WIRING METHODS

A. General Requirements 300-1. Scope. (a) All Wiring Installations. This article covers wiring methods for all wiring installations. Exception No. 1: As permitted in Article 504, Intrinsically Safe Systems. I Exception No. 2: Only those sections referenced in Article 725 shall apply to Class 1, Class 2, and Class 3 circuits. Exception No. 3: Only those sections referenced in Article 760 shall apply to fire protective signaling circuits. Exception No. 4: Only those sections referenced in Article 770 shall apply to optical fiber cables. Exception No. 5: Only those sections referenced in Article 800 shall apply to communications systems. Exception No. 6: Only those sections referenced in Article 810 shall apply to radio and television equipment. Exception No. 7: Only those sections referenced in Article 820 shall apply to community antenna television and radio distribution systems. (b) Integral Parts of Equipment. The provisions of this article are not intended to apply to the conductors that form an integral part of equipment, such as motors, controllers, motor control centers, or factoryassembled control equipment. 300-2. Limitations. (a) Voltage. Wiring methods specified in Chapter 3 shall be used for voltages 600 volts, nominal, or less where not specifically limited in some section of Chapter 3. They shall be permitted for voltages over 600 volts, nominal, where specifically permitted elsewhere in this Code. (b) Temperature. Temperature limitation of conductors shall be in accordance with Section 310-10. 300-3. Conductors. (a) Single Conductors. Single conductors specified in Table 310-13 shall only be permitted to be installed where part of a recognized wiring method of Chapter 3. (b) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the neutral and all equipment grounding conductors shall be contained within the same raceway, cable tray, trench, cable, or cord. 70-144 NATIONAL ELECTRICAL CODE Exception No. 1: The conductors of single-conductor Type MI cable with a nonmagnetic sheath and installed in accordance with Section 330-16 shall be permitted to be run in separate cables. Exception No. 2 to (b): Column type panelboards using auxiliary gutters and pull boxes with neutral terminations. Exception No. 3 to (a) and (b): As permitted in Sections 250-57(b), 250- 79(f), 300-5(i), 300-20(a), 318-8(d), and 339-3(a)(2). (c) Conductors of Different Systems. (1) 600 Volts, Nominal, or Less. Conductors of 600 volts, nominal, or less, alternating-current circuits, and direct-current circuits shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway. Exception: For solar photovoltaic systems in accordance with Section 690-4(b). (FPN): See Section 725-52(a)(2) for Class 2 and Class 3 circuit conductors. (2) Over 600 Volts, Nominal. Conductors of circuits rated over 600 volts, nominal, shall not occupy the same equipment wiring enclosure, cable, or raceway with conductors of circuits rated 600 volts, nominal, or less. (FPN): See Section 300-32, Conductors of Different Systems - over 600 volts, nominal. Exception No. 1: Secondary wiring to electric-discharge lamps of 1000 volts or less, if insulated for the secondary voltage involved, shall be permitted to occupy the same fixture enclosure as the branch-circuit conductors. Exception No. 2: Primary leads of electric-discharge lamp ballasts, insulated for the primary voltage of the ballast, where contained within the individual wiring enclosure, shall be permitted to occupy the same fixture enclosure as the branch-circuit conductors. Exception No. 3: Excitation, control, relay, and ammeter conductors used in connection with any individual motor or starter shall be permitted to occupy the same enclosure as the motor circuit conductors. 300-4. Protection Against Physical Damage. Where subject to physical damage, conductors shall be adequately protected. (a) Cables and Raceways Through Wood Members. (1) Bored Holes. In both exposed and concealed locations, where a cable or raceway-type wiring method is installed through bored holes in joists, rafters, or wood members, holes shall be bored so that the edge of the hole is not less than 1½ inches (31.8 mm) from the nearest edge of the wood member. Where this distance cannot be maintained, the cable or raceway shall be protected from penetration by screws or nails by a steel plate or bushing, at least ¹/₁₆ inch (1.59 mm) thick, and of appropriate length and width installed to cover the area of the wiring. Exception: Raceways as covered in Articles 345, 346, 347, and 348. ARTICLE 300 - WIRING METHODS 70-145 (2) Notches in Wood. Where there is no objection because of weakening the building structure, in both exposed and concealed locations, cables or raceways shall be permitted