Class 2 and 3 circuits are defined as the portion of the wiring system between the power source and the connected equipment. Because of the power limitations of Class 2 circuits, many consider them to be safe from a fire initiation standpoint and to provide an acceptable level of protection from electrical shock. Class 3 circuits limit the output power to a level that usually will not initiate fires. But, they can and do operate at higher voltage levels and, therefore, can present a shock hazard.
Class 2 and 3 circuits
Class 2 circuits power temperature controls, doorbells, door openers, lighting controls, irrigation controls, communications accessories, etc., in many types of occupancies. Class 3 circuits are not as common but are occasionally used to power equipment that requires more energy than a Class 2 power source delivers. For example, a Class 3 circuit normally is used in home theater and sound systems. Such circuits also show up in commercial sound and public address systems as well as central fire and security systems. Class 2 and 3 circuit power sources must be listed and marked to indicate their class of supply and electrical rating.
The National Electrical Code (NEC) defines these circuits based on their power supplies, which limit the total energy to a defined maximum value that will not be exceeded even should a short-circuit occur on the load side of the power supply. For information purposes and not for design criteria nor for field-constructed installed power sources, the actual permitted values of voltage and current that Class 2 and 3 circuits are allowed to deliver can be found in Table 11(A) for AC systems and Table (B) for DC systems in Chapter 9 of the NEC.
In some installations, it may be necessary to reclassify a Class 2 or Class 3 circuit and install it as a Class 1 circuit. Sections 725.8 and 725.52(A), Example 2, outline the conditions and requirements permitting reclassification. Installers must employ an ordinary wiring method that provides protection of circuit conductors when reclassification is done. For example, when the failure of a circuit component can create a significant hazard, such as an explosion or fire, a Class 1 circuit, and not a Class 2 or 3 circuit, must be used. Bottom line, a component failure that is capable of producing a direct hazard is required to be installed in conduit or be provided with physical protection. Think about it: If a control circuit used to monitor room temperature fails to operate either open or closed, the results may cause discomfort but certainly do not create a hazardous condition. However, the authority having jurisdiction would certainly consider a circuit failure to a nurse call system in a hospital to be deemed a hazardous condition.
Class 2 and 3 circuit conductors
Three choices for selecting wiring methods are as follows:
1. The circuits may or may not need to be reclassified. But, when they are, they must be wired using Class 1 wiring methods as outlined in 725.52(A), Example 2 and 725.42.
2. Specifically listed for the circuit type
3. Substitute cables for the circuit type
The minimum size circuit is 18 AWG when a Class 2 or 3 circuit is reclassified and used as a Class 1 circuit. But remember, most Class 2 or 3 circuits usually are wired using listed cable types specified by the NEC. UL 13, Standard for Safety for Power-Limited Circuit Cables, which lists 30 AWG as the smallest for Class 2 and 24 AWG for Class 3 circuits, determines the cable size.
Generally speaking, Class 2 and 3 cables are readily available with conductors in sizes between 24 and 12 AWG, respectively. Class 2 and 3 cables also are available in larger sizes to supply specialized applications such as audio equipment. Many times, a manufacturer will specify a certain size conductor based on the length needed for the circuit. In other words, between source and load, smaller conductors may be used for shorter lengths and larger conductors for longer lengths.
Section 725.51 in the NEC seems to establish the necessary requirements for wiring on the supply side of Class 2 and 3 power sources. The overcurrent protection device must be 20 amps or less and be installed ahead of the power source. Under certain conditions, the example of 725.51 permits the input leads of a transformer to be 18 or 14 AWG.
In next month’s Code Comments, look for more on the topic of Class 2 and 3 circuits. EC
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.