A. Scope and General

•370-1. Scope. This article covers the installation and use of all boxes and conduit bodies used as outlet, junction, or pull boxes, depending on their use. Cast, sheet metal, nonmetallic, and other boxes such as FS, FD, and larger boxes are not classified as conduit bodies. This article also includes installation requirements for fittings used to join raceways and to connect raceways and cables to boxes and conduit bodies.

(FPN): For systems over 600 volts, nominal, see Part D of this article.

•370-2. Round Boxes. Round boxes shall not be used where conduits or connectors requiring the use of locknuts or bushings are to be connected to the side of the box.

•370-3. Nonmetallic Boxes. Nonmetallic boxes shall be permitted only with open wiring on insulators, concealed knob-and-tube wiring, nonmetallic-sheathed cable, and nonmetallic raceways.

Exception No. 1: Where internal bonding means are provided between all entries, nonmetallic boxes shall be permitted to be used with metal raceways or metal-jacketed cables.

Exception No. 2: Where integral bonding means with a provision for attaching an equipment grounding jumper inside the box are provided between all threaded entries in nonmetallic boxes listed for the purpose, nonmetallic boxes shall be permitted to be used with metal raceways or metal-jacketed cables.

•370-4. Metal Boxes. All metal boxes shall be grounded in accordance with the provisions of Article 250.

•370-5. Short Radius Conduit Bodies. Conduit bodies such as capped elbows and service-entrance elbows enclosing conductors No. 6 or smaller, and that are only intended to enable the installation of the raceway and the contained conductors, shall not contain splices, taps, or devices and shall be of sufficient size to provide free space for all conductors enclosed in the conduit body.

•370-15. Damp, Wet, or Hazardous(Classified) Locations.

(a) Damp or Wet Locations. In damp or wet locations, boxes, conduit bodies, and fittings shall be so placed or equipped as to prevent moisture from entering or accumulating within the box, conduit body, or fitting. Boxes, conduit bodies, and fittings installed in wet locations shall be listed for use in wet locations.

(FPN No. 1): For boxes in floors, see Section 370-27(b).

(FPN No. 2): For protection against corrosion, see Section 300-6.

(b) Hazardous (Classified) Locations. Installations in hazardous (classified) locations shall conform to Articles 500 through 517.

•370-16. Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies. Boxes and conduit bodies shall be of sufficient size to provide free space for all enclosed conductors. In no case shall the volume of the box, as calculated in (a) below, be less than the fill calculation as calculated in (b) below. The minimum volume for conduit bodies shall be as calculated in (c) below.

The provisions of this section shall not apply to terminal housings supplied with motors. See Section 430-12.

Boxes and conduit bodies enclosing conductors, size No. 4 or larger, shall also comply with the provisions of Section 370-28.

(a) Box Volume Calculations. The volume of a wiring enclosure (box) shall be the total volume of the assembled sections, and, where used, the space provided by plaster rings, domed covers, extension rings, etc., that are marked with their volume in cubic inches or are made from boxes the dimensions of which are listed in Table 370-16(a).

1. Standard Boxes. The volumes of standard boxes that are not marked with a cubic inch capacity shall be as given in Table 370-16(a).
   
   
2. Other Boxes. Boxes 100 cubic inch (1640 cu in.) or less, other than those described in Table 370-16(a), and nonmetallic boxes shall be durably and legibly marked by the manufacturer with their cubic inch capacity. Boxes described in Table 370-16(a) that have a larger cubic inch capacity than is designated in the table shall be permitted to have their cubic inch capacity marked as required by this section.

(b) Box Fill Calculations. The volumes in paragraphs (1) through (5) below, as applicable, shall be added together. No allowance shall be required for small fittings such as locknuts and bushings.

1. Conductor Fill. Each conductor that originates outside the box and terminates or is spliced within the box shall be counted once, and each conductor that passes through the box without splice or termination shall be counted once. The conductor fill, in cubic inches, shall be computed using Table 370-16(b). A conductor, no part of which leaves the box, shall not be counted.
   
   Exception: Where an equipment grounding conductor or not over four fixture wires smaller than No. 14, or both, enter a box from a domed fixture or similar canopy and terminate within that box, it shall be permitted to omit these conductors from the calculations.
   
