David Barnett
Essex GroupInc.
With the growth of structured wiring solutions for campus installations, our engineering department has received numerous calls about installing premises cables outdoors. Typically, the system designer or user wants to extend the unshielded twisted-pair cabling as backbone cabling between buildings.
Can you install premises wiring outdoors? Sure, and it will work--at least for a while. But, except for a few specific products, premises cabling is not designed for outdoor applications and, therefore, is not suitable for that environment.
Article 800-51 of the National Electrical Code categorizes communications cables for premises use according to levels of flame or smoke resistance. Materials that meet NEC requirements are usually not suitable for use outdoors.
Exposure to the elements
Cables installed outdoors are exposed to water, which adversely affects the transmission properties of any cable not constructed of, or protected by, materials that limit its intrusion. Premises cables designed for indoor installation do not need to be constructed for protection against water intrusion; therefore, if you use this type of cable outdoors, you will be inviting signal transmission problems.
Depending on placement, cables installed outdoors can be exposed to sunlight. Ultraviolet radiation degrades plastic unless special additives are included in the compounds. Outside plant, or OSP, cables are usually black because carbon black is added to the jacket material to absorb UV energy.
The UL 444 specification for premises cables has optional requirements related to UV protection. These optional requirements are mainly exercised for residential station wires, where the jacket compound is modified to include an additive to meet these requirements. Most off-the-shelf premises cabling intended for commercial installations is not designed for, or tested to, these requirements. If the cable that you run between buildings in an aerial plant is rated communications general-purpose, or CM; communications riser, or CMR; or communications plenum, or CMP; that cable may be damaged by sunlight over time.
It is also inappropriate to install premises cable outdoors in an area where the temperature drops below freezing. UL 444 specifies nominal temperature requirements that are usually applied to station wiring. Impact and cold bend testing are performed at -10oC (14oF) and -20oC (-4oF). By comparison, OSP cables are tested at -40oC (-40oF). At temperatures below freezing, typical CM, CMR and CMP cable may crack under bending load or impact.
Proper grounding is another concern when installing premises twisted-pair cables between buildings (see "Ask Donna", November 1995, page 67). For example, the typical OSP cable has a metallic shielding in the sheath, which serves not only as physical protection for the cable but also provides an effective way to ground the cable.
This shielding protects the cable, termination equipment, electronics and personnel against high-voltage surges from sources such as lightning strikes. Grounding is essential but, because of the additional labor, effectively grounding premises cables outdoors can increase installation costs.
Choosing an OSP cable
For outdoor applications that require Category 3 transmission performance, you can use standard 83 nanofarad/mile, 24 American Wire Gauge, OSP, twisted-pair cable. Although this cable is not tested or marked as Category 3, its transmission properties should meet or exceed Category 3 requirements. However, you should always check with the cable manufacturer.
For higher bandwidth--Category 5, for example--an OSP fiber-optic cable is recommended. At this time, there is no widely available twisted-pair alternative. In either case, choose the OSP cable construction--aerial, direct buried or duct--best-suited to your installation.
Because OSP cables are not rated by the NEC for flame retardancy, where do you make the transition from outdoor to indoor cabling? Exception 3 to Article 800-50 of the NEC allows you to bring a nonlisted cable inside a building to a maximum length of 50 feet for termination. And that termination must be in an enclosure or a primary protector listed by a nationally recognized testing laboratory.
This complex subject cannot be covered in a short article. You should refer to the sections of the NEC mentioned in this article and any other related sections. Also, before you begin the installation project, consult and understand all state and local codes. Consult appropriate local inspectors to resolve questions or to clarify issues.