A Code Guide reader recently sent a list of various types of instrumentation circuits and asked how the circuits should be classified. He wanted to know if the various circuit types were Class 1, Class 2, Class 3, or some other classification covered by the NEC, or if the NEC covered the circuits at all. The classification of instrumentation circuits and other remote control, signaling, and power limited circuits is a common source of problems for users of the NEC. This Article will attempt to clarify some of the confusion that surrounds Article 725.
We should first establish what Article 725 is about and what it is not about. For instance, a common misconception about Article 725 is that it allows CL2 or similar "thermostat wire" or "bell wire" to be used on "low voltage" circuits. (In this context, "low voltage" is usually taken to mean less than 50 V or less than 100 V. This definition of low voltage is not universal. For example, with regard to switchgear, low voltage often means less than 1000 V.) Article 725 is not just about low voltage, nor is it the only article concerned with low voltage circuits.
A number of articles cover low voltage circuits and equipment. Article 720 covers circuits and equipment that operate at less than 50 V. Section 720-1, Exception lists other articles that contain provisions for low-voltage circuits and equipment. Articles 411, 551, 650, 669, 690, 725, and 760 are mentioned. These articles cover low voltage lighting, recreational vehicles, pipe organs, electroplating, solar photovoltaic systems, remote control and signaling, and fire alarm systems, respectively. Other applications of what may be low voltage equipment and circuits are covered by articles 504, 640, 727, 770, 780, 800, 810, and 820. These articles cover intrinsically safe systems, sound recording, instrumentation tray cable, optical fiber cables, closed-loop power distribution, communications, radio and television, and community antenna TV and radio.
Of the many articles listed above, a few articles other than Article 725 may apply to instrumentation and control circuits. In particular, Article 504 covering intrinsically safe circuits, Article 727 covering instrumentation tray cable, Article 770 covering fiber-optic cables, and Article 800 covering communications, often apply to some parts of instrumentation installations.
So what is Article 725 really about? The title of Article 725 is "Class 1, Class 2, and Class 3 Remote-Control, Signaling, and Power-Limited Circuits." Section 725-1 says Article 725 covers such circuits where they "are not an integral part of a device or appliance." Certainly the description "remote-control, signaling, and power-limited" covers most instrumentation and control circuits. However, some such systems, such as those using optical-fiber cable, are not Class 1, Class 2, or Class 3 circuits.
An important distinction to be made about circuits covered by Article 725 is given in the Fine Print Note to Section 725-1. Remote control, signaling, and power-limited circuits are quite different from power and lighting circuits. Because of the limited loads and limited power sources involved, Article 725 offers modifications of the general rules of Chapters 1 through 4. As noted in Section 90-3, Chapter 7 provides rules for Special Conditions which modify the usual rules. Article 725 allows for special wiring methods,different wire sizes and insulations, and distinct rules about such things as overcurrent protection and derating factors. In turn, Article 725 also restricts the intermixing of remote control, signaling, and power limited circuits with power and lighting circuits.
Class 1, Class 2, and Class 3 circuits are differentiated from each other by power limitations. Section 725-2 provides definitions of Class 1, 2 and 3 circuits. Class 1 circuits may or may not be supplied by power-limited sources. Class 2 circuits are limited to voltage and current values that will not usually present a shock or fire hazard. Class 3 circuits are allowed to have higher of voltage and current than Class 2 circuits. The voltage and current levels of Class 3 circuits may present a shock hazard but generally do not present a fire hazard.
Section 725-21 says Class 1 circuits may be either Class 1 power-limited circuits or Class 1 remote-control and signaling circuits. Notice that Class 1 power-limited circuits are restricted to not over 30 V and not over 1000 VA. The power limitation may be provided by overcurrent devices in conjunction with transformers or other power sources. Class 1 remote-control and signaling circuits are not required to be supplied by power-limited sources. Remote-control and signaling circuits are differentiated only by use. This section does not say that Class 1 power-limited circuits cannot be used for remote-control or signaling. Rather, it says that Class 1 circuits used for remote-control or signaling are not required to be power-limited.