to be laid in notches in wood studs, joists, rafters, or other wood members where the cable or raceway at those points is protected against nails or screws by a steel plate at least 1/16 inch (1.59 mm) thick installed before the building finish is applied. Exception: Raceways as covered in Articles 345, 346, 347, and 348. (b) Nonmetallic-Sheathed Cables and Electrical Nonmetallic Tubing through Metal Framing Members. (1) Nonmetallic-Sheathed Cable. In both exposed and concealed locations where nonmetallic-sheathed cables pass through either factory or field punched, cut or drilled slots or holes in metal members, the cable shall be protected by bushings or grommets securely fastened in the opening prior to installation of the cable. (2) Nonmetallic-Sheathed Cable and Electrical Nonmetallic Tubing. I Where nails or screws are likely to penetrate nonmetallic-sheathed cable, or electrical nonmetallic tubing, a steel sleeve, steel plate, or steel clip not less than ¹/₁₆ inch (1.59 mm) in thickness shall be used to protect the cable or tubing. (c) Cables through Spaces Behind Panels Designed to Allow Access. Cables, or raceway-type wiring methods, installed behind panels designed to allow access shall be supported according to their applicable articles. (d) Cables and Raceways Parallel to Framing Members. In both exposed and concealed locations, where a cable- or raceway-type wiring method is installed parallel to framing members, such as joists, rafters, or studs, the cable or raceway shall be installed and supported so that the nearest outside surface of the cable or raceway is not less than 1½ inches (31.8 mm) from the nearest edge of the framing member where nails or screws are likely to penetrate. Where this distance cannot be maintained, the cable or raceway shall be protected from penetration by nails or screws by a steel plate, sleeve, or equivalent at least 1/16 inch (1.59 mm) thick. Exception No. 1: Raceways as covered in Articles 345, 346, 347, and 348. Exception No. 2: For concealed work in finished buildings, or finished panels for prefabricated buildings where such supporting is impracticable, it shall be permissible to fish the cables between access points. Exception No. 3: For mobile homes and recreational vehicles. 300-5. Underground Installations. (a) Minimum Cover Requirements. Direct buried cable or conduit or other raceways shall be installed to meet the minimum cover requirements of Table 300-5. (b) Grounding. All underground installations shall be grounded and bonded in accordance with Article 250 of this Code. (c) Underground Cables Under Buildings. Underground cable installed under a building shall be in a raceway that is extended beyond the outside walls of the building. Table 300-5. Minimum Cover Requirements, 0 to 600 Volts, Nominal, Burial in Inches (Cover is defined as the shortest distance measured between a point on the top surface of any direct buried conductor, cable, conduit, or other raceway and the top surface of finished grade, concrete, or similar cover.) 70-146 Type of Wiring Method or Circuit 4 5 3 Residential Branch Circuits for Control Rigid Nonmetallic Circuits Rated of Irrigation and Conduit 120 Volts or less Landscape Lighting 2 Approved for with GFCI Limited to Not More Rigid Metal Direct Burial Protection than 30 Volts and 1 Conduit or without Concrete and Maximum Installed with Type Location of Direct Burial Intermediate Encasement or Overcurrent UF or in Other Wiring Method Cables or Metal Other Approved Protection of Identified Cable or Circuit Conductors Conduit Raceways 20 Amperes or Raceway All Locations Not Specified Below 24 6 18 12 6 In Trench Below 2-Inch Thick Concrete or Equivalent 18 6 12 6 6 Under a Building 0 (In 0 0 0 0 NATIONAL ELECTRICAL CODE Raceway (In (In Only) Raceway Raceway Only) Only) Under Minimum of 4-Inch Thick Concrete Exterior Slab with No Vehicular Traffic and the Slab 18 4 4 6 (Direct 6 (Direct Extending Not Less than 6 Burial) Burial) Inches beyond the 4 (In 4 (In Underground Installation Raceway) Raceway) Table 300-5. (Continued) Under Streets, Highways, Roads, Alleys, Driveways, and Parking Lots 24 24 24 24 24 One- and Two-Family Dwelling Driveways and Outdoor Parking Areas, and Used Only for Dwelling- Related Purposes 18 18 18 12 18 In or under Airport Runways, Including Adjacent Areas Where Trespassing Prohibited 18 18 18 18 18 In Solid Rock Where 2 2 2 2 2 Covered by Minimum of (In (In (In 2 Inches Concrete Raceway Raceway Raceway Extending Down to Rock Only) Only) Only) ARTICLE 300 WIRING METHODS Note 1. For SI units: one inch = 25.4 millimeters Note 2. Raceways approved for burial only where concrete encased shall require concrete envelope not less than 2 inches thick. Note 3. Lesser depths shall be permitted where cables and conductors rise for terminations or splices or where access is otherwise required. Note 4. Where one of the wiring method types listed in columns 1-3 is combined with one of the circuit types in columns 4 and 5, the shallower depth of burial shall be permitted. 