
•Ladder cable tray cannot pipe hazardous or explosive gasses from one area to another as happens with conduit systems.
•In areas where there is the potential for dust to accumulate, ladder cable trays should be installed. The dust buildup in ladder cable trays will be less than the dust buildup in ventilated trough or solid bottom cable trays.
Ladder cable trays are available in widths of 6, 9, 12, 18, 24, 30, 36, and 42 inches with rung spacings of 6, 9, 12, or 18 inches. Wider rung spacings and wider cable tray widths decrease the
overall strength of the cable tray. Specifiers should be aware that some cable tray manufacturers do not account for this load reduction in their published cable tray load charts.
With one exception, the specifier selects the rung spacing that he or she feels is the most desirable for the installation. The exception is that 9 inches is the maximum allowable rung spacing for a ladder cable tray supporting any 1/0 through 4/0 single conductor cables [See Section 392.3(B)(1)(a)].
Where the ladder cable tray supports small diameter multiconductor control and instrumentation cables; 6, 9, or 12 inch rung spacings should be specified. Quality Type TC, Type PLTC, or Type ITC small diameter multiconductor control and instrumentation cables will not be damaged due to the cable tray rung spacing selected, but the installation may not appear neat if there is significant drooping of the cables between the rungs.
For ladder cable trays supporting large power cables, 9 inch or wider rung spacings should be selected. For many installations, the cable trays are routed over the top of a motor control center (MCC) or switchgear enclosure. Cables exit out the bottom of the cable trays and into the top of the MCC or switchgear enclosure. For these installations, the cable manufacturer's recommended minimum bending radii for the specific cables must not be violated. If the rung spacing is too close, it may be necessary to remove some rungs in order to maintain the proper cable bending radii. This construction site modification can usually be avoided by selecting a cable tray with 12 or 18 inch rung spacing.
If you are still uncertain as to which rung spacing to specify, 9 inch rung spacing is the most common and is used on 80% of the ladder cable tray sold.
Cable Tray Manual | Cooper |
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