Cable-tray rung spacing

Q: We are installing Enhanced Category 5 cable throughout a project. The electrical contractor ordered a tray 6 inches wide by 4 inches deep, with 9-inch rung spacing. Our university standard for cable tray calls for 6-inch rung spacing, but nobody seems to know why. The customer questions how the rung spacing might affect the cable.

Q: We are installing Enhanced Category 5 cable throughout a project. The electrical contractor ordered a tray 6 inches wide by 4 inches deep, with 9-inch rung spacing. Our university standard for cable tray calls for 6-inch rung spacing, but nobody seems to know why. The customer questions how the rung spacing might affect the cable.

Does rung spacing really make a difference? Will the cable sag over time? How about the crush factor with the weight of all the cable? I believe that the largest number of cables in any one segment of tray will only be about 60.

Jay Meeker

CCSO Plant Engineering

University of Illinois at Urbana-Champaign

Urbana, IL

A: According to The Cable Tray Institute, an association of companies involved in the development of cable-tray systems, 9 inches is the most common rung spacing for ladder cable tray, which may be used to support all sizes of cables. Your university standard of 6 inches was likely chosen to avoid any visible drooping of the 4-pair cables between rungs.

After reading ansi/tia/eia-569a, the Commercial Building Standard for Telecommunications Pathways and Spaces, published by the Telecommunications Industry Association and the Electronic Industries Alliance (tia/ eia--Arlington, VA), your customer may decide that the 6-inch spacing is overly cautious.

tia/eia-569a, Section 4.6.5.1, says that where cable tray is not available, telecommunications cables can be placed in open-top cable supports, located on 48- to 60-inch centers. We also are advised to install special supports to carry the additional weight where larger quantities of cables (50 to 75 cables) are bunched together.

For additional free cable-tray-design expertise, fax your questions to The Cable Tray Institute`s technical director, Richard Buschart, PE, at (314) 872-8686.

Even though cable tray is fabricated in numerous styles--ladder, ventilated trough, channel, and solid-bottom-- ladder cable tray is used for about 75% of the cable-tray wiring system installations. Why? Because cables may enter or exit through the top or the bottom of the cable tray; moisture will not accumulate; the rungs provide convenient anchors for tying down the cables in the sloping sections of cable-tray runs; and the space between the rungs provides hand access through the cable-tray bottom, which may be the only access available after the other building trades have completed their work.

Donna Ballast is a communications analyst at the University of Texas at Austin and a BICSI registered communications distribution designer (rcdd). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, the University of Texas, Austin, TX 78713; tel: (512) 471-0112, fax: (512) 471-8883, e-mail: ballast@utexas.edu.

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