Thomas Edison, three years after developing the first usable light bulb in 1879, established electrical generating and distribution equipment in lower Manhattan and began selling power to residential and commercial customers. Although he had conceived and put into operation a system of fusing that protected wired from overheating based on ampacity, nevertheless electrical fires were numerous and it became clear that something more had to be done.

Led by insurance underwriters, who were losing vast amounts of money, a group of electrical, architectural and other interested individuals consolidated the five existing regional codes and created the original National electrical Code (NEC), which came out in 1897. In 1911 it came under control of the National Fire Protection Association (NFPA). NEC 2008 is the 51st edition.

Currently, new editions come out every three years, invariably with numerous changes and new requirements. As far as NFPA is concerned, NEC is purely advisory. It is made available to countries, states, municipalities and others to adopt with or without amendments as they see fit, at which point it acquires legal standing. NEC has nine chapters. Chapters 1-4 apply generally. Chapters 5, 6 and 7 apply to Special Occupancies, Special Equipment and Special Conditions.

Chapters 1-4 apply to all electrical work except where modified in these later chapters. Chapter 7, Special Conditions, and Chapter 8, Communications Circuits, exempt low-voltage cabling from some requirements in Chapters 1-4 and in some instances impose more stringent requirements.

Many people assume NEC does not cover installations under a certain voltage, but that is not the case. Is purpose is "the practical safeguarding of persons and property from hazards arising from the use of electricity". Why, then, should it address low-voltage cabling where the energy level is incapable to injury to persons by shock and incapable of igniting flammable material? The answer is that additional hazards abound.

• In the event of fire, the insulation of low-voltage cabling can contribute to the early spread and intensity of the inferno and generate large amounts of toxic smoke.
• Improperly installed low-voltage cabling can contact power and light conductors and inadvertently become energized to a higher level.
• Poor workmanship can render other wiring and equipment inaccessible. An example is the non-compliant practice of laying cable directly on a suspended ceiling so that the panels cannot be lifted out of the way to work on equipment above.

Ongoing accumulation of discontinued cabling can intensify the problems listed above and also make maintenance and troubleshooting very difficult.

Certain low-voltage systems, such as fire alarm and nurse call can compromise life and property if they fail to operate as intended. Cable is not defined in the Code, but elsewhere it has been described as either a single conductor that is stranded or two of more conductors insulated from one another, frequently within a common jacket. This formulation includes all kinds of power wiring, even some high-voltage transmission lines.

But cabling has come to mean low-voltage wiring, including such diverse items as coaxial cable for video, Category 5e for Ethernet, 24-volt thermostat wire and the increasingly relevant fiber optic. How does NEC address these and similar cabling and wiring modes? Primarily in Chapter 7, Articles 720 through 770, and Chapter 8. Great emphasis must be placed on Article 725, titled Class 1, Class 2 and Class 3 Remote-control, Signaling and power-limited Circuits. while it is true that lower voltage and energy levels are present in these circuits, you are missing the point if you think that is their essence. the fundamental concept in Article 725 is that what is being conducted is not power to create, for example, light, heat or motive force, but rather information, either prescriptive as in control or descriptive as in signaling.

The protocol for designing and building these circuits, laid out in Article 725, clearly states what constitutes each of the three classes and what wiring methods and materials are required for each. Class 1 wiring is subdivided into Class 1, power-limited, supplied by a source that is not more than 30 volts and 1000 volt-amperes, and non-power-limited, technically known as Class 1 remote control and signaling, supplied by a source that does not exceed 600 volts and has no volt-amp limitation. Additionally, any remote control or signaling circuit that, if it went into failure, would constitute a direct fire or life safety hazard, falls into the Class 1 category. The Code makes the point that thermostat control circuits for heating and cooling do not qualify as Class 1. Nor do fire alarm and nurse call because they just report hazardous conditions, they do not create them. (A nurse call system can be Class 1, however, if it exceeds the Class 2 power limits.) A Class 2 circuit extends from the output of a listed Class 2 power source, usually a transformer or electronic power supply to the load. Other sources can be a thermocouple or battery, neither of which has to be listed.

A Class 3 circuit is similar but has different maximum voltage and power levels. These limits are found in NEC Chapter 9, Tables 11(A) for alternating current and 11(B) for direct current. Scrutinizing these tables, you will see that Class 3 circuits have somewhat higher power limits than Class 2 circuits for various voltages. Class 2 is considered safe from an electric shock and fire standpoint, where Class 3 is considered fire safe only. for most work, it is not necessary to go into the rather intricate Chapter 9 tables. The listed power source will have the class number right on it and in the accompanying documentation, which is part of the listing. The basic rule is that Class 2 and Class 3 circuits are not to occupy the same cable or raceway as Class 1 circuits or power and light wiring.

