Ecmweb 17163 Nec Code Quandaries 1117 Pr
Ecmweb 17163 Nec Code Quandaries 1117 Pr
Ecmweb 17163 Nec Code Quandaries 1117 Pr
Ecmweb 17163 Nec Code Quandaries 1117 Pr
Ecmweb 17163 Nec Code Quandaries 1117 Pr

Stumped by the Code? Rules Regarding Wiring Installed in Ducts Fabricated for Environmental Air

Nov. 20, 2017
Your most pressing National Electrical Code (NEC) questions answered

All questions and answers are based on the 2017 NEC.

Underlined text indicates a Code change.

Q. What are the Code rules regarding wiring installed in ducts specifically fabricated for environmental air?

A. Equipment is only permitted within the duct specifically fabricated to transport environmental air if necessary for the direct action upon, or sensing of, the contained air. Equipment, devices, and/or illumination are only permitted to be installed in the duct if necessary to facilitate maintenance and repair. Luminaires must be of the enclosed gasketed-type [300.22(B)].

Permitted wiring methods include Type MC cable that has a smooth or corrugated impervious metal sheath without an overall nonmetallic covering, electrical metallic tubing (EMT), flexible metallic tubing (FMT), intermediate metal conduit (IMC), or rigid metal conduit (RMC) without an overall nonmetallic covering can be installed in ducts specifically fabricated to transport environmental air. Flexible metal conduit (FMC) in lengths not exceeding 4 ft can be used to connect physically adjustable equipment and devices within the fabricated duct.

Exception: Wiring methods and cables listed for plenum spaces can be installed in ducts specifically fabricated for environmental air‑handling purposes under the following conditions:

(1) The wiring method or cabling is necessary to connect to equipment or devices associated with the direct action upon or sensing of the contained air, and

(2) The total length of such wiring method or cabling doesn’t exceed 4 ft.

Class 2 and Class 3 cables selected in accordance with Table 725.154 and installed in accordance with

Sec. 725.135(B) are permitted to be installed in ducts specifically fabricated for environmental air [725.3(C) Exception 1].

Power-limited fire alarm cables selected in accordance with Table 760.154 and installed in accordance with Sec. 760.135(B) are permitted to be installed in ducts specifically fabricated for environmental air [760.3(B) Exception 1].

Q. What are the Code rules regarding wiring installed in plenum spaces, such as above suspended ceilings or below a raised floor used for environmental air?

A. Section 300.22(C) applies to spaces not specifically fabricated for environmental air-handling but used for air-handling as a plenum (for example, the space above a suspended ceiling or below a raised floor used for environmental air). It does not apply to habitable rooms or areas of buildings, the prime purpose of which is not air handling.

Informational Note 1: The space or cavity above a suspended ceiling or below a raised floor used for environmental air are examples of the type of plenum space to which this Section applies.

Exception: In a dwelling unit, this section of the Code doesn’t apply to the space between joists or studs where the wiring passes through that space perpendicular to the long dimension of that space (Fig. 1).

Wiring methods permitted in plenum spaces include electrical metallic tubing (EMT), rigid metal conduit (RMC), intermediate metal conduit (IMC), armored cable, metal-clad cable (MC cable) without a nonmetallic cover, and flexible metal conduit (FMC). Surface metal raceways or metal wireways with metal covers can also be installed in a plenum space [300.22(C)(1)].

Cable ties for securing and supporting must be listed for use in a plenum space.

PVC conduit [Art. 352], electrical nonmetallic tubing (ENT) [Art. 362], liquidtight flexible conduit (LFC), and nonmetallic cables aren’t permitted to be installed in plenum spaces because they give off deadly toxic fumes when burned or superheated.

Plenum-rated control, signaling, and communications cables and raceways are permitted in plenum spaces according to the following:

• Communications [Sec. 800.3(C) and Table 800.154(a)]

• Control and Signaling (Sec. 725.3(C), Exception No. 2 and Table 725.154]

• Coaxial Cable [Sec. 820.3(C) and Table 820.154(a)]

• Fire Alarm Sec. 760.3(C), Exception No. 2 and Table 760.154]

• Optical Fiber Cables and Raceways [Table 770.154(a)]

• Sound Systems [Sec. 640.9(C) and Table 725.154]

Any wiring method suitable for the condition can be used in a space not used for environmental air-handling purposes.

Metal cable tray systems can be installed to support the wiring methods and equipment permitted to be installed in a plenum space [300.22(C)(2)(a)].

Electrical equipment with a metal enclosure can be installed in a plenum space [300.22(C)(3)]. Examples of electrical equipment permitted in plenum spaces are air‑handlers, junction boxes, and dry‑type transformers; however, transformers must not be rated over 50kVA when located in hollow spaces [450.13(B)].

Wiring methods beneath raised floors for information technology equipment can be installed as permitted in Art. 645 [300.22(D)].

Q. What are the NEC requirements when installing splices, taps, and feed-through conductors in a cabinet?

A. A cabinet is an enclosure designed for surface or flush mounting that has a frame, mat or trim in which a swinging door or doors can be hung [Art. 100]. Cabinets and cutout boxes are often used to enclose panelboards, switches, meters, current transformers, and control equipment. Cabinets are permitted to contain overcurrent protection devices and other wiring and equipment as provided in Sec. 312.8(A) and (B).

The wiring space within cabinets can be used for conductors feeding through, spliced, or tapping where all of the following conditions are met [312.8(A)]:

(1) The area of all conductors at any cross section doesn’t exceed 40% of the cross‑sectional area of the space.

(2) The area of all conductors, splices, and taps installed at any cross section doesn’t exceed 75% of the cross-sectional area of that space (Fig. 2).

(3) A permanently affixed warning label having sufficient durability to withstand the environment involved and comply with Sec. 110.21(B) must be applied on the cabinet to identify the location of the disconnecting means for the feed‑through conductors.

Q. What NEC conditions must be met to install power monitoring equipment within cabinets?

A. The wiring space within cabinets can contain power monitoring equipment where all of the following conditions are met [312.8(B)]:

(1) The power monitoring equipment is identified as a field installable accessory as part of listed equipment, or a listed kit for field installation in the overcurrent protection device enclosures.

(2) The area of all conductors, splices, taps, and equipment at any cross section does not exceed 75% of the cross-sectional area of that space.

Holt is the owner of Mike Holt Enterprises, Inc. in Leesburg, Fla. He can be reached at www.mikeholt.com.
About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

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