70-147 70-148 NATIONAL ELECTRICAL CODE (d) Protection from Damage. Direct buried conductors and cables emerging from the ground shall be protected by enclosures or raceways extending from the minimum cover distance required by Section 300-5(a) below grade to a point at least 8 feet (2.44 m) above finished grade. In no case shall the protection be required to exceed 18 inches (457 mm) below finished grade. Conductors entering a building shall be protected to the point of entrance. Where the enclosure or raceway is subject to physical damage, the conductors shall be installed in rigid metal conduit, intermediate metal conduit, Schedule 80 rigid nonmetallic conduit, or equivalent. (e) Splices and Taps. Direct buried conductors or cables shall be permitted to be spliced or tapped without the use of splice boxes. The splices or taps shall be made by approved methods and with identified materials. (f) Backfill. Backfill containing large rock, paving materials, cinders, large or sharply angular substance, or corrosive material shall not be placed in an excavation where materials may damage raceways, cables, or other substructures or prevent adequate compaction of fill or contribute to corrosion of raceways, cables, or other substructures. Where necessary to prevent physical damage to the raceway or cable, protection shall be provided in the form of granular or selected material, suitable running boards, suitable sleeves, or other approved means. (g) Raceway Seals. Conduits or raceways through which moisture may contact energized live parts shall be sealed or plugged at either or both ends. (FPN): Presence of hazardous gases or vapors may also necessitate sealing of underground conduits or raceways entering buildings. (h) Bushing. A bushing, or terminal fitting, with an integral bushed opening shall be used at the end of a conduit or other raceway that terminates underground where the conductors or cables emerge as a direct burial wiring method. A seal incorporating the physical protection characteristics of a bushing shall be permitted to be used in lieu of a bushing. (i) Single Conductors. All conductors of the same circuit and, where used, the neutral and all equipment grounding conductors shall be installed in the same raceway or shall be installed in close proximity in the same trench. Exception No. 1: Conductors in parallel in raceways shall be permitted, but each raceway shall contain all conductors of the same circuit including grounding conductors. Exception No. 2: Isolated phase installations shall be permitted in nonmetallic raceways in close proximity where conductors are paralleled as permitted in Section 310-4 and the conditions of Section 300-20 are met. 300-6. Protection Against Corrosion. Metal raceways, cable armor, boxes, cable sheathing, cabinets, elbows, couplings, fittings, supports, and support hardware shall be of materials suitable for the environment in which they are to be installed. ARTICLE 300 - WIRING METHODS 70-149 (a) General. Ferrous raceways, cable armor, boxes, cable sheathing, cabinets, metal elbows, couplings, fittings, supports, and support hardware shall be suitably protected against corrosion inside and outside (except threads at joints) by a coating of approved corrosion-resistant material such as zinc, cadmium, or enamel. Where protected from corrosion solely by enamel, they shall not be used out-of-doors or in wet locations as described in (c) below. Where boxes or cabinets have an approved system of organic coatings and are marked "Raintight," "Rainproof" or "Outdoor Type," they shall be permitted out-of-doors. Exception: Threads at joints shall be permitted to be coated with an identified electrically conductive compound. (b) In Concrete or in Direct Contact with the Earth. Ferrous or nonferrous metal raceways, cable armor, boxes, cable sheathing, cabinets, elbows, couplings, fittings, supports, and support hardware shall be permitted to be installed in concrete or in direct contact with the earth, or in areas subject to severe corrosive influences where made of material judged suitable for the condition, or where provided with corrosion protection approved for the condition. (c) Indoor Wet Locations. In portions of dairies, laundries, canneries, and other indoor wet locations, and in locations where walls are frequently washed or where there are surfaces of absorbent materials, such as damp paper or wood, the entire wiring system, where installed