Where it is desirable from a standpoint of economy or functionality to combine them, 725.52(A) Exception No. 2 permits Class 2 or Class 3 circuits to be reclassified and installed as Class 1. If this is done, it is required that the Class 2 or Class 3 marking be removed, and that Class 1 overcurrent protection b provided. The new reclassified circuit actually becomes a Class 1 circuit, even though it remains connected to a Class 2 or Class 3 power source. Consequently, it must maintain separation from any other Class 2 or Class 3 circuits. Once you have mastered the definitions of Class 1, Class 2 and Class 3 Remote-Control and Signaling circuits, you are well positioned to understand the wiring requirements.

Let's consider what some or all of these have in common: Abandoned Class 2 and Class 3 cable that is accessible must be removed. Abandoned is defined as not terminated at equipment and not identified for future use with a tag. With the rapid ongoing advances in technology and periodic changes is occupancy, a great amount of abandoned cable, notably above suspended ceilings and below raised floors, is being generated. The fact is that a lot of it is not being removed. This is, in part, because it is unclear who precisely is responsible for the removal. If you go into a customer's facility and run a couple of Ethernet lines or do a small troubleshooting job, are you responsible for removing all of the abandoned cable in the building? The owner and tenant may not want to pay for this work. Moreover, there is the risk of inadvertently cutting out a live circuit or, when pulling out a cable, you could disturb an adjacent cable and bring out a latent fault. There is the potential for bringing down a complete network and putting a lot of people out of work for the rest of the day.

And yet it is vital that this old material be removed. It adds greatly to a building's fire load and a large amount of toxic smoke can be generated. Also, especially under a raised floor or within a plenum, excess cabling impedes cooling. Additionally, maintenance and troubleshooting are hampered by the presence of abandoned cable. And so, besides complying with the Code, there are very real reasons for removing old cable. Strangely, the Code does not require the removal of abandoned electrical power cable or Class 1 remote control and signaling cable (non-power-limited), the the requirement in Article 725 also appears with nearly identical wording in articles dealing with:

• Audio Signal Processing, Amplification and Reproduction Equipment (640).
• Information Technology Equipment (645).
• Fire Alarm Systems (760).
• Optical Fiber Cable and Raceways (770).
• Community Antenna Television and Radio Distribution Systems (820).
• Network-Powered Broadband Communications systems (830)

Since only accessible abandoned cable must be removed, the requirement does not apply to cable in raceways.

Another mandate that applies to article 725 wiring and additionally to other types of low-voltage cabling is that these circuits must be installed in a neat and workmanlike manner. Cabling on ceilings and walls must be supported by the building structure so that it will not be damage by normal building use. Straps, staples, hangers, cable ties and the like may be used. Notice that unlike power wiring, which is assigned minimum support intervals for each type of cable in Articles 320 through 398, low-voltage cable support intervals are left to the worker's discretion, as long as the end result is adequate for the type of cable and environment in which it is installed. (See other building standards such as TIA-569-B for additional installation practices). It is further mandated that the installation conforms with Article 300.4(D), which addresses protection from screw and nail penetration.

Another overall requirement for Class 1, 2 and 3 circuits is that they be identified at terminal and junction locations so as to prevent unintentional interference with other circuits during testing and servicing.

Class 1 circuits, in 725.25, are required to comply with Part I of Article 300 and with the appropriate articles in Chapter 3. An important provision is contained in 300.15, which requires all splices to be in enclosures. Since there is no such provision in Part III of Article 725, which covers Class 2 and Class 3 circuits, it is clear that these circuits are not required to have splices enclosed in boxes. All three classes are exempted from the Chapter 3 minimum wire size of 14 AWG. Low-voltage cable can employ 16 and 18 AWG conductors if they are appropriate to the load.

The remainder of Article 725 addresses two major areas:

• Which combinations of Class 1, Class 2, Class 3, electric light and power, non-power-limited fire alarm, power-limited fire alarm, medium and low powered, network-powered broadband communications, non-conductive and conductive optical fiber cables, audio, etc. are permitted within the same cable, raceway or enclosure.
• The hierarchy of cable types -- Class 3 plenum, Class 3 riser, Class 2 riser, power-limited tray cable, Class 3 cable, Class 2 cable, Class 2 and 3 limited-use cable.

• It is beyond the scope of this article to discuss the above topics in detail. It should suffice to say that a design error in either of these areas can be costly and dangerous. But Article 725 lays out the requirements in full. The Code should be consulted on a case-by-case basis before going into uncharted waters.

  There are a number of low-voltage cabling topics, which are treated outside of Article 725, notably in Articles 640, 645, 760, 770, 820 and 830. Many of the issues raised are similar. Abandoned cable, workmanship, cable hierarchy and permitted substitutions echo the provisions in Article 725. Because these are not precisely remote control and signaling circuits, some of the wording is a little different. The Code should be consulted for specific job applications.

  As we have seen, NEC requirements for low-voltage cabling differ from those for power and light wiring. In some instances, the mandates are less stringent, in some instances more stringent, Overall, there is the potential for substantial cost savings without compromising safety if you go by the book.