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This article originally appeared in Cabling Business Magazine--

NEC Chapter 8, Communications Systems-4


By David Herres


This is the second of a four-part series dealing with National Electrical Code (NEC) Chapter 8, Communications systems. Each piece will examine one of the four articles within that chapter: Article 800, Communications Circuits, Article 810, Radio and Television Equipment, Article 820, Community Antenna Television and Radio Distributions Systems, and Article 830, Network-Powered Broadband Communications Systems. Article 820, Community Antenna and Radio Distribution Equipment, provides safety guidelines for these installations and focuses on coaxial cable.-

NEC Article 820, Community Antenna and Radio Distribution, focuses on the installation and use of coaxial cable. Coax has a few other applications such as instrumentation and internal equipment wiring, but its most common uses are for CATV, antenna lead-ins in satellite dish systems, video surveillance transmission lines, and the like. In the early 1970's it was the primary Ethernet medium, but in that application it was entirely shoved aside by the cheaper and more versatile UTP Category cable.

Coaxial cable derives its name from the fact that the two wires, inner signal carrying conductor and outer grounded signal reference, share a common axis. These elements are separated by a tubular insulating layer, which is quite flexible and has a substantial dielectric constant. The outer layer is usually a woven braid and/or foil assembly designed for flexibility and good shielding characteristics.

The signal is actually carried in the dielectric as a fluctuating force field. This is the most useful propagation mode. Above a distinct cutoff frequency, signal propogation can jump into different modes whereupon the cable functions as a waveguide. Propogation above cutoff is generally undesirable since multiple modes with varying phase velocities invariably interfere with one another.

Coax is a very old medium -- it actually predates Edison's lightbulb. First known patents were acquired by Oliver Heaviside in 1880, Ernst Werner von Siemens in 1884 and Nikola Tesla in 1894.

1936 was a great year for coaxial cable -- milestones included first transmission of TV signals, a 300 km underwater coaxial cable near Australia and a coaxial telephone line between New York and Philadelphis capable of carrying 240 simultaneous telephone links.

Today coaxial cable has been partially eclipsed by optical fiber and UTP (Cat 5e and beyond) but it holds its own in cable TV and video distribution. Because it is easy to run and terminate and provides excellent RF and magnetic shielding, there is every reason to believe it will be around for a great many years.

NEC Article 820 begins with a statement of scope and specialized definitions applicable only to this article. The statement of scope says that the focus of the article is coaxial cable distribution of radio frequency signals typically employed in CATV systems. Thus, the door is left open for general (non-CATV) mandates as long as radio frequency signals are involved.

A major aspect of coax usage is that it is capable of carrying AC power, usually just under 60 volts, in addition to the radio frequency signal. Simple capacitor-inductor networks can easily separate these two waveforms as needed. The AC is introduced at either end or anywhere along the transmission line, taken from a small utility-powered transformer. This voltage is available in order to power amplifiers, typically located every 400 feet along the transmission line, where it might not be feasible to provide electrical services.

If you need to troubleshoot a CATV system the standard pathology is loss of signal. You can use a voltmeter to check for AC power at various points since any problem along the line will probably take out both the AC power and the RF signal. Of course loss of AC power at the source, as with a tripped circuit breaker, could be the problem. The RF component can be checked with a field strength meter or a small prtable TV.

NEC 820.2, Definitions, starts out by defining abandoned coaxial cable as installed coax not terminated at equipment other than a coaxial connector and not identified for future use with a tag. Later on in Article 820, it is required that the accessible portion of abandoned coax be removed. Coax that is within raceway or behind wall surfaces is not considered accessible and so may remain in place. Coaxial cable above suspended ceiling panels is accessible and must be removed. This requirement pertains to other types of low voltage, communication and Class 2 and Class 3 remote control and signaling wire but currently does not apply to power and light wiring. The safety concern is certainly valid because in the event of fire a large amount of such abandoned wiring would generate huge amounts of toxic smoke.

Not addressed by the Code is who is responsible for this removal. If you install a short run of coax, are you obliged to remove all abandoned data, voice and signaling cable from the room in which you worked, or even from the entire building? There is the further issue of who should finance the operation, tenant or building owner. Cable removal moreover is not for the faint-hearted, since inadvertant removal of live circuitry would have significant impact on operations. Even moving old still active cabling a slight amount could introduce errors into the data flow.

Another important definition is Exposed (to accidental contact): A circuit in such a position that, in case of failure of supports and or insulation, contact with another circuit could result. Coaxial cable, with an upper limit of 60 on AC voltage, is not considered a significant shock hazard under most conditions. But in an outdoor installation, for one example, if mounting straps were to fail and chafing occur between the coax and a higher voltage line, the results could be catastrophic at the indoor point of usage. For that reason, it is important to see that all coax installations are adequately secured and protected. Unlike power and light wiring, coax does not have definite securing intervals, so it is left to the installer to look into the future and determine whether a given configuration will be stable for years to come even if building usage or other changes occur.

