Three factors needing to be considered for performing a cable installation are nature of material, voltage level and current carrying capacity. Also fire protection. The support system for cabling and equipment is a vital component of a properly designed data-communications system. The acceptance and use of the eia/tia-569 Commercial Building Standard for Telecommunications Pathways and Spaces by architects and engineers has proven the importance of a carefully designed and maintained cable-support system. There are six major types of cable supports: cable runway, center-rail systems, cable tray, wire basket, bridle rings or cable hooks, and wireway. Each is designed for specific purposes. ...view middle of the document...
For example, for runway that is 6 to 12 inches wide, the maximum cable fill is the width of the runway. For 15- to 25-inch-wide runway, the maximum fill is 12 inches, and for 30-inch wide runway, the maximum fill is 10 inches. Pile-up is limited to minimize damage to cables at the bottom of the stack, which can be crushed against rung supports. For high-performance cables such as Category 5, 6 inches of cable fill is a more realistic specification. NEMA`s VE-1 standard permits spans of 8, 12, 16, and 20 feet. Permitting longer spans means fewer supports are required, which can lower installed costs. Loading is spelled out in pounds per foot. Cable fill or fill depth can be 3, 4, 5, or 6 inches. In the data-communications market, 6 inches is usually preferred due to tight space restraints on tray width.
NEMA-based load and span designations, called class designations, are as follows:
•Load class A is 50 lbs/ft.
•Load class B is 75 lbs/ft.
•Load class C is 100 lbs/ft.
There are 12 classes under the NEMA designation system. For example, a 20-foot support span with a 100-lb/ft maximum load capacity uses the designation 20C. This rating is widely used in industrial applications to specify cable-support systems.
The NEC addresses cable fill but gives no span limitations. Cable fill for communications cable is limited to 50%, but that does not mean the cable tray is only half full. Rather, the fill is based on the cross-sectional area of the cable. The space between cables will take up the other 50% of the space. A good rule of thumb for determining when the tray is full is the following: the number of cables x the diameter of the cable x 2. Remember that 50% fill is full. Grounding is also important. Underwriters Laboratories (UL) and the NEC have specific requirements to ensure metallic systems are properly grounded, whether power conductors are present. A system that is grounded--even though it contains only data-communications wiring--must be capable of carrying away (grounding) accidental power sources or surges. Another important consideration is material. Steel is cost- effective but heavy. Aluminum is more expensive but easy to work with, lightweight, and aesthetically pleasing without having to be coated with additional finishes. Nonmetallic materials can be expensive but are lightweight and easy to work with. However, they can be hazardous in plenum spaces because of the smoke and fumes they emit in a fire.
Finishes offer protection
There are three types of finishes: painted, zinc-plated, and polyvinyl chloride (PVC). Painted finishes offer satisfactory protection against corrosion but do not offer grounding protection. Zinc-plated finishes are impervious to corrosion and offer grounding and static protection. PVC, an anodized finish, offers both corrosion resistance and grounding. When specifying a finish, consider its thickness, adhesion properties, and corrosion resistance. A minimum thickness for the coating is 1.5 mil for paint...