Current Affairs: Wiring Basics, Part 2 (June 2000)
Follow Approved Wiring MethodsLow-voltage wiring, like all electrical circuits, must use conductors that are listed for the voltage level and application. Conductors approved for control circuits are identified as type CL2 or CL3, for Class 2 and Class 3 wiring respectively. These conductors may be installed in conduit systems, but may also be exposed in most situations. An approved communications cable is identified as type CM, for cable meeting the requirements of Article 800, or type MP (multipurpose), which also meets the requirements of Article 760 ¿ Fire Alarm Systems.
When CL2, CL3, CM, or MP conductors are installed without conduit, they must be supported from the building structure and installed in a neat and workmanlike manner. They may not simply be draped across ductwork, piping, and other above-ceiling construction, and they should not be hung in vertical runs without support. Article 725 states: ¿Access to equipment shall not be denied by an accumulation of conductors and cables that prevents removal of panels, including suspended ceiling panels.¿ With a single exception, low-voltage wiring may not be supported from or strapped to other conduit systems; Class 2 conductors, solely for the purpose of connection to the control circuits of a piece of equipment, may be supported from a conduit that carries the power supply conductors for that equipment. Circuits in elevator hoistways must be installed in metal conduit.
Power-limited circuits must also be segregated from line-voltage circuits and other circuits that operate at higher voltage and current levels to prevent inadvertent cross-connection of hazardous energy levels. In general, they may not occupy the same conduit, raceway, junction box, or compartment with these other circuits. When required to connect to the same equipment, Class 2 or 3 circuits may enter a common equipment enclosure with, but should be segregated from, higher-power wiring within the enclosure. If Class 2 and Class 3 circuits are installed together in conduits, junction boxes, or compartments, all conductors must be approved for Class 3 application.
Select the Correct Cable TypeBecause the conductors of low-voltage wiring are usually installed without conduit, the contribution of their insulation and jacket materials to the spread of fire and smoke must be limited. This is the basis for distinctions made in the above table.
In general, a cable type may be used in the application shown, as well as any application with less-stringent requirements. For example, plenum application being the most stringent, a type CL2P cable may be used not only in the plenum space, but for vertical risers or general use as well. Similarly, a type CL2 cable may be used in place of CL2X in a dwelling unit or in conduit, but cannot be used for a vertical riser or plenum application. A type CM cable may be used for CL3 or CL2 applications, and a type CL3 cable may be used for CL2 applications.
Power-limited tray cable (PLTC) is provided with a durable jacket capable of withstanding the greater exposure to moisture, chemicals, and physical contact expected in industrial cable tray installations. This ¿heavy-duty¿ construction also makes this cable type suitable for use in locations where an explosion hazard from ignition of flammable vapor, dust, or fiber exists, by providing a higher level of protection of the conductors from physical damage. The insulation level of PLTC cable is adequate for either Class 2 or Class 3 application.
Finally, outdoor applications where cable is exposed require that the jacket be resistant to moisture and degradation from ultraviolet (UV) radiation.
Both the NEC and NRTL standards require cable type designations to be marked on the outer jacket at regular intervals. This makes it relatively easy during construction observation to determine whether the correct cable type has been used.