Point of Entrance is defined as the point within a building at which coaxial cable emerges from an external wall, from a concrete floor slab or from a rigid or intermediate metal conduit. Such conduit, to define the point of entrance, must be connected by a grounding conductor to a ground electrode, and both ends of the pipe have to be bonded to the grounding conductor.

Point of Entrance becomes important in 820.48, which covers unlisted cables entering buildings. Unlisted cable is permitted to be installed outside buildings and can extend a maximum of 50' from the point of entrance as defined previously provided that the cable enters from outside and is terminated at a grounding block.

It is required that where coaxial cable enters a building, the outer conductive shield be grounded as close as practicable to the point of entrance.

820.3 makes reference to other articles. In the rare instances that CATV equipiment is installed in classified (hazardous) areas, the applicable requirements of Chapter 5, Special Occupancies, apply.

Section 300.22 is applicable where coaxial cable is installed in ducts, plenums or other spaces used for environmental air. It is worth noting that no wiring of any type can be installed in ducts used to transport dust, loose stock or flammable vapors. No wiring system of any type is to be installed in any duct or shaft containing only such ducts, used for vapor removal or ventilation of commercial-type cooking equipment.

For installation of optical fiber, communications circuits and network-powered broadband communication, those sections apply. Article 830, Network-Powered Broadband Communications, has more stringent requirements than Article 820, so employing those wiring methoeds would facilitate a future upgrade.

Another stipulation for coaxial cable is that it is permitted to deliver power to equipment that is directly associated with the radio frequency system provided the voltage is not over 60 volts and provided the current is supplied by a transformer or other device that has power-limiting characteristics. Unlike Class 2 and Class 3 wiring, an exact power limitation is not given.

A further requirement is that power is to be blocked from premises devices on the network that are not intended to be powered by the coaxial cable.

Part I concludes with four obligatory sections which have to be observed for all low-voltage cabling:

Access to electrical equipment must not be denied by an accumulation of coaxial cables that prevents removal of panels, including suspended ceiling panels." It is a definite NEC violation to lay coax or any other wiring across suspended ceiling panels, even in retrofit work. This would be the easiest way to accomplish a run, but eventually such a practice would cause the panels to be locked in place making future maintenance on any of the above ceiling mechanical systems vwery difficult. Another Code violation is to strap cabling to existing electrical conduit, which then becomes compromised in terms of heat loss and weight. Coax and other cabling above a suspended ceiling must be independently mounted and secured to the building structure even if it is necessary to dril into wood or concrete. If theere is open metal trusswork or a similar structure, you are fortunate.

The second requirement is that CATV and radio distribution cabling are to be installed in a neat and workmanlike manner. Coaxial cable must be supported so as not to be subject to damage. The cable must be secured by hardware including straps, staples, cable ties, hangers or similar fittings designed and installed so as not to damage the cable. In the case of coax and other low voltage cabling, there is a little more flexibility than with power and light wiring, where minimum support intervals are given.

A third universal low-voltage mandate has been previously mentioned -- abandoned cable must be removed.

The last of these cautionary mandates is of great importance and is increasingly an issue with electrical inspectors.

Installation must not compromise fire-resistant rated walls, partitions, floors or ceilings. If you drill through any of these, it is necessary to firestop the openings so that the fire resistance rating is maintained. Building and fire codes in various locations discuss firestopping in great detail. A typical requirement is that boxes installed on opposite sides of a fire-rated wall must have a 24" horizontal separation so that a hot jet of flame won't blast through.

Part II, Coaxial Cables Outside and Entering Buildings, covers aerial and underground installations up to the point of entrance.

On poles prior to the point of grounding, conductors where practicable are to be located below electric light, power, Class 1 or non-power-limited fire alarm circuit conductors. They are not to be attached to a cross-arm that carries electric light or power conductors.

Coaxial cable that comprises lead-in or aerial drop conductors must be kept away from electric light, power, Class 1 or non-power-limited fire alarm conductors so as to avoid contact. An exception, where close approach cannot be avoided, sets 12" as an absolute minimum clearance, and states that this clearance is to increase to 40" at the pole.

Aerial coaxial cable is permitted to be attached to an above the roof raceway mast that does not enclose or support conductors of electric light or power circuits. This setup is rarely seen since the typical raceway mast is used exclusively for service entrance conductors.

Part II continues with requirements for running coaxial cable above roofs (8' minimum from all points with the standard three exceptions for garages, overhangs and steep roofs not subject to foot traffic).

Between buildings, coax must be supported to withstand anticipated loads. Here again the Code is more flexible than is the case with light and power wiring. Also, messenger cable is permitted for long spans and extreme loading conditions.

Of great importance, coaxial cable on buildings must have a separation of at least 4" from electric light, power, Class 1 or non-power-limited fire alarm circuit conductors not in raceway or cable. Alternately, it can be permanently separated from those conductors by means of a continuous nonconductive barrier in addition to the existing wire insulation.

