iring methods are addressed in Chapter 3 of the Code.  Article 300 Scope states all wiring installations are covered unless modified by other articles.  Section 725.3 states that circuits and equipment shall comply with the articles or sections listed in  725.3(A) through (F). Only those sections of Article 300 referenced in this article shall apply to Class 1, Class 2, and Class 3 circuits.   Which NEC Wiring Methods Apply to Low Voltage Wiring?  here has been a controversy about the application of Chapter 3 (where requirements for Raceways and Auxiliary Gutters can be found) to the wiring of Class 2 and Class 3 circuits.  The 24 volts used for a thermostat circuit is usually Class 2 wiring.  The NEC does not require that thermostat wire or bell wire be installed in raceways , boxes, and auxiliary gutters etc. when these conductors are for Class 2 circuits as defined in Article 725.  But are we required to follow the rules in the NEC when Class 2 and Class 3 circuits are  installed in raceway systems which are covered in Chapter 3?  These requirements are addressed in Section 725.3: Circuits and equipment shall comply with the articles or sections listed in  725.3(A) through (F). Only those sections of Article 300 referenced in this article shall apply to Class 1, Class 2, and Class 3 circuits. (A) Number and Size of Conductors in Raceway. Section 300.17. (B) Spread of Fire or Products of Combustion. Section 300.21. The accessible portion of abandoned Class 2, Class 3, and PLTC cables shall not be permitted to remain. (C) Ducts, Plenums, and Other Air-Handling Spaces. Section 300.22 for Class 1, Class 2, and Class 3 circuits installed in ducts, plenums, or other space used for environmental air. Type CL2P or CL3P cables shall be permitted for Class 2 and Class 3 circuits. D) Hazardous (Classified) Locations. Articles 500 through  516 and Article 517, Part IV, where installed in hazardous (classified) locations. (E) Cable Trays. Article 392, where installed in cable tray. (F) Motor Control Circuits. Article 430, Part VI, where tapped from the load side of the motor branch-circuit protective device(s) as specified in 430.72(A).	Some General Considerations Power circuits are wired using a minimum size conductor of No. 14 copper as required by Section 310.4.  There are several exceptions such as using No. 16 for fixture taps.  However, for Class 2, 3 and power-limited circuits No. 16, No. 18 and even No. 22 conductors for Class 2 circuits are often used.  Obviously, there are many economic advantages to using small wire sizes.    However, these advantages are diminished by the separation requirements.  For instance Class 2 circuits cannot be installed in the same cable or raceway as power, Class 1 or power limited circuits.   When an inspector sees a box or raceway filled with both No. 14 or larger wire sizes and No. 16 or No. 18 conductors he is immediately tipped.  This usually leads to an investigation to find why the smaller wire sizes are being used and many cases leads to a citation.  Also, the listing for the location where the cables are used is a requirement as stated in Section 310.8.  If class 2 cables are used out of doors in raceways or in a slab they must be listed for wet locations.  The inside a raceway installed out of doors or in a slab is considered a wet location as stated in the NEC  under the definition of location, wet. It should be noted that in listing cables for wet locations only the wire insulation is tested not the cable sheath.  Direct buried under ground cables have to be identified for direct burial. Tthe direct burial listing tests include a wet location test and a crushing test. A problem with high powered speakers may also disallow Class 2 cables.  If the speakers have over 100 watts of power delivered to them the power exceeds the maximum rating for a Class 2 100 VA power supply as given in either Table Chapter 9 11(A) or 10(B). Power supplies for Class 2 circuits have to be listed as Class 2.  These are usually small transformers like the ones used for boiler thermostat circuits.  Class 2 circuits cannot be derived from a larger transformer such as a 450 VA 120V/24V type by using fuses.  Using a 4 ampere fuse at 24 volts would limit the power to 4 times 24 or 96 volt-amperes, but the fault current is too high and does not conform to the listing tests for Class 2 circuits.   Also, Intrinsically safe circuits are not necessarily Class 2.  The two types of circuits are independent from each other since two separate test are use to list the power supplies.  A Class 2 circuit can be supplied through an intrinsically safe module to make it both Class 2 and intrinsically safe.  This is often done to supply automatic door closure edge switches for commercial garages since the door switch is within 18 inches of the floor in a Class 1, Division 2 area.