All Class 2 and Class 3 circuits are limited energy circuits. As mentioned previously, Class 2 circuits are required to be limited to lower levels of power than Class 3 circuits. The voltage and current limitations permitted and required for Class 2 and Class 3 circuits are given in Tables 11(a) and 11(b) in Chapter 9. However, the values given in the tables are for listing purposes only and are not intended for use with field-constructed power supplies. Except for certain dry cell batteries and thermocouples, Section 725-41 requires all Class 2 and Class 3 power supplies to be listed or to be derived from other listed equipment. This is a very important point. Section 725-41 was revised significantly in the 1996 NEC in an attempt to put an end to field designs and alterations of power supplies. Tables 11(a) and 11(b) were moved from Article 725 to Chapter 9 to emphasize their applicability only to listing and to remove them as far as possible from the installation requirements.
Prior to 1996, Section 725-31 (now 725-41) simply referred to the tables of power source limitations [formerly 725-31(a) and (b)] for classification of Class 2 and Class 3 circuits. These tables were commonly interpreted to allow field construction of power supplies to meet Class 2 and 3 power limitations. However, the notes to the power limitation tables also included some information about testing of the power supplies. The testing requirements go well beyond the capabilities of most code users and can only be adequately demonstrated through the listing process. Many control and instrumentation systems exist that were wired as Class 2 or Class 3 circuits and supplied by power supplies that seem to meet the power limitations but are not listed or properly tested.
Section 725-41 now requires listed power sources in most cases, but it also provides information on how some common circuits are to be classified. 725-41(a)(1) and (2) simply require transformers or power supplies to be listed as Class 2 or Class 3. 725-41(a)(3) and (4) provide for other acceptable sources. Fine Print Notes that are new in the 1996 code also provide examples of common circuit types that are considered to be Class 2 or Class 3. For example, a circuit card in a controller or PLC may be listed as a Class 2 or Class 3 source. Also, data circuits used to interconnect computers are typically treated as Class 2 circuits. Such circuits are sometimes supplied by equipment marked "Listed ITE Power Supply." ("ITE" stands for Information Technology Equipment.)
We have seen that a Class 1 circuit may be classified as power-limited or remote-control and signaling, based on use and power limitations. Class 2 and Class 3 circuits are classified based on the power source. In general, Class 2 and Class 3 circuits must be supplied by listed transformers or power supplies. Listed equipment that contains a listed power source and is so identified or listed information technology equipment with limited power circuits may also supply Class 2 or Class 3 circuits.
As mentioned above, circuits that meet the requirements of Class 1, Class 2, or Class 3 circuits may be treated differently from power and lighting circuits. Special wiring methods, conductors sizes, and overcurrent protection requirements may apply. In addition, restrictions may be placed on circuits using the special rules. We will examine the rules for Class 1 circuits first, followed by the rules for Class 2 and Class 3 circuits.
Class 1 methods and materials
Class 1 circuits are required to be installed using ordinary wiring methods of Chapter 3 in accordance with Section 725-25. This means that ordinary raceways and wire or cable assemblies such as NM cable, MC cable, or AC cable must be used. However, smaller conductors are permitted. Conductors are allowed to be No. 16 or No. 18 protected by 10 A and 7 A overcurrent devices respectively. In addition, according to 725-23 Exception No. 3(725-24 Exception No. 4 in 1999), No. 14 and larger conductors may be protected by overcurrent devices rated at up to 300% of the conductor rating. Because loads are relatively small and overloading is unlikely, the required overcurrent protection is essentially only short-circuit and ground-fault protection. These overcurrent protection requirements nearly match the requirements for similar motor control circuits as covered in Section 430-72 Exception No. 2. For sizes No. 16 and No. 18, fixture wires are permitted to be used for Class 1 circuits. The permissible insulation types are listed in Section 725-27.
When more than three Class 1 circuit conductors or power and lighting conductors are present in a raceway, Section 725-28 allows the derating factors of Note 8 to the ampacity tables of Article 310 to be disregarded for Class 1 circuit conductors as long as the load on the conductors does not exceed 10% of the ampacity of the Class 1 conductors. For example, imagine a three wire control circuit for a motor that is derived from a central control panel rather than being tapped from the motor branch circuit. The three Class 1 conductors may share a raceway with the branch circuit power conductors to the motor. If the control circuit current is 1 A and the conductors are No. 16, the control conductors need not be counted in applying Note 8 to the ampacity of the power conductors.
