When you install low-voltage or limited-energy systems, understanding a few key terms is essential.
Anyone installing power distribution systems now or planning to do so in the future should know the related NEC rules. Last month, we looked at the definitions that form the basis of what I call “technology wiring,” including “bonded,” “bonding jumper,” and “Class 3 circuit.” Building from that discussion, let’s move forward and address a new group of terms important to anyone involved in power-distribution system installation.
Grounded. The NEC defines this as, “connected to earth or to some conducting body that serves in place of the earth.” Metal parts of the electrical system are grounded to the earth to prevent the destruction of electrical components, as well as electric shock that can occur from superimposed voltage from lightning and voltage transients [Sec. 250-2(b)]. In addition, earth grounding of metal parts can help prevent the buildup of static charges on metal equipment and material.
Intrinsically safe systems (Art. 504). This is an assembly of interconnected intrinsically safe apparatus, associated apparatus, and interconnecting cables that may be used in hazardous (classified) locations. An intrinsically safe circuit is a circuit in which any spark or thermal effect is incapable of causing ignition of a mixture of flammable or combustible material in air under prescribed test conditions.
Intrinsically safe systems and devices such as switches, thermocouples, light-emitting diodes, connectors, and resistance temperature devices limit spark or thermal temperatures to a level that prevents ignition of flammable or combustible material.
You can wire and install intrinsically safe apparatus using exposed wiring techniques outlined in Chapter 7 and Chapter 8. This would include cables such as CL2 (Class 2), CL3 (Class 3), MP (multipurpose coaxial), or PLTC (power-limited tray cable) [Sec. 725-61].
Low-voltage circuit, less than 50V (Art. 720). A low-voltage circuit that falls within the scope of Art. 720 operates at less than 50V, and has a power supply rating of less than 1000VA. Circuits operating at less than 30V and less than 1000VA are Class 1 circuits.
Chapters 1 through 4 list acceptable methods for wiring that falls within the scope of Art. 720, except that the minimum conductor size is No. 12 (Sec. 720-4). Based on this requirement, you must install 600V insulated conductors in accordance with a Chapter 3 wiring method, install splices in outlet boxes (Sec. 300-15), and provide overcurrent protection as specified in Sec. 240-3.
Motor control circuit. This is the circuit that carries the electric signals directing the performance of a motor controller (Sec. 430-71). Motor control circuit conductors are tapped to the motor branch-circuit supply conductors. The overcurrent for these conductors must accord with Table 430-72(b).
Network-powered broadband communications systems (Art. 830). Network-powered broadband communications systems provide voice, audio, video, data, and interactive services through a network interface unit (NIU). An example of a network-powered broadband communications system would be hybrid fiber-coaxial (HFC) cable used for either video/audio conferencing or interactive multimedia entertainment systems.
The 1999 Code allows two classifications of network-powered broadband communications system circuits. Both types involve some risk of electric shock. The intent of Art. 830 is that the classification limits (together with wiring methods and mechanical protection) should result in an installation equivalent in safety to those now permitted in the NEC.
Low-power circuits. Low-power circuits are essentially the same as “Not Inherently Limited Class 3 circuits up to 100V and 100VA.” These circuits are intended to power one NIU installed in a single-family residence.
Prior to Jan. 1, 2000, you could use existing coaxial cable types for low-power broadband systems. Today, you can use only listed Type-BL (broadband low-power), BM (broadband medium-power), CM, or MP coaxial cables.
Medium-power circuits. Medium-power circuits are similar to “Class 3 circuits up to 150V and 100VA.” These circuits are intended to provide power for multiple NIUs or a single NIU with expanded capabilities. The circuit voltage of 150V permits greater distances between supply locations (or longer transmission lines). You can use only listed BM, CM, and MP coaxial cables for these applications.
Optical-fiber cables and raceways (Art. 770). Art. 770 covers optical-fiber cables and raceways used to transmit light for control, signaling, and communications. Optical-fiber cable does not carry power or voltage; therefore, you can install the non-conductive optical-cable cable with power conductors, or with other low-voltage or limited-energy circuits (Sec. 770-52). Optical-fiber cables must be marked “OFC.”
Plenum cable. Plenum cable is any technology-wiring cable listed as being suitable for use in ducts, plenums, and other spaces used for environmental air. It must also be listed as having adequate fire-resistant and low smoke-producing characteristics. You can install plenum cable exposed in spaces used to move environmental air, such as the space above a suspended ceiling for return air [Sec. 300-22(c)].
Radio and television equipment (Art. 810). Art. 810 covers antenna systems for radio and television receiving equipment, and amateur radio transmitting and receiving equipment. This article includes antennas such as multielement, vertical-rod, and dish, as well as wiring and cabling that connects them to the receiving equipment.
Type-CATV. A community antenna and television system or CM (Sec. 820-53) installed in accordance with Art. 820 (Sec. 810-2) or Type-OF optical-fiber cable installed in accordance with Art. 770.
Telecommunications (telephone) circuits (Art. 800). Art. 800 covers the installation requirements for telephones and wiring for other related telecommunications purposes, such as phone lines to computers and fire- and burglar-alarm systems connected to central stations.
Because the incoming ringing voltage from the telephone central office to the premises is 70V to 105V (20 Hz to 30 Hz), telecommunications wires must have a voltage rating of not less than 300V (Secs. 800-50 and 800-51). Cables that meet this requirement are marked CM (communications) or MP (multipurpose).