exposed, includ- 1 ing all boxes, fittings, conduits, and cable used therewith, shall be mounted so that there is at least ¹/₄-inch (6.35-mm) air space between it and the wall or supporting surface. Exception: Nonmetallic raceways, boxes, and fittings shall be permitted to be installed without the airspace on a concrete, masonry, tile, or similar surface. (FPN): In general, areas where acids and alkali chemicals are handled and stored may present such corrosive conditions, particularly when wet or damp. Severe corrosive conditions may also be present in portions of meat-packing plants, tanneries, glue houses, and some stables; installations immediately adjacent to a seashore and swimming pool areas; areas where chemical de-icers are used; and storage cellars or rooms for hides, casings, fertilizer, salt, and bulk chemicals. 300-7. Raceways Exposed to Different Temperatures. (a) Sealing. Where portions of an interior raceway system are exposed to widely different temperatures, as in refrigerating or cold-storage plants, circulation of air from a warmer to a colder section through the raceway shall be prevented. (b) Expansion Joints. Raceways shall be provided with expansion joints where necessary to compensate for thermal expansion and contraction. 300-8. Installation of Conductors with Other Systems. Raceways or cable trays containing electric conductors shall not contain any pipe, tube, or equal for steam, water, air, gas, drainage, or any service other than electrical. 70-150 NATIONAL ELECTRICAL CODE 300-9. Grounding Metal Enclosures. Metal raceways, boxes, cabinets, cable armor, and fittings shall be grounded as required in Article 250. 300-10. Electrical Continuity of Metal Raceways and Enclosures. Metal raceways, cable armor, and other metal enclosures for conductors shall be metallically joined together into a continuous electric conductor and shall be so connected to all boxes, fittings, and cabinets as to provide effective electrical continuity. Raceways and cable assemblies shall be mechanically secured to boxes, fittings, cabinets, and other enclosures. Exception No. 1: As provided in Section 370-17(c) for nonmetallic boxes. Exception No. 2: As provided in Section 250-33, Exception No. 2 for metal enclosures. Exception No. 3: As provided in Section 250-75, Exception where permitted for reduction of electrical noise. 300-11. Securing and Supporting. (a) Secured in Place. Raceways, cable assemblies, boxes, cabinets, and fittings shall be securely fastened in place. Support wires that do not provide secure support shall not be permitted as the sole support. Branch-circuit wiring associated with equipment that is located within, supported by, or secured to a fire-rated floor or roof/ceiling assembly shall not be secured to the ceiling support wires. Branch-circuit wiring associated with equipment that is located within, supported by, or secured to a non-fire-rated floor or roof/ceiling assembly shall be permitted to be supported by the ceiling support wires. Exception: As permitted elsewhere in this Code. (b) Raceways Used as Means of Support. Raceways shall not be used as a means of support for other raceways, cables, or nonelectric equipment. Exception No. 1: Where the raceways or means of support are identified for the purpose. (FPN): See Article 318 for cable trays. Exception No. 2: Raceways containing power supply conductors for electrically controlled equipment shall be permitted to support Class 2 circuit conductors or cables that are solely for the purpose of connection to the equipment control circuits. Exception No. 3: As permitted in Section 370-23 for boxes or conduit bodies or Section 410-16(f) for fixtures. 300-12. Mechanical Continuity-Raceways and Cables. Metal or nonmetallic raceways, cable armors, and cable sheaths shall be continuous between cabinets, boxes, fittings, or other enclosures or outlets. Exception: Short sections of raceways used to provide support or protection of cable assemblies from physical damage. ARTICLE 300 - WIRING METHODS 70-151 300-13. Mechanical and Electrical Continuity-Conductors (a) General. Conductors in raceways shall be continuous between outlets, devices, etc., and there shall be no splice or tap within a raceway itself. Exception No. 1:- As provided in Section 374-8 for auxiliary gutters. Exception No. 2: As provided in Section 362-7 for wireways. Exception No. 3: As provided in Section 300-15(a), Exception No. 1 for boxes or fittings. Exception No. 4: As provided in Section 352-7 for surface metal raceways. Exception No. 5: Busways as covered in Article 364. (b) Device Removal. In multiwire branch circuits, the continuity of a grounded conductor shall not be dependent upon device connections, such as lampholders, receptacles, etc., where the removal of such devices would interrupt the continuity. 