   
2. Clamp Fill. Where one or more internal cable clamps, whether factory or field supplied, are present in the box, a single volume allowance in accordance with Table 370-16(b) shall be made based on present in the box. No allowance shall be required for a cable connector with its clamping mechanism outside the box.
   
   
3. Support Fittings Fill. Where one or more fixture studs or hickeys are present in the box, a single volume allowance in accordance with Table 370-16(b) shall be made for each type of fitting based on the largest conductor present in the box.
   
   
4. Device or Equipment Fill. For each yoke or strap containing one or more devices or equipment, a double volume allowance in accordance with Table 370-16(b) shall be made for each yoke or strap based on the largest conductor connected to a device(s) or equipment supported by that yoke or strap.
   
   
5. Equipment Grounding Conductor Fill. Where one or more equipment grounding conductors enters a box, a single volume allowance in accordance with Table 370-16(b) shall be made based on the largest equipment grounding conductor present in the box. Where an additional set of equipment grounding conductors, as permitted by Section 250-74, Exception No. 4, is present in the box, an additional volume allowance shall be made based on the largest equipment grounding conductor in the additional set.

(c) Conduit Bodies. Conduit bodies enclosing No. 6 conductors or smaller other than short radius conduit bodies as described in Section 370-5 shall have a cross-sectional area not less than twice the cross-sectional area of the largest conduit or tubing to which it is attached. The maximum number of conductors permitted shall be the maximum number permitted by Table 1 of Chapter 9 for the conduit to which it is attached.

Conduit bodies shall not contain splices, taps, or devices unless they are durably and legibly marked by the manufacturer with their cubic inch capacity. The maximum number of conductors shall be computed using the same procedure for similar conductors in other than standard boxes. Conduit bodies shall be supported in a rigid and secure manner.

•370-17. Conductors Entering Boxes, Conduit Bodies, or Fittings. Conductors entering boxes, conduit bodies, or fittings shall be protected from abrasion and shall comply with (a) through (d) below.

(a) Openings to Be Closed. Openings through which conductors enter shall be adequately closed.

(b) Metal Boxes and Conduit Bodies. Where metal boxes or conduit bodies are installed with open wiring or concealed knob-and-tube wiring, conductors shall enter through insulating bushings or, in dry locations, through flexible tubing extending from the last insulating support and firmly secured to the box or conduit body. Where raceway or cable is installed with metal boxes or conduit bodies, the raceway or cable shall be secured to such boxes and conduit bodies.

(c) Nonmetallic Boxes. Nonmetallic boxes shall be suitable for the lowest temperature rated conductor entering the box. Where nonmetallic boxes are used with open wiring or concealed knob-and-tube wiring, the conductors shall enter the box through individual holes. Where flexible tubing is used to encase the conductors, the tubing shall extend from the last insulating support to no less than 1/4 in. (6.35 mm) inside the box. Where nonmetallic-sheathed cable is used, the cable assembly, including the sheath, shall extend into the box no less than 1/4 in. (6.35 mm) through a nonmetallic-sheathed cable knockout opening. In all instances, all permitted wiring methods shall be secured to the boxes.

Exception: Where nonmetallic-sheathed cable is used with boxes no larger than a nominal size 2 1/4 in. by 4 in. mounted in walls or ceilings and where the cable is fastened within 8 in. (203 mm) of the box measured along the sheath and where the sheath extends through a cable knockout no less than 1/4 in. (6.35 mm), securing the cable to the box shall not be required. Multiple cable entries shall be permitted in a single cable knockout opening.

(d) Conductors No. 4 or Larger. Installation shall comply with Section 300-4(f).

•370-18. Unused Openings. Unused cable or raceway openings in boxes and conduit bodies shall be effectively closed to afford protection substantially equivalent to that of the wall of the box or conduit body. Metal plugs or plates used with nonmetallic boxes or conduit bodies shall be recessed at least 1/4 in. (6.35 mm) from the outer surface.

•370-19. Boxes Enclosing Flush Devices. Boxes used to enclose flush devices shall be of such design that the devices will be completely enclosed on back and sides, and that substantial support for the devices will be provided. Screws for supporting the box shall not be used in attachment of the device contained therein.

