13.1 Box Installation, Support, Accessibility & Luminaire and Fan Boxes (Article 314)
Key Takeaways
In walls or ceilings of noncombustible material, the front edge of a box may be set back not more than ¼ in. from the finished surface; in combustible material it must be flush or project (314.20).
Gaps or open spaces around boxes in plaster, drywall, or plasterboard may not exceed ⅛ in. at the edge of the box (314.21).
A box supported only by raceways needs two or more threaded conduits secured within 3 ft of the box, or within 18 in. if the box contains a device or supports a luminaire (314.23(E) and (F)).
Outlet boxes for luminaires may support up to 50 lb unless listed and marked for a greater weight, and paddle fans require boxes listed for fan support, up to 70 lb (314.27).
Boxes must be installed so the wiring inside can be reached without removing any part of the building, sidewalks, paving, or earth (314.29).
Why box installation is tested
DLI's knowledge areas include outlet, device, pull, and junction boxes and their support. Inspectors write correction orders for recessed boxes, loose boxes, and fans hung on standard boxes, and exam questions often turn on one dimension: ¼ in., ⅛ in., 18 in., 3 ft, 50 lb, or 70 lb.
Box types and materials
- 314.3 Nonmetallic boxes: used only with open wiring on insulators, concealed knob-and-tube, cabled wiring methods with entirely nonmetallic sheaths, flexible cords, and nonmetallic raceways, unless internal bonding is provided between all entries or the box is listed for metal raceways.
- 314.4 Metal boxes: must be grounded and bonded under Article 250.
- 314.15 Damp or wet locations: boxes, conduit bodies, and fittings must prevent moisture from entering or accumulating, and boxes in wet locations must be listed for wet locations. Field drainage openings between ⅛ in. and ¼ in. are permitted in boxes listed for damp or wet locations. The 2026 NEC 250.64(G) prohibits routing a grounding electrode conductor through such openings.
Conductors entering boxes (314.17)
- Openings through which conductors enter must be closed in a manner identified for the application.
- Where cable is used, each cable must be secured to the box, except as follows: with nonmetallic-sheathed cable and single-gang boxes not larger than 2¼ in. × 4 in. mounted in walls or ceilings, the cable may be fastened within 8 in. of the box instead of with a clamp, if the sheath extends at least ¼ in. inside the box.
- In nonmetallic boxes, the cable sheath must extend at least ¼ in. inside the box beyond any cable clamp.
Box position in walls and ceilings (314.20 and 314.21)
| Wall or ceiling material | Rule |
|---|---|
| Noncombustible (such as drywall over steel studs, plaster, masonry) | The box or extension ring may be set back not more than ¼ in. |
| Combustible (such as wood paneling) | The box must be flush with the finished surface or project from it |
| Repairing gaps (314.21) | Plaster, drywall, or plasterboard surfaces that are broken or incomplete around boxes must be repaired so there are no gaps larger than ⅛ in. at the box edge |
| Surface extensions (314.22) | Made by mounting and mechanically securing an extension ring over the box, with EGC continuity maintained |
Supporting boxes (314.23)
| Mounting | Requirement |
|---|---|
| (A) Surface | Fastened to the surface; if the surface cannot provide support, use another method |
| (B) Structural mounting | Nails and screws passing through the box interior must be within ¼ in. of the back or ends of the box; braces must be metal at least 0.020 in. thick or wood at least 1 in. × 2 in. |
| (C) Mounting in finished surfaces | Secured by clamps, anchors, or fittings identified for the application |
| (D) Suspended ceilings | Fastened to the ceiling framing members, which are securely fastened to each other and to the building structure, or supported by identified support wires |
| (E) Raceway-supported, no devices or luminaires | Supported by two or more conduits threaded wrenchtight into the box or hubs, each secured within 3 ft of the box |
| (F) Raceway-supported, with devices or luminaires | Supported by two or more conduits threaded wrenchtight, each secured within 18 in. of the box |
| (G) Concrete or masonry | Identified as suitably protected from corrosion and securely embedded |
| (H) Pendant boxes | Supported by multiconductor cord or cable with strain relief, or by conduit stems with set screws or locknuts |
A single EMT or a single conduit is never enough to support a box by itself under (E) or (F).
Box depth (314.24)
Outlet and device boxes must have enough depth for the equipment installed. A box that does not enclose devices or utilization equipment must be at least ½ in. deep. A box that encloses devices or utilization equipment and conductors 12 AWG or smaller must be at least 15/16 in. deep, and larger equipment and conductors need more depth.
Luminaire, fan, and floor boxes (314.27)
| Use | Requirement |
|---|---|
| Wall luminaire boxes | Marked on the interior with the maximum weight if they support more than 50 lb; otherwise a wall box may support a luminaire of up to 50 lb |
| Ceiling luminaire outlets | Boxes designed to support luminaires of at least 50 lb; heavier luminaires need a box listed and marked for the weight, or must be supported independently |
| Floor boxes | Listed specifically for floor applications |
| Ceiling-suspended paddle fans | Outlet boxes listed for the sole support of fans; a box must be marked if listed for fans heavier than 35 lb, and the maximum supported weight is 70 lb |
| Utilization equipment | Boxes must be designed to support the equipment's weight, such as heavy ceiling-mounted equipment |
| Spare fan-ready outlets in dwellings | Where spare, separately switched ungrounded conductors are run to a ceiling outlet box in a location acceptable for a paddle fan in a one-family, two-family, or multifamily dwelling, the box must be listed for the sole support of a paddle fan |
Accessibility (314.29)
Boxes, conduit bodies, and handhole enclosures must be installed so the wiring in them can be reached without removing any part of the building, or, in underground circuits, without excavating sidewalks, paving, earth, or other substances used to establish finished grade. Listed boxes and handhole enclosures may be covered by gravel, light aggregate, or noncohesive granulated soil if their location is effectively identified and accessible for excavation. A box above a lift-out ceiling panel is accessible; a box buried behind drywall is not.
Handhole enclosures (314.30) must be designed and installed to withstand all expected loads; covers must carry an identifying mark or logo showing electrical use, and covers over 100 lb or otherwise requiring tools to open.
Worked example: two boxes on a warehouse wall
- A junction box with no devices is supported only by two ¾ in. RMC conduits threaded into its hubs. Each conduit must be secured within 3 ft of the box. A single conduit is not enough.
- A weatherproof receptacle box on the same wall is supported only by two threaded RMC conduits. Because it contains a device, each conduit must be secured within 18 in. of the box.
A device box is installed in a drywall wall over steel studs. What is the maximum distance the box may be set back from the finished surface?
0 in.; it must be flush
⅛ in.
½ in.
¼ in.
A cast box containing a receptacle is supported only by two threaded rigid metal conduits. Under 314.23(F), within what distance of the box must each conduit be secured?
18 in.
12 in.
3 ft
6 ft
What is the maximum weight of a ceiling-suspended paddle fan that may be supported by an outlet box listed for fan support?
35 lb
70 lb
50 lb
100 lb
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