300-14. Length of Free Conductors at Outlets, Junctions, and Switch Points. At least 6 inches (152 mm) of free conductor shall be left at each outlet, junction, and switch point for splices or the connection of fixtures or devices. Exception: Conductors that are not spliced or terminated at the outlet, junction, or switch point. 300-15. Boxes or Fittings-Where Required. (a) Box or Fitting. A box or fitting shall be installed at each conductor splice connection point, outlet, switch point, junction point, or pull point for the connection of conduit, electrical metallic tubing, surface raceway, or other raceways. Exception No. 1: A box or fitting shall not be required for a conductor splice connection in surface raceways, wireways, header-ducts, multioutlet assemblies, auxiliary gutters, cable trays, and conduit bodies complying with Sections 370-16(c) and 370-28 and having removable covers that are accessible after installation. Exception No. 2: As permitted in Section 410-31 where a fixture is used as a raceway. (b) Box Only. A box shall be installed at each conductor splice connection point, outlet, switch point, junction point, or pull point for the connection of Type AC cable, Type MC cable, mineral-insulated, metal-sheathed cable, nonmetallic-sheathed cable, or other cables, at the connection point between any such cable system and a raceway system and at each outlet and switch point for concealed knob-and-tube wiring. Exception No. 1: Where cables enter or exit from conduit or tubing that is used to provide cable support or protection against physical damage. A fitting shall be provided on the end(s) of the conduit or tubing to protect the wires or cables from abrasion. 70-152 NATIONAL ELECTRICAL CODE Exception No. 2: As permitted by Section 336-16 for insulated outlet devices supplied by nonmetallic-sheathed cable. Exception No. 3: Where accessible fittings are used for straight-through splices in mineral-insulated, metal-sheathed cable. Exception No. 4: A wiring device with integral enclosure identified for the use having brackets that securely fasten the device to walls or ceilings of conventional on-site frame construction for use with nonmetallicsheathed cable shall be permitted without a separate box. (FPN): See Sections 336-15, Exception No. 2, 545-10, 550-10(j), and 551-47(e), Exception No. 1. Exception No. 5: Where metallic manufactured wiring systems are used. Exception No. 6: A conduit body shall be permitted in lieu of a box where installed to comply with Section 370-16(c) and Section 370-28. Exception No. 7: Where a device identified and listed as suitable for installation without a box is used with a closed-loop power distribution system. Exception No. 8: A fitting identified for the use shall be permitted in lieu of a box where accessible after installation and where the conductors are not spliced or terminated. (c) Fittings and Connectors. Fittings and connectors shall be used only with the specific wiring methods for which they are designed and listed. (d) Equipment. An integral junction box or wiring compartment as part of listed equipment shall be permitted at an outlet in lieu of a box. 300-16. Raceway or Cable to Open or Concealed Wiring. (a) Box or Fitting. A box or terminal fitting having a separately bushed hole for each conductor shall be used wherever a change is made from conduit, electrical metallic tubing, nonmetallic-sheathed cable, Type AC cable, Type MC cable, or mineral-insulated, metal-sheathed cable and surface raceway wiring to open wiring or to concealed knob-and-tube wiring. A fitting used for this purpose shall contain no taps or splices and shall not be used at fixture outlets. (b) Bushing. A bushing shall be permitted in lieu of a box or terminal fitting at the end of a conduit or electrical metallic tubing where the raceway terminates behind an open (unenclosed) switchboard or at an unenclosed control and similar equipment. The bushing shall be of the insulating type for other than lead-sheathed conductors. 300-17. Number and Size of Conductors in Raceway. The number and size of conductors in any raceway shall not be more than will permit dissipation of the heat and ready installation or withdrawal of the conductors without damage to the conductors or to their insulation. (FPN): See the following sections of this Code: electrical nonmetallic tubing, 331-6; conduit, 345-7 and 346-6; electrical metallic tubing, 348-6; rigid nonmetallic conduit, 347-11; flexible metallic tubing, 349-12; flexible metal conduit, 350-3; liq- ARTICLE 300 - WIRING METHODS 70-153 uidtight flexible metal conduit, 351-6; liquidtight nonmetallic flexible conduit, 351-25; surface raceways, 352-4 and 352-25; underfloor raceways, 354-5; cellular metal floor raceways, 356-5; cellular concrete floor raceways, 358-11; wireways, 362-5; auxiliary gutters, 374-5; fixture wire, 402-7; theaters, 520-5; signs, 600-21(d); elevators, 620-33; sound recording, 640-3 and 640-4; Class 1, Class 2, and Class 3 circuits, Article 725; fire protective signaling circuits, Article 760, and optical fiber cables, Article 770. 