In regard to other communication system wires, there is to be no interference in access for maintenance, and no contact that could cause abrasion.

Where practicable, a separation of 6' is to be maintained from lightning conductors.

Concerning underground conductors entering a building, coax in duct, pedestal, hand hole enclosure or manhole that also contains electric light or power conductors or Class 1 circuits is to be kept separate by means of a suitable barrier.

Direct-buried coax is to be kept 12" from light, power or Class 1 circuits unless one or the other is in raceway or has metal armor.

Section III, Protection, and Section IV, Grounding Methods, address some critical issues. Everyone knows that coax outer shield is to be grounded. But often this connection is made in an improper manner.

Where coaxial cable enters a building, the outer conductive shield is to be grounded as close as practicable to the point of attachment or termination.

Grounding may be accomplished by installing a listed primary protector next to or integral with a ground block. This equipment is not to be located in a hazardous location or in the vicinity of easily ignitable material.

Grounding of the coaxial cable outer shield is to be accomplished according to a definite set of specifications --

The grounding conductor performs the same function as the ground electrode conductor ("ground wire") in power and lighting work, but it takes a somewhat different form. It is to be insulated and listed. It is to be copper or other corrosion-resistant material, stranded or solid. It must be 14 AWG minimum and have current-carrying capacity approximately equal to the outer conductive shield of the coax. But it does not have to exceed 6 AWG, that would be overkill in view of ground electrode impedance. It is to be as short as practicable, in one- and two-family dwellings not longer than 20 feet.

If it is not possible to have a grounding conductor less than 20 feet, drive a separate communication ground rod close to the ground block. But this is not to be an isolated "floating" ground. Connect the grounding conductor to the power grounding electrode system by means of a 6 AWG copper conductor, which will not exceed 20 feet in length. Here you are not so much connecting to the power system ground rod as you are bonding to the service neutral bar and main bonding jumper.

The key concept is that all grounding systems including lightning protection are to be bonded together by means of a low-impedance current path.

Section 820.106 covers somewhat specialized grounding methods for coaxial cable installed in mobile homes.

Part V, Installation Methods Within Buildings, covers material that is familiar territory for low-voltage cable installers. The sections are similar to text in the articles on other types of communications systems, fiber optic cable, fire alarm systems and Class 1, Class 2 and Class 3 Remote Control, Signaling and Power-Limited Circuits.

Some requirements are relaxed, for example conduit fill restrictions do not apply. But it is critical that plenum, riser, general purpose and dwelling applications each see the correct cable. Substitutions are permitted according to charts in each of the low-voltage articles. Plenum type cable may be used in risers, for example, and plenum or riser cable may be used in general purpose applications.

These are the principle NEC Article 820 requirements regarding coaxial cable. Observing all NEC mandates will insure a safe installation from the point of view of fire and electric shock hazards.

Next: The fourth part in this series will deal with Article 830, Network-Powered Broadband Communications Systems.
--END--


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Here is a selection of the most significant electricians' books available online today, at the best prices around. Clicking on any logo provides access to reviews and ratings by electricians. A good place to start is with the 2008 NEC Handbook, which contains the complete text of the current code plus extensive commentary, diagrams and illustrations. Other books of interest for the electrician are available as well.

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This site is created and conducted By David Herres, NH Master Electrician License #11335M

E-mail: electriciansparadise@hughes.net


HOME | Best Web Host | Question of the Week | Archived Questions | More Archived NEC Questions | Still More Archived Questions | Still More Archived Questions-2 | Still More Archived Questions-3 | Articles | Electrical Deficiencies | More Electrical Deficiencies | Electricians Tools | Online computers | Cybercorner | Electrician's License | Electronics Tutorials | Electricians' worksaving ideas | Electronic Theorems | Satellite Dish | Digital Cameras and Equipment | HTML Color Chart | Electronic Acronyms | Electronic Definitions | Electrician's Soldering Tutorial | Photovoltaic Power | Wind Power | Fire Alarm Basics | More Fire Alarm Info | Working with MC and EMT | Electricians' Color Code | Wiring Commercial Garages | Managing Your Emergency Lights | Lighting Design | Industrial Wiring | Wiring Ethernet | Residential Wiring | Low Voltage Wiring | PLC Overview | Electrical Troubleshooting Techniques | Using Loop Impedance Meter | Ten Common Grounding Errors |NEC and Low-Voltage Wiring | Raceway Protection and NEC | Working with Metal Raceway | Inductance and Characteristic Impedance | Understanding Capacitance | History of the Ethernet | Twisting Data Conductors | NEC Article 800, Communications Circuits | NEC Article 810, Radio and Television Equipment | NEC Article 820, Community Antenna and Radio Distribution Equipment | NEC Article 830, Network-Powered Broadband | Troubleshooting Submersible Well Pumps | Wiring Healthcare Facilities | First Edition National Electrical Code 1897 | Books for Electricians | Links


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