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 only
those sections of Article 300 referenced in Section 725.3 shall apply to Class
1, Class 2, and Class 3 circuits. 725.3 subsections (A) through (J)
identify the applicable sections of Article 300 that apply.
Which NEC Raceway 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, boxes, and auxiliary gutters etc. the rules for the
respective Article apply.
Sections and Articles that Apply to Control Wiring
(A) Number and Size of Conductors in Raceway. Section 300.17.
This means that for over three conductors in a raceway we are limited to
40 percent fill.
(B) Spread of Fire or Products of Combustion. Section 300.21. This
section addresses fire stopping where electrical raceways or cable
penetrate fire barriers. Openings around electrical penetrations
into or through fire-resistant-rated walls, partitions, floors, or
ceilings shall be fire stopped using approved methods to maintain the
fire resistance rating.
(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. Factory-assembled multi
conductor control or power cable specifically listed for use in in an air
handling space is permitted.
D) Hazardous (Classified) Locations. Articles 500 through
516 and Article 517, Part IV, where installed in hazardous (classified) locations.
This is a big one. When flammable gases or vapors are
encountered or combustible dusts such as coal dust, or combustible
filings such as aluminum or magnesium are encountered special wiring
methods have to be used to prevent ignition, spread of fire and products
of combustion, and transmitting of flammable vapors or dusts from one
area to another. A low power, low voltage Class 2 circuit can
provide enough energy to cause an explosion in these areas.
Many occupancies are classified as hazardous locations by the NEC such as
commercial garages, paint spray booths, anesthetizing areas in hospitals,
aircraft hangers, and bulk storage plants. Surprisingly, garages where
only diesel equipment is repaired is not classified as a commercial garage
because diesel has a flash point above 100 degrees F. while areas where JP4,
a military fuel, is handled is classified because JP4 has a flashpoint below
100 degrees F.
(E) Cable Trays. Article 392, where installed in cable tray.
The rules here cover bonding of cable trays and cable tray fill.
(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). Motor control circuits are special and have their own
sections in the motor Article.
(G) Instrumentation Cable shall comply with Article 727.
(H) Raceways exposed to different temperatures shall comply with
300.7(A). This section requires sealing of raceways to prevent the
circulation of warm air to colder sections of a raceway or sleeve especially
where a raceway or sleeve passes from the interior to the exterior of a
building.. Also expansion fittings may be required to compensate for
thermal expansion and contraction.
(I) Vertical Support for Fire Rated Cables and Conductors shall comply
with 300.19.
(J) A bushing shall be installed where cables emerge from a raceway
used for mechanical support or protection in accordance with 300.15(C)
Also, Section 725.25 located in Part 1 of Article 725 requires that 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.
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General Considerations
In general power circuits are wired using a minimum size conductor of No. 14
copper although this requirement has been removed from the Code. 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 310.10. Per 310.10(C) and the definition of Location,
Wet in Article 100 cables used out of doors
in raceways in a wet location, in masonry, or in a concrete slab, or underground 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 over 100 watts of power is delivered to the speakers the power
exceeds the maximum rating for a Class 2 100 VA power supply as given in
either Table Chapter 9 11(A) or 11(B). Additionally, audio system cables
are not allowed in the same raceway or cable as Class 2 conductors (725.139(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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