As noted above, conductors of Class 1 circuits are permitted to share a raceway or other enclosure with power or lighting circuit conductors only where the Class 1 circuit is functionally associated with the powered equipment. This permission is found in Section 725-26. Exceptions apply to control centers and underground installations.
If a Class 1 circuit is limited to 150 V and 5 A, and the installation is in an industrial establishment maintained and supervised by qualified persons, Type ITC (Instrumentation Tray Cable) may be used as a wiring method. Type ITC cable is not permitted to be installed with power, lighting, or nonpower-limited circuits. Article 727 covers Type ITC. Article 727 is another article that modifies the provisions of Chapters 1 through 4. It permits a wiring method that would not be appropriate in other circumstances.
Class 1 conductors that extend beyond one building are required by Section 725-29 to meet the requirements for outside branch circuits as provided in Article 225. Article 225 covers such things as clearances from ground or roofs, wiring methods, circuit entries to buildings, and so on.
Class 2 and Class 3 methods and materials
Unlike Class 1 circuits, Class 2 and Class 3 circuits are permitted to utilize special wiring methods in lieu of the usual methods of Chapter 3. This general permission is found in Section 725-52. The specific methods are listed in Sections 725-61 and 725-71. Section 725-51 and Figure 725-41 provide a reminder that Class 2 or 3 circuits begin at the output of the Class 2 or 3 power source and that the supply side of the power supply is still an ordinary circuit that requires ordinary wiring methods. Separate overcurrent protection is not required for Class 2 and 3 circuits because of the power-limited sources.
The wiring methods of Sections 725-61 and 725-71 are special cable types. Section 725-61 lists the cable types and their permitted uses along with acceptable substitutions. For instance, type CL3 cable may be used as CL2 cable, but the reverse is not permitted. As another example, Type PLTC may be used for any Class 2 or 3 circuit except for riser and plenum applications. Listing and marking requirements for the various cable types are given in 725-71. (Incidentally, proposals were accepted for the 1999 code that dramatically simplify the substitution table, reduce the number of permitted substitutions, and eliminate Figure 725-61, the Cable Substitution Hierarchy illustration.)
Class 2 and Class 3 cable types differ from ordinary conductors and wiring methods in the size of the wires and the insulation ratings. Most ordinary conductors and wiring methods have 600 V or higher rated insulation. Class 3 cables are rated at 300 V. Class 2 cables may be rated less than 300 V. Class 2 and Class 3 cables usually use conductors sized No. 14 or smaller, size Nos. 18, 20, and 22 being quite common.
Separation from power and lighting circuits is required for Class 2 and Class 3 circuits. Class 2 and 3 circuits are not permitted in the same raceway or enclosure with power, lighting, Class 1 or nonpower-limited fire alarm circuits. Exceptions provide for cases where the Class 2 and 3 conductors are separated by barriers, installed in separate raceways, or other separation is provided. Outside enclosures or raceways, a separation of at least two inches is generally required between Class 2 or Class 3 circuits and nonpower-limited circuits.
Class 2 and Class 3 circuits are permitted in the same enclosures or raceways with other power-limited circuits, communications circuits, or jacketed cables of some other systems. Some restrictions are provided. For example, Class 2 circuits may be mixed with Class 3 circuits only if the insulation on the Class 2 circuits is equal to that required for the Class 3 circuits. If Class 2 or 3 circuits are mixed with communications circuits, the Class 2 or 3 circuits must be reclassified as communications circuits. In addition, Class 2 and 3 circuits that extend beyond one building may be required to meet certain requirements of Article 800. The restrictions on mixing of Class 2 and Class 3 circuits with circuits of other systems are found in Section 725-54.
The required separation of Class 2 and Class 3 circuits from other circuits is commonly violated. Often such violations are justified on the basis of Section 300-3(c)(1). This section permits circuits of different systems to be mixed if all the circuits are 600 V or less and all insulation is equal to the maximum voltage applied to any of the circuits. However, Section 300-1(a) Exception No. 2 prohibits the application of 300-3(c)(1) to Article 725 circuits because 300-3(c)(1) is not referenced in Article 725. (This rule explaining the applicability of Article 300 to Article 725-3 in the 1999 NEC.) A common violation is installing a CL2 cable in a raceway with power conductors supplying rooftop HVAC equipment. Separation is important because inadvertent contact between power-limited and nonpower-limited circuits can bypass the limited energy power supply and subject the power-limited circuit to unsafe voltages and currents.