•370-20. In Wall or Ceiling. In walls or ceilings of concrete, tile, or other noncombustible material, boxes shall be installed so that the front edge of the box will not be set back of the finished surface more than 1/4 in. (6.35 mm). In walls and ceilings constructed of wood or other combustible material, boxes shall be flush with the finished surface or project therefrom.

•370-21. Repairing Plaster and Dry Wall or Plasterboard. Plaster, dry wall, or plasterboard surfaces that are broken or incomplete shall be repaired so there will be no gaps or open spaces greater than 1/8 in. (3.18 mm) at the edge of the box or fitting.

•370-22. Exposed Surface Extensions. Surface extensions from a box of a concealed wiring system shall be made by mounting and mechanically securing a box or extension ring over the concealed box. Where required, equipment grounding shall be in accordance with Article 250.

Exception: A surface extension shall be permitted to be made from the cover of a concealed box where the cover is designed so it is unlikely to fall off, or be removed if its securing means becomes loose. The wiring method shall be flexible and arranged so that any required grounding continuity is independent of the connection between the box and cover.

•370-23. Supports. Enclosures within the scope of Article 370 shall be rigidly and securely fastened in place in accordance with (a) through (g) below.

(a) Surface Mounting. Enclosures shall be fastened to the surface upon which they are mounted unless such surface does not provide adequate support, in which case they shall be supported in accordance with (b).
(b) Structural Mounting. Enclosures shall be rigidly supported from a structural member of the building or from grade either directly or by using a metal, polymeric, or wood brace. Support wires that do not provide rigid support shall not be permitted as the sole support.

1. Nails, where used as a fastening means, shall be permitted to pass through the interior of the enclosure if located within 1/4 in. (6.35 mm) of the back or ends of the enclosure.
   
   
2. Metal braces shall be protected against corrosion and formed from metal not less than .020 in. (508 micrometers) thick uncoated. Wood braces shall have a cross section not less than nominal 1 in. by 2 in. Wood braces in wet locations shall be treated for the conditions. Polymeric braces shall be identified as being suitable for the use.

(c) Nonstructural Mounting. It shall be permissible to make a flush installation in existing covered surfaces where adequate support is provided by clamps, anchors, or fittings. Framing members of suspended ceiling systems shall be permitted as the support if the framing members are adequately supported and securely fastened to each other and to the building structure. Enclosures so supported shall be fastened to the framing member by mechanical means such as bolts, screws, or rivets. Clips identified for use with the type of ceiling framing member(s) and enclosure(s) shall also be permitted.

(d) Raceway Supported Enclosure(s), Without Devices or Fixtures. Enclosures that are not over 100 cu in. (1640 cu cm) in size and that have threaded entries or have hubs identified for the purpose, and that do not contain devices or support fixtures, shall be considered to be adequately supported where two or more conduits are threaded wrenchtight into the enclosure or hubs, and where each conduit is supported within 3 ft (914 mm) of the enclosure on two or more sides so as to provide the rigid and secure installation intended by this section of the Code.

Exception: Rigid metal, intermediate metal, or rigid nonmetallic conduit or electrical metallic tubing shall be permitted to support conduit bodies, provided the conduit bodies are not larger than the largest trade size of the conduit or electrical metallic tubing. Such enclosures shall also be considered to be adequately supported if the enclosures comply with Subsection 370-23(e):

(e) Raceway Supported Enclosures, With Devices or Fixtures. Enclosures that are not over 100 cu in. (1640 cu cm) in size and that have threaded entries or have hubs identified for the purpose, and that support fixtures or contain devices, or both, shall be considered adequately supported where two or more conduits are threaded wrenchtight into the enclosure or hubs and where each conduit is supported within 18 in. (457 mm) of the enclosure so as to provide the rigid and secure installation intended by this section of the Code.

Exception No. 1: Rigid metal or intermediate metal conduit shall be permitted to support conduit bodies, provided the conduit bodies are not larger than the largest trade size of the conduit.