300-18. Raceway Installations. Raceways shall be installed complete between outlet, junction, or splicing points prior to the installation of conductors. Exception No. 1: Exposed raceways having a removable cover. Exception No. 2: Where required to facilitate the installation of utilization equipment. Exception No. 3: Prewired assemblies in accordance with Articles 349 and 350. 300-19. Supporting Conductors in Vertical Raceways. (a) Spacing Intervals-Maximum. Conductors in vertical raceways shall be supported if the vertical rise exceeds the values in Table 300-19(a). One cable support shall be provided at the top of the vertical raceway or as close to the top as practical. Intermediate supports shall be provided as necessary to limit supported conductor lengths to not greater than those values specified in Table 300-19(a). Exception: Steel wire armor cable shall be supported at the top of the riser with a cable support that clamps the steel wire armor. A safety device shall be permitted at the lower end of the riser to hold the cable in the event there is slippage of the cable in the wire armored cable support. Additional wedge-type supports shall be permitted to relieve the strain on the equipment terminals caused by expansion of the cable under load. Table 300-19(a). Spacings for Conductor Supports Conductors Support of AWG or Conductors in Aluminum or Circular-Mil Vertical Copper-Clad Size of Wire Raceways Aluminum Copper 18 AWG through 8 AWG Not greater than 100 feet 100 feet 6 AWG through 1/0 AWG " " " 200 feet 100 feet 2/0 AWG through 4/0 AWG " " " 180 feet 80 feet Over 4/0 AWG through 350 kcmil " " " 135 feet 60 feet Over 350 kcmil through 500 kcmil " " " 120 feet 50 feet Over 500 kcmil through 750 kcmil " " " 95 feet 40 feet Over 750 kcmil " " " 85 feet 35 feet For SI units: one foot = 0.3048 meter. 70-154 NATIONAL ELECTRICAL CODE (b) Support Methods. One of the following methods of support shall be used: (1) By clamping devices constructed of or employing insulating wedges inserted in the ends of the conduits. Where clamping of insulation does not adequately support the cable, the conductor also shall be clamped. (2) By inserting boxes at the required intervals in which insulating supports are installed and secured in a satisfactory manner to withstand the weight of the conductors attached thereto, the boxes being provided with covers. (3) In junction boxes, by deflecting the cables not less than 90 degrees and carrying them horizontally to a distance not less than twice the diameter of the cable, the cables being carried on two or more insulating supports, and additionally secured thereto by tie wires if desired. Where this method is used, cables shall be supported at intervals not greater than 20 percent of those mentioned in the preceding tabulation. (4) By a method of equal effectiveness. 300-20. Induced Currents in Metal Enclosures or Metal Raceways. (a) Conductors Grouped Together. Where conductors carrying alternating current are installed in metal enclosures or metal raceways, they shall be so arranged as to avoid heating the surrounding metal by induction. To accomplish this, all phase conductors and, where used, the neutral and all equipment grounding conductors shall be grouped together. Exception No. 1: As permitted in Section 250-50, Exception for equipment grounding connections. Exception No. 2: As permitted in Section 426-42 and Section 427-47 for skin effect heating. (b) Individual Conductors. Where a single conductor carrying alternating current passes through metal with magnetic properties, the inductive effect shall be minimized by (1) cutting slots in the metal between the individual holes through which the individual conductors pass, or (2) passing all the conductors in the circuit through an insulating wall sufficiently large for all of the conductors of the circuit. Exception: In the case of circuits supplying vacuum or electric-discharge lighting systems or signs, or X-ray apparatus, the currents carried by the conductors are so small that the inductive heating effect can be ignored where these conductors are placed in metal enclosures or pass through metal. (FPN): Because aluminum is not a magnetic metal, there will be no heating due to hysteresis; however, induced currents will be present. They will not be of sufficient magnitude to require grouping of conductors or special treatment in passing conductors. through aluminum wall sections. 