Published In February 2000
The National Electrical Code (NEC), Class 2 and Class 3 covers remote-control, signaling, and power-limited circuits covered by Chapter 7 in Article 725. They are considered to be a special condition other than a normal power circuit. In Class 2 and Class 3 systems, either the voltage or the current, depending upon circuit design, is limited to a level that will not cause a fire and, under normal installation requirements, would not be a shock hazard. All of this could change if the Class 2 or Class 3 circuits were mixed with normal field-installed power circuits. Section 725-52 requires cables to be installed on the load side of a Class 2 or Class 3 power source. This section also requires that cables used for wiring within buildings be listed as being resistant to the spread of fire. General-purpose cable (CL2 and CL3), riser cable (CL2R and CL3R), plenum cable (CL2P and CL3P), limited-use cable (CL2X and CL3X), and power-limited tray cable (Type PLTC) are the types of cables normally used for wiring Class 2 and Class 3 circuits. While these cables have insulation on the individual conductors and an outside sheath surrounding the internal conductors in the cables, the cables will not normally have a voltage rating marked on the cable jacket. Voltage markings on cables could be misapplied by someone on the field and misused on normal power circuits. Class 3 cables are required by Section 725-71(f) to have a minimum voltage rating of 300 volts, but again, this rating would not be marked on the cable. However, a cable may have voltage markings if it has multiple listings. Even though these multiple-listed cables are marked with a voltage rating, they could be used for Class 2 or Class 3 circuits. Even though these Class 2 and Class 3 conductors and cables often have insulation ratings as high as 300 volts, Section 725-54 does not allow these cables and conductors to be mixed with electric light, power, Class 1 conductors, nonpower-limited fire alarm circuits, or medium-powered broadband circuits. There are exceptions to this, but the general rule is to not mix the various circuits in raceways, cables, cable trays, enclosures, manholes, or boxes. By preventing contact between low voltage power-limited circuits and other higher- voltage circuits, the possibility of a short circuit or inadvertent interconnection to higher-voltage circuits is minimized. For example, if a Class 2 thermostat cable for an air conditioning unit was installed in the same raceway as the branch circuit conductors feeding the unit, a short from one of the power conductors to the low- voltage thermostat conductor could create a dangerous fire and shock hazard. Therefore, thermostat cable and the power conductors should be installed in separate raceways terminating at the A/C unit. Within the A/C unit, the thermostat cable has a barrier separating the low-voltage cables from the higher voltage conductors. The thermostat cable does operate a low-voltage coil for a contactor located within the power side of the unit. Section 725-54(a)(1), Exception No. 2 in the NEC allows low-voltage cables and higher-voltage conductors to be in the same enclosure where the higher voltage conductors are not greater than 150 volts to ground and are introduced solely to connect to the equipment. Where using this exception, the low-voltage conductors must be installed and insulated as Class 1 conductors. Some installations require that Class 2 or Class 3 circuits enter an enclosure, an outlet box, or certain raceways, such as wireways and surface raceways as covered in Article 352, to enable connecting the circuit to the equipment. These Class 2 or Class 3 conductors or cables must be separated from the power conductors by either a barrier or a raceway within the enclosure. The barrier is normally made of the same material as the enclosure but the NEC does not specify the type of material for the barrier. If an internal raceway is used in the enclosure for routing the low- voltage conductors, it could be a nonmetallic, surface-mounted raceway with solid, enclosed, or slotted sides. The slotted sides would allow individual conductors to be routed out to their connection points within the enclosure. Where routing the low-voltage conductors inside the enclosure, Section 725-54(a)(1), Exception No. 2(a) allows the Class 2 or Class 3 conductors to be routed to maintain a permanent quarter-inch separation between the low-voltage and the higher-voltage conductors. In an application where there is only a single knockout entrance into an enclosure, barriers, quarter-inch separation, or interior-mounted raceways are not possible since both power and low-voltage control conductors must enter the enclosure. Section 725-54(a)(1), Exception No. 3 allows a continuous length of nonconductive tubing to be installed around the low-voltage conductors so it provides a permanent separation of the two systems. In summary, most Class 2 and Class 3 systems must be isolated from power circuits to protect the low-voltage system. Mixing power circuits and power-limited systems should only be considered by carefully following the limited number of exceptions in the NEC. In all other cases, strict separation of the two systems is required. ODE is a staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at email@example.com.