Most high-speed cable system failures are caused by excessive bending, twisting, or pulling stress, or by improper termination procedures. For example, excessive force on the cable (due to stress during installation or an improper cable support device), can alter the pair geometry within the cable assembly, causing an increase in return loss, attenuation, or near-end crosstalk.

An installer who recently installed Category 5 cable using bridle rings and tie-wraps as the main means of support came upon a difficult scenario. Despite his care in installation, the cable did not work because the ring and tie-wraps allowed excessive bending and stress at specific points of the cable. After reworking the job using a recommended support device, the cabling system performed according to specs.

Code requirements

The National Electrical Code specifies that in a horizontal cable run from a telephone closet to a work area, only approved cable support methods should be used, such as J-hook fasteners, cable tray, and wireway. All of these provide the broad, continuous support required by high-performance cable to avoid bending stress or failure.

J-hook fasteners typically are the most widely used of the three methods because they offer speed and economy in installation and are easy to reconfigure, if necessary. These J-hook hangers are available in two sizes: The large J-hook can accommodate up to 75 four-pair UTP cables; while the small J-hook can accommodate up to 50 four-pair UTP cables. The J-hook can also hold fiberoptic cable or inner duct.

J-hooks recommended for use with Category 5 copper cables have a wide base loop with rounded smooth curves to spread the weight of the cable bundle and reduce the possibility of snag as cabling is pulled. A closure tie prevents cables from slipping. For main cabling routes, "tree" assemblies made of multiple J-hooks can be used.

Installation options

The easiest way to install cable supports is by connecting them with prefabricated clips on ceiling support wires or rods. However, due to recent NEC changes (Sec. 300-11), you may no longer be able to install cable supports on existing ceiling support wires, rods, or acoustical tees. So, you may have to install additional dedicated drop rods or drop wires. (Check with the Authority Having Jurisdiction [AHJ] or local inspector.) You can hang cable supports from existing beams and trusses, using prefabricated I-beam clamps, C- or Z-purlin clips, and other fasteners. For example, an I-beam clamp (100-lb load rating) is attached directly to the bottom cord of a web truss or a solid I-beam in the ceiling.

Generally, J-hook fasteners should be placed at 4-ft to 5-ft intervals so the cables are properly supported and lower cables are not stressed and deformed due to the weight of the cable bundle.

While attaching cable supports, the installer should maintain the minimum distances from sources of electromagnetic interference: 6 in. from power circuits of 2 kVA or less, 1 ft from high-voltage (arc discharge) lighting, such as fluorescent and HID lamps, 3 ft from power conductors of 5kV or higher, and 3.5 ft from transformers and motors.