As noted for Class 1 circuits, Type ITC cable may be used in certain industrial facilities. Type ITC cable is essentially the same as Type PLTC. (Compare the descriptions of Sections 725-71(e) and 727-4.) In fact, a proposal has been accepted for the 1999 code that would permit Type PLTC to be substituted for Type ITC. Thus, under the 1999 NEC, Type PLTC could be used for Class 1 circuits that meet the requirements of Sections 727-2 and 727-3, as well as for Class 2 and Class 3 circuits. In such a case, all three circuit types could be installed in the same cable tray if suitable barriers were provided.
Class 1, Class 2, and Class 3 circuits are distinguished from power and lighting circuits by use and power limitations. Because of the limited power provided by the source, Class 2 and Class 3 circuits can use wiring methods that would not be suitable for other uses. All three classes of circuits can use conductors smaller than would be permitted for branch circuits. Overcurrent protection may not include overload protection, and in the case of Class 2 or 3 circuits overcurrent protection is provided by the power source. Although Class 1 circuits can be produced from ordinary power sources and appropriate overcurrent devices, Class 2 and Class 3 circuits are defined primarily by listed power supplies. Since Class 1 circuits are allowed to use small conductors and different overcurrent protection, they must be kept from contact with power and lighting circuits unless the power and lighting circuits are functionally associated. Finally, because of the differing wiring methods and insulation requirements, Class 2 and Class 3 circuits must be kept separated by distance or barriers from power, lighting or Class 1 circuits.
Examples in instrumentation
Now, back to the original question. Seven different types of instrumentation circuits are described below, along with their likely classifications.
1. 4-20 mA signals derived from 24-28 V dc power supplies. The circuits are typically fused at 0.10 A to 0.25 A. Such circuits may be Class 2 or Class 3, but the classification is dependant on a listed power supply, circuit card, or the like. Circuits installed prior to 1996 may have been determined to be Class 2 or 3 based on the information now in Tables 11(a) or 11(b). Under the current code, without a listed power supply, Class 1 is most appropriate. Some such circuits may be fed through the associated apparatus of intrinsically safe systems, in which case Article 504 would apply.
2. Dc voltage signals of 0-10 V or
0-1 V. These are probably Class 2, but again, the power source determines the classification. The discussion in item 1 applies here as well.
3. 120 V ac discrete voltage signals derived from 15 A or 20 A branch circuits, but fused at 1 ampere or less. These are Class 1 circuits.
4. 24 V dc discrete voltage signals fused at 0.25 A or below. These fall into the same general category and are subject to the same issues as Items 1 and 2.
5. 120 V ac and 24 V dc discrete control signals used to operate relays, solenoids, valves, and so on. Again, these could be Class 2 or 3 circuits with listed power supplies, but the 120 V ac circuits are more likely Class 1.
6. Digital Signals-under 25, V RS-232, 422, and the like. Th@ese are most likely derived from some form of information technology equipment and are likely Class 2, although some proprietary or customized equipment may not carry a listing.
7. Thermocouples. The power source in this case is the bimetallic junction, which is considered to be a Class 2 power source.
In all of the above, cable selection may be a major issue. As noted previously, for large industrial facilities, Types PLTC and ITC may be good choices for Class 2 and 3 and some Class 1 circuits. Class 1 circuits in other occupancies should use ordinary wiring methods, with MC cable being a common choice for flexibility and range of available configurations. CM (communications) or MP (multipurpose) cables may be substituted for Class 2 and 3 cables as permitted in Section 725-61. Type MP cables are being removed from all substitution tables in the 1999 code, but CM types will remain. CM type substitutions canbe found for many other cable types. A look at Figure 725-53 will reveal that with the elimination of the MP family of cables, CM cables will become the "master of substitutions," or the top of the hierarchy. Therefore, CM cable types are also likely choices for Class 2 and 3 circuits that need coaxial or triaxial cable or some other specialized cable.