Ducts, Plenums, and Air-Handling Spaces
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The designations "National Electrical Code” and “NEC” refer to the
National Electrical Code®, which is a registered
trademark of the
National Fire Protection Association.
Top 50 NEC Rules
Ducts, Plenums, and Air-Handling Spaces
By Mike Holt
Ducts that transport dust, loose stock, or vapors must
not have any wiring method installed within them. What about ducts or
plenums used for environmental air?
Where necessary for the direct action upon, or sensing of, the
contained air, Type MI cable, Type MC cable that has a smooth or
corrugated impervious metal sheath without an overall nonmetallic
covering, electrical metallic tubing, flexible metallic tubing,
intermediate metal conduit, or rigid metal conduit without an overall
nonmetallic covering can be installed in ducts or plenums specifically
fabricated to transport environmental air.
Flexible metal conduit in lengths not exceeding 4 feet can be used
connect physically adjustable equipment and devices, provided any
openings are effectively closed.
Where equipment or devices are installed and illumination is
necessary to facilitate maintenance and repair, enclosed gasketed-type
luminaires are permitted.
What about space used for environmental air? Wiring and equipment in
spaces used for environmental air-handling purposes must comply with
300.22(C)(1) and (2). This requirement doesn't apply to habitable rooms
or areas of buildings, the prime purpose of which isn't air handling.
FPN: The spaces above a suspended ceiling or below a raised
that are used for environmental air are examples of the type of space
which this section applies.
As per 300.22(C)(1), electrical metallic tubing, rigid metal
intermediate metal conduit, armored cable, metal-clad cable without a
nonmetallic cover, and flexible metal conduit can be installed in other
environmental air spaces. Where accessible, surface metal raceways,
metal wireways with metal covers, or solid bottom metal cable tray with
solid metal covers can be installed in other environmental air spaces.
Control, signaling, and communication cables installed in surface
metal raceways, metal wireways with solid metal covers, or solid bottom
metal cable trays with solid metal covers are not required to be
Rigid nonmetallic conduit (Art. 352), electrical nonmetallic tubing
(Art. 362), and nonmetallic cables are not permitted to be installed in
spaces used for environmental air because they give off deadly toxic
fumes when burned or super heated.
However, control, signaling, and communications cables, and
nonmetallic raceways installed in spaces used for environmental air
be plenum rated. A space not used for environmental air-handling
purposes has no wiring method restrictions.
As per 300.22(C)(2), electrical equipment with a metal enclosure is
permitted in other environmental air spaces, unless prohibited
in the Code. For example, dry-type transformers with a metal enclosure,
rated not over 50kVA, can be installed above suspended ceilings used
environmental air [450.13(B)].
What are the requirements when working in an information technology
equipment room? Wiring under a raised floor in an information
room must comply with 645.5(D). Signal and communications cables under
raised floor are not required to be plenum rated [645.5(D)(5)(c)],
because ventilation is restricted to that room/space [645.5(D)(3)].
Editor's note: This information was extracted from Mike
the National Electrical Code
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What's Wrong Here?
By Joe Tedesco
Think you know how this installation violates the
NEC? Visit EC&M's
Web site to see the answer.
Hint: Pull the plug!
By Mike Holt
Q. A conductor passing through a box unbroken
counts as one conductor, so if I loop a conductor through the box
unbroken and strip off enough insulation to terminate on a receptacle
without cutting the wire, does the conductor count as one or two
conductors for box sizing?
Visit EC&M's Web
site to see the answer.
By Steven Owen
Q. A 60-foot metal pole supporting luminaires is
found to be deficient with respect to the 2005 version of the NEC. The
installation consists of the following: The interior of the pole is
being used as a raceway. A properly sized handhole has been provided. A
grounding terminal within the pole has been provided. The size of the
branch-circuit equipment-grounding conductor, which also serves to bond
the metal pole to the metal raceway supporting the branch circuit
conductors, is 12 AWG. The branch circuit rating is 30A. What could
possibly be the deficiency in this particular installation?
- There is no raceway inside the pole.
- The pole does not feature a hinged base.
- There is a lack of vertical conductor support per 300.19.
- Improperly sized equipment grounding and bonding
Web site for the answer and explanation.
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Code News Update
Code Committee Call-Up
Got some extra time on your hands? Looking to put some
of your vast knowledge of the electrical field to use? NFPA is looking
for new members for several of its committees, including the
Anyone interested in serving can download the application form at NFPA's Web
- Committee on Electrical Equipment in Chemical Atmospheres(seeking
members in all categories). This committee is responsible for NFPA 496,
Standard for Purged and Pressurized Enclosures for Electrical
Equipment; NFPA 497, Recommended Practice for the Classification
of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified)
Locations for Electrical Installations in Chemical Process Areas;
and NFPA 499, Recommended Practice for the Classification of
Combustible Dusts and of Hazardous (Classified) Locations for
Installations in Chemical Process Areas
- Committee on Electrical Equipment Evaluation (seeking members in
- Committee on Electrical Systems Maintenance (seeking members in all
categories, except special experts). This committee is responsible for
NFPA 73, Electrical Inspection Code for Existing
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Seminars have been the preferred sources of this information for more
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of our 2008 Code change conferences will receive Professional
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The conferences are also approved by every state that has a continuing
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For additional information on the dates and locations of these
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