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 (G)
Therefore, Only
those sections of Article 300 referenced in Article 725.3 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?
The general consensus on this is that where Class 2 conductors are
installed in a raceway the rules for that particular raceway apply.
Sections that Apply
(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
be removed. Abandoned Class 2, Class 3, and PLTC cable is defined as that
cable that is not terminated at equipment and not identified for future
use with a tag.
(C) Ducts, Plenums, and Other Air-Handling Spaces. Class 1, Class 2, and
Class 3 circuits installed in ducts, plenums, or other space used for
environmental air shall comply with 300.22. Type CL2P or CL3P cables and
plenum signaling raceways shall be permitted for Class 2 and Class 3
circuits installed in other spaces used for environmental air.
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).
(G) Cable Trays. Article 392, where installed in cable tray.
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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
alerted. This usually leads to an investigation to find why the smaller
wire sizes are being used and many cases leads to a citation for installing
Class 2 circuit conductors in the same enclosure or raceway as power and
lighting or Class 1 circuits.
Also, the listing for the location where the cables are used is a requirement
as stated in Section 310.8 (C). If class 2 cables are used out of doors
in raceways in a wet location, in masonry, or in a concrete slab, or underground they must be listed for wet locations. 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. The 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). Additionally, audio system cables
are not allowed in the same raceway as Class 2 conductors (725.56(F)).
Power supplies for Class 2 circuits shall 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.
Class 2 cables are often used for controls and signaling in nonhazardous
locations
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