300-21. Spread of Fire or Products of Combustion. Electrical installations in hollow spaces, vertical shafts, and ventilation or air-handling ducts shall be so made that the possible spread of fire or products of combustion will not be substantially increased. Openings around electrical penetrations through fire-resistant-rated walls, partitions, floors, or ceilings shall be firestopped using approved methods to maintain the fire resistance rating. ARTICLE 300 - WIRING METHODS 70-155 (FPN): Directories of electrical construction materials published by qualified testing laboratories contain many listing installation restrictions necessary to maintain the fire resistive rating of assemblies where penetrations or openings are made. An example is the 24-inch (610-mm) minimum horizontal separation between boxes on opposite sides of the wall. Assistance in complying with Section 300-21 can be found in these directories and product listings. 300-22. Wiring in Ducts, Plenums, and Other Air-Handling Spaces. The provisions of this section apply to the installation and uses of electric wiring and equipment in ducts, plenums, and other air-handling spaces. (FPN): See Article 424, Part F, Duct Heaters. (a) Ducts for Dust, Loose Stock, or Vapor Removal. No wiring systems of any type shall be installed in ducts used to transport dust, loose stock, or flammable vapors. No wiring system of any type shall be installed in any duct, or shaft containing only such ducts, used for vapor removal or for ventilation of commercial-type cooking equipment. (b) Ducts or Plenums Used for Environmental Air. Only wiring methods consisting of Type MI cable, Type MC cable employing 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 shall be installed in ducts or plenums specifically fabricated to transport environmental air. Flexible metal conduit and liquidtight flexible metal conduit shall be permitted, in lengths not to exceed 4 feet (1.22 m), to connect physically adjustable equipment and devices permitted to be in these ducts and plenum chambers. The connectors used with flexible metal conduit shall effectively close any openings in the connection. Equipment and devices shall be permitted within such ducts or plenum chambers only if necessary for their direct action upon, or sensing of, the contained air. Where equipment or devices are installed and illumination is necessary to facilitate maintenance and repair, enclosed gasketed-type fixtures shall be permitted. (c) Other Space Used for Environmental Air. Section 300-22(c) applies to space used for environmental air-handling purposes other than ducts and plenums as specified in Sections 300-22(a) and 300-22(b). Only totally enclosed nonventilated insulated busway having no provisions for plug-in connections and wiring methods consisting of Type MI cable, Type MC cable without an overall nonmetallic covering, Type AC cable, or other factory-assembled multiconductor control or power cable that is specifically listed for the use shall be installed in such other space. Other type cables and conductors shall be installed in electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, rigid metal conduit, flexible metal conduit, or, where accessible, surface metal raceway or wireway with metal covers or solid bottom metal cable tray with solid metal covers. Electric equipment with a metal enclosure or with a nonmetallic enclosure listed for the use and having adequate fire-resistant and low-smokeproducing characteristics, and associated wiring material suitable for the ambient temperature, shall be permitted to be installed in such other space unless prohibited elsewhere in this Code. (FPN): The space over a hung ceiling used for environmental air-handling purposes is an example of the type of other space to which Section 300-22(c) applies. 70-156 NATIONAL ELECTRICAL CODE Exception No. 1: Liquidtight flexible metal conduit in single lengths not exceeding 6 feet (1.83 m). Exception No. 2: Integral fan systems specifically identified for such use. Exception No. 3: This section does not include habitable rooms or areas of buildings, the prime purpose of which is not air handling. Exception No. 4: Listed prefabricated cable assemblies of metallic manufactured wiring systems without nonmetallic sheath shall be permitted where listed for this use. Exception No. 5: This section does not include the joist or stud spaces in dwelling units where wiring passes through such spaces perpendicular to the long dimension of such spaces. (d) Data Processing Systems. Electric wiring in air-handling areas beneath raised floors for data processing systems shall comply with