Exception No. 2: An unbroken length(s) of rigid or intermediate metal conduit shall be permitted to support a box used for fixture support, or to support a wiring enclosure within a fixture and used in lieu of a box in accordance with Section 300-15(d), where all of the following conditions are met:

1. The conduit is securely fastened at a point so that the length of conduit beyond the last point of conduit support does not exceed 3 ft (914 mm).
2. The conduit length before the last point of conduit support is 12 in. (305 mm) or greater.
3. Where accessible to unqualified persons, the fixture, measured to its lowest point, is at least 8 ft (2.44 m) above grade or standing area and at least 3 ft (914 mm) measured horizontally to the 8 ft (2.44 m) elevation from windows, doors, porches, fire escapes, or similar locations.
4. A fixture supported by a single conduit does not exceed 12 in. (305 mm) in any direction from the point of conduit entry.
5. The weight supported by any single conduit does not exceed 20 lb (9.08 kg).
6. At the fixture end, the conduit(s) is (are) threaded wrenchtight into the box or wiring enclosure, or into hubs identified for the purpose.

(f) Enclosures in Concrete or Masonry. An enclosure(s) shall be permitted to be supported by being embedded.

(g) Pendant Boxes. Boxes shall be permitted to be supported on pendants in accordance with the following:

1. Flexible Cord. Boxes shall be permitted to be supported from a multiconductor cord or cable in an approved manner that protects the conductors against strain, such as a strain-relief connector threaded into a box with a hub.
   
   
2. Conduit. Boxes supporting lampholders or lighting fixtures, or wiring enclosures within fixtures used in lieu of boxes in accordance with Section 300-15(d), shall be permitted to be supported by rigid or intermediate metal conduit stems where, for stems longer than 18 in. (457 mm), the stems are connected to the wiring system with flexible fittings suitable for the location. At the fixture end, the conduit(s) shall be threaded wrenchtight into the box or wiring enclosure, or into hubs identified for the purpose.

Where supported by only a single conduit, the threaded joints shall be prevented from loosening by the use of set-screws or other effective means, or the fixture, at any point, shall be at least 8 ft (2.44 m) above grade or standing area and at least 3 ft (914 mm) measured horizontally to the 8 ft (2.44 m) elevation from windows, doors, porches, fire escapes, or similar locations. A fixture supported by a single conduit shall not exceed 12 in. (305 mm) in any horizontal direction from the point of conduit entry.

•370-24. Depth of Outlet Boxes. No box shall have an internal depth of less than 1/2 in. (12.7 mm). Boxes intended to enclose flush devices shall have an internal depth of not less than 15/16 in. (23.8 mm).

•370-25. Covers and Canopies. In completed installations, each box shall have a cover, faceplate, or fixture canopy.

(a) Nonmetallic or Metal Covers and Plates. Nonmetallic or metal covers and plates shall be permitted with nonmetallic boxes. Where metal covers or plates are used, they shall comply with the grounding requirements of Section 250-42.

(FPN): For additional grounding requirements, see Section 410-18(a) for metal fixture canopies, and Sections 380-12 and 410-56(d) for metal faceplates.

(b) Exposed Combustible Wall or Ceiling Finish. Where a fixture canopy or pan is used, any combustible wall or ceiling finish exposed between the edge of the canopy or pan and the outlet box shall be covered with noncombustible material.

(c) Flexible Cord Pendants. Covers of outlet boxes and conduit bodies having holes through which flexible cord pendants pass shall be provided with bushings designed for the purpose or shall have smooth, well-rounded surfaces on which the cords may bear. So-called hard rubber or composition bushings shall not be used.

•370-27. Outlet Boxes.

(a) Boxes at Lighting Fixture Outlets. Boxes used at lighting fixture outlets shall be designed for the purpose. At every outlet used exclusively for lighting, the box shall be so designed or installed that a lighting fixture may be attached.

(b) Floor Boxes. Boxes listed specifically for this application shall be used for receptacles located in the floor.

Exception: Boxes located in elevated floors of show windows and similar locations where the authority having jurisdiction judges them to be free from physical damage, moisture, and dirt.

(c) Boxes at Fan Outlets. Outlet boxes shall not be used as the sole support for ceiling (paddle) fans.

Exception: Boxes listed for the application shall be permitted as the sole means of support.

•370-28. Pull and Junction Boxes. Boxes and conduit bodies used as pull or junction boxes shall comply with (a) through (d) of this section.

