A variety of solutions for copper and fiber cable support systems offers increased flexibility and lower life cycle costs.
With today's fast-track projects and design/build jobs, selecting the best wiring method to support extensive data communications cabling is a real concern. The support system for cabling and equipment, whether overhead, perimeter, infloor, or underfloor, is a vital component of a properly designed data-communications system. Architects and engineers generally use EIA/TIA-569 “Commercial Building Standard for Telecommunications Pathways and Spaces” in designing and specifying a cable support system, but this first level of specification at the designer's office satisfies only some of the cable placement concerns.
In general, seven types of systems support cables within a building: cable runway, center-rail systems, cable tray, wire basket, flexible steel cable tray, bridle ring or cable hooks, and wireway or surface raceway. When selecting a cable raceway or cable tray support system, consider the load capacity and grounding and plenum requirements.
Load capacity is the maximum cable load allowed on a system. You use the distance between supports, or span, and the maximum cable weight, or loads, to determine a cable-support system's load capacity. You can find standards that use different terms in describing load capacity, but the basic centers around span and cable weight.
Proper grounding is important on a metal cable tray system — even though it contains only datacom wiring. It must safely carry fault current or an accidental electrical discharge like lightning.
If the cables run in an area carrying environmental air in an open cable-support system, such as cable runway or center-rail or ladder-type cable tray, then you must use plenum-rated cable. If you're using a closed system, such as an enclosed raceway or solid-bottom tray with solid covers, then the NEC doesn't require plenum-rated cable.
The three types of cable runway are solid bar, tubular, and C-channel. While solid bar construction is strong, it's also heavy and expensive. Tubular is easier to install and more economical. Many users find C-channel to be a good compromise between the other two because it offers a good price-to-performance ratio.
The center rail, or center spine, support system is growing in popularity. It allows for easy cable lay-in, eliminating the need for cable pulling in many cases. A center-rail system is available as a rod-mounted track or vertical rack, as well as wall-mounted, half-rack, and multitiered vertical-rack. Rungs are available in a variety of widths and depths.
NEMA specifies fill capacities for center rail supports at 3 in., 4 in., 5 in., and 6 in. deep and as wide as 24 in. These systems can be modified in the field and are available with a variety of components for altering the pathway and getting around field obstructions.
Available in ladder, vented, or solid construction, cable tray systems are suited for supporting network cabling that will be upgraded, reconfigured, or relocated. They're available in aluminum, steel, fiberglass, or stainless steel constructions with galvanized or painted finishes.
In addition to providing a high load-bearing capacity, a solid-bottom cable tray keeps cable organized and secure, and uses dividers to separate power and data communications cables. For added cable protection, you can specify a solid-bottom cable tray with a cover. You can also specify ventilated louvers to allow cooling by natural convection, while maintaining cable protection. This type of tray can be hung on a center rod support, a C-hanger, or a two-hanger trapeze configuration. Brackets provide for wall mounting.
Although it's a relatively new product, the wire basket or wire mesh cable tray has grown quickly in popularity. Since the components are field configured and cut, pathways no longer need to be designed before the components are ordered. The metal can be cut, bent, and joined with splice connectors to make an L-, T-, or cross-shaped layout. Wire mesh systems are ideal for small installations or those that will undergo several moves, adds, and changes.
This welded steel-wire product can be installed using a few tools. It has the flexibility to adapt to any routing layout. Available for 1-in., 2-in., or 4-in. cable fills, it comes in even increments from 2-in. to 24-in. widths, with a zinc-plated or painted basket.
A wireway or surface raceway system can be placed along any desired route within a space, or suspended from hangers within a space. Almost two dozen vendors offer raceway products.
One manufacturer has special outside corner fittings, and another company has a patented symmetrical hinge that allows you to retain the cable during installation.
Bridle rings or cable hooks are simple hardware items that attach to structural building members and are available in a variety of load capacities. A variety of new cable hook designs that provide a greater cable-bearing surface than a bridle ring are very popular where only a small number of cables are run.
As a general rule, you should suspend Cat. 5 cable on appropriate J-hook, or cable-hook hardware, with a spacing of about 4-ft or 5-ft on center to minimize cable sag. However, the distance between these hardware items can be best determined in the field, since the installer should consider the weight and number of cables in each bundle.
A variation on the J-hook is a cost-effective alternative to cable tray because it can hold as many as 425 UTP or fiber optic cables, and it attaches to any main structure, including a beam or purlin. In addition, you can pull the cables smoothly through the closed-support sling system. This device is a 4-in.×12-in. strip of plastic-coated fabric; one end of the fabric strip attaches to a bar that is secured to any main structure, such as a beam flange or purlin. This metal bar, which is slightly wider than the fabric strip, has a hook on both ends. The two hooks slip down on a pair of tabs located on the ends of the bar, creating the large sling. The sling is opened by lifting the bar off the tabs, allowing a large number of cables to be placed inside the sling where they're well supported.
For underfloor cable distribution, a surface-mount, raised-floor raceway system offers a number of features: it's portable, reusable, expandable, and interchangeable. Composed of either plastic or metal, the modular panels are only a few inches high. This low-profile system provides a high cable capacity. And you don't need any special tools to install the UL-Listed system, which includes a full line of accessories.
A variety of wire management and support systems are available to support low-voltage communications needs, including DDC control panels and building automation systems. The specifier must select a system that serves today's needs as well as the requirements of tomorrow.
Sidebar: Connecting Multiple Buildings
With the explosion of growth in a variety of communications services, customers and contractors are demanding greater capacity and easier placement methods when multiple buildings are connected on a single operational network. As a result, conduit and duct manufacturers are introducing products to meet those demands.
In outside plant construction, conduit is typically located underground to protect cables from damage and to facilitate cable placement for immediate and future need. It's a good idea to install innerduct, which is available as outdoor type, riser-rated, or plenum-rated, inside a larger-diameter conduit to protect, isolate, and identify the cables. Innerduct is classified by outside diameter (OD), whereas trade size conduit is classified by inside diameter (ID).
In addition to the smooth-wall type, innerduct is also available with a ribbed wall to reduce friction between the cable sheath and the innerduct wall. A wave rib on the inside of the duct reduces surface contact between the cable and the wall of the duct, thus reducing the coefficient of friction and allowing longer cable pulls. One manufacturer offers an ultra-low friction innerduct coated with low-friction ribbons made of Teflon and silicon molecules in a polyethylene matrix.
Sidebar: Seismic Considerations.
Seismic bracing should be applied in any areas susceptible to earthquake damage. Ladder rack supports with transverse braces and longitudinal braces attached to racks and trays provide the necessary extra support in an earthquake. After identifying the seismic force in the specification process, the next step is to choose a bracing system to accommodate it. The system must transfer the horizontal seismic force to the structure. Essentially, you can analyze any kind of support runway or tray, wireway, conduit on a trapeze, or hook system in this manner.