Network Building: Twisted-Pair Cable Specifics

After deciding what category of twisted-pair cable you need, you will need to consider features like shielding, type of shielding and solid or stranded. Its kind of like buying a new car, choosing the make and model is just the beginning of the decisions.

To shield or not to shield

Unshielded twisted-pair cable, or UTP, is designed for normal office environments. When you use UTP cable, you must be careful not to route cable close to fluorescent light fixtures, air conditioners, or electric motors (such as automatic door motors or elevator motors). UTP is the least expensive type of cable.

In environments that have a lot of electrical interference, such as factories, you may want to use shielded twisted-pair cable, also known as STP. Because STP can be as much as three times more expensive than regular UTP, you won’t want to use STP unless you have to. With a little care, UTP can withstand the amount of electrical interference found in a normal office environment.

Most STP cable is shielded by a layer of aluminum foil. For buildings with unusually high amounts of electrical interference, you can use more expensive, braided copper shielding for even more protection.

When to use plenum cable

The outer sheath of both shielded and unshielded twisted-pair cable comes in two varieties: PVC and Plenum. PVC cable is the most common and least expensive type. Plenum cable is a special type of fire-retardant cable that is designed for use in the plenum space of a building.

Plenum cable has a special Teflon coating that not only resists heat, but also gives off fewer toxic fumes if it does burn. Unfortunately, plenum cable costs more than twice as much as ordinary PVC cable.

Most local building codes require that you use plenum cable whenever the wiring is installed within the plenum space of the building. The plenum space is a compartment that’s part of the building’s air distribution system and is usually the space above a suspended ceiling or under a raised floor.

Note that the area above a suspended ceiling is not a plenum space if both the delivery and return lines of the air-conditioning and heating system are ducted. Plenum cable is required only if the air-conditioning and heating system are not ducted. When in doubt, it’s best to have the local inspector look at your facility before you install cable.

Sometimes solid, sometimes stranded

The actual copper wire that composes the cable comes in two varieties: solid and stranded. Your network will have some of each.

  • In stranded cable, each conductor is made from a bunch of very small wires that are twisted together. Stranded cable is more flexible than solid cable, so it doesn’t break as easily. However, stranded cable is more expensive than solid cable and isn’t very good at transmitting signals over long distances.

    Stranded cable is best used for patch cables, such as the cable used to connect a computer to a wall jack or the cable used to connect patch panels to hubs and switches.

    Strictly speaking, the cable that connects your computer to the wall jack is called a station cable — not a patch cable. Patch cables are used in the wiring closet, usually to connect patch panels to switches.

  • In solid cable, each conductor is a single solid strand of wire. Solid cable is less expensive than stranded cable and carries signals farther, but it isn’t very flexible. If you bend it too many times, it will break. Solid cable is usually used for permanent wiring within the walls and ceilings of a building.