Exception: Terminal housings supplied with motors shall comply with the provisions of Section 430-12.

(a) Minimum Size. For raceways containing conductors of No. 4 or larger, and for cables containing conductors of No. 4 or larger, the minimum dimensions of pull or junction boxes installed in a raceway or cable run shall comply with the following:

1. Straight Pulls. In straight pulls, the length of the box shall not be less than eight times the trade diameter of the largest raceway.
   
   
2. Angle or U Pulls. Where angle or U pulls are made, the distance between each raceway entry inside the box and the opposite wall of the box shall not be less than six times the trade diameter of the largest raceway in a row. This distance shall be increased for additional entries by the amount of the sum of the diameters of all other raceway entries in the same row on the same wall of the box. Each row shall be calculated individually, and the single row that provides the maximum distance shall be used.
   
   Exception: Where a raceway or cable entry is in the wall of a box or conduit body opposite to a removable cover and where the distance from that wall to the cover is in conformance with the column for one wire per terminal in Table 373-6(a). The distance between raceway entries enclosing the same conductor shall not be less than six times the trade diameter of the larger raceway.
   
   When transposing cable size into raceway size in (a)(1) and (a)(2) above, the minimum trade size raceway required for the number and size of conductors in the cable shall be used.
   
   
3. Boxes or conduit bodies of dimensions less than those required in (a)(1) and (a)(2) above shall be permitted for installations of combinations of conductors that are less than the maximum conduit or tubing fill (of conduits or tubing being used) permitted by Table 1 of Chapter 9, provided the box or conduit body has been approved for and is permanently marked with the maximum number and maximum size of conductors permitted.

(b) Conductors in Pull or Junction Boxes. In pull boxes or junction boxes having any dimension over 6 ft (1.83 m), all conductors shall be cabled or racked up in an approved manner.

(c) Covers. All pull boxes, junction boxes, and conduit bodies shall be provided with covers compatible with the box or conduit body construction and suitable for the conditions of use. Where metal covers are used, they shall comply with the grounding requirements of Section 250-42. An extension from the cover of an exposed box shall comply with Section 370-22, Exception.

(d) Permanent Barriers. Where permanent barriers are installed in a box, each section shall be considered as a separate box.

•370-29. Conduit Bodies, Junction, Pull, and Outlet Boxes to Be Accessible. Conduit bodies, junction, pull, and outlet boxes shall be so installed that the wiring contained in them can be rendered accessible without removing any part of the building or, in underground circuits, without excavating sidewalks, paving, earth, or other substance that is to be used to establish the finished grade.

Exception: Listed boxes shall be permitted where covered by gravel, light aggregate, or noncohesive granulated soil if their location is effectively identified and accessible for excavation.

C. Construction Specifications

•370-40. Metal Boxes, Conduit Bodies, and Fittings.

(a) Corrosion Resistant. Metal boxes, conduit bodies, and fittings shall be corrosion resistant or shall be well-galvanized, enameled, or otherwise properly coated inside and out to prevent corrosion.

(FPN): See Section 300-6 for limitation in the use of boxes and fittings protected from corrosion solely by enamel.

(b) Thickness of Metal. Sheet steel boxes not over 100 cu in. (1640 cu cm) in size shall be made from steel not less than 0.0625 in. (1.59 mm) thick. The wall of a malleable iron box or conduit body and a die-cast or permanent-mold cast aluminum, brass, bronze, or zinc box or conduit body shall not be less than 3/32 in. (2.38 mm) thick. Other cast metal boxes or conduit bodies shall have a wall thickness not less than 1/8 in. (3.17 mm).

Exception No. 1: Listed boxes and conduit bodies shown to have equivalent strength and characteristics shall be permitted to be made of thinner or other metals

Exception No. 2: The walls of listed short radius conduit bodies, as covered in Section 370-5, shall be permitted to be made of thinner metal.

(c) Metal Boxes Over 100 Cu In. Metal boxes over 100 cu in. (1640 cu cm) in size shall be constructed so as to be of ample strength and rigidity. If of sheet steel, the metal thickness shall not be less than 0.053 in. (1.35 mm) uncoated.

(d) Grounding Provisions. A means shall be provided in each metal box for the connection of an equipment grounding conductor. The means shall be permitted to be a tapped hole or equivalent.

•370-41. Covers. Metal covers shall be of the same material as the box or conduit body with which they are used, or they shall be lined with firmly attached insulating material not less than 1/32 in. (0.79 mm) thick, or they shall be listed for the purpose. Metal covers shall be the same thickness as the boxes or conduit body for which they are used, or they shall be listed for the purpose. Covers of porcelain or other approved insulating materials shall be permitted if of such form and thickness as to afford the required protection and strength.

•370-42. Bushings. Covers of outlet boxes and conduit bodies having holes through which flexible cord pendants may pass shall be provided with approved bushings or shall have smooth, well-rounded surfaces upon which the cord may bear. Where individual conductors pass through a metal cover, a separate hole equipped with a bushing of suitable insulating material shall be provided for each conductor. Such separate holes shall be connected by a slot as required by Section 300-20.

•370-43. Nonmetallic Boxes. Provisions for supports or other mounting means for nonmetallic boxes shall be outside of the box, or the box shall be so constructed as to prevent contact between the conductors in the box and the supporting screws.

•370-44. Marking. All boxes and conduit bodies, covers, extension rings, plaster rings, and the like shall be durably and legibly marked with the manufacturer’s name or trademark.

D. Pull and Junction Boxes for Use on Systems Over 600 Volts, Nominal.

•370-70. General. In addition to the generally applicable provisions of Article 370, the rules in Sections 370-71 and 370-72 shall apply.

•370-71. Size of Pull and Junction Boxes. Pull and junction boxes shall provide adequate space and dimensions for the installation of conductors, and they shall comply with the specific requirements of this section.

Exception: Terminal housings supplied with motors shall comply with the provisions of Section 430-12.
(a) For Straight Pulls. The length of the box shall not be less than 48 times the outside diameter, over sheath, of the largest shielded or lead-covered conductor or cable entering the box. The length shall not be less than 32 times the outside diameter of the largest nonshielded conductor or cable.

(b) For Angle or U Pulls.

1. The distance between each cable or conductor entry inside the box and the opposite wall of the box shall not be less than 36 times the outside diameter, over sheath, of the largest cable or conductor. This distance shall be increased for additional entries by the amount of the sum of the outside diameters, over sheath, of all other cables or conductor entries through the same wall of the box.
   
   Exception No. 1: Where a conductor or cable entry is in the wall of a box opposite a removable cover, the distance from that wall to the cover shall be permitted to be not less than the bending radius for the conductors as provided in Section 300-34.
   
   Exception No. 2: Where cables are nonshielded and not lead covered, the distance of 36 times the outside diameter shall be permitted to be reduced to 24 times the outside diameter.
   
   
2. The distance between a cable or conductor entry and its exit from the box shall not be less than 36 times the outside diameter, over sheath, of that cable or conductor.
   
   Exception: Where cables are nonshielded and not lead covered, the distance of 36 times the outside diameter shall be permitted to be reduced to 24 times the outside diameter.

(c) Removable Sides. One or more sides of any pull box shall be removable.

•370-72. Construction and Installation Requirements.

(a) Corrosion Protection. Boxes shall be made of material inherently resistant to corrosion or shall be suitably protected, both internally and externally, by enameling, galvanizing, plating, or other means.

(b) Passing Through Partitions. Suitable bushings, shields, or fittings having smooth, rounded edges shall be provided where conductors or cables pass through partitions and at other locations where necessary.

(c) Complete Enclosure. Boxes shall provide a complete enclosure for the contained conductors or cables.

(d) Wiring Is Accessible. Boxes shall be so installed that the wiring is accessible without removing any part of the building. Working space shall be provided in accordance with Section 110-34.

(e) Suitable Covers. Boxes shall be closed by suitable covers securely fastened in place. Underground box covers that weigh over 100 lb (45.4 kg) shall be permanently marked “DANGER — HIGH VOLTAGE — KEEP OUT.” The marking shall be on the outside of the box cover and shall be readily visible. Letters shall be block type at least 1/2 in. (12.7 mm) in height.

(f) Suitable for Expected Handling. Boxes and their covers shall be capable of withstanding the handling to which they may likely be subjected.