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Wiring Methods - Part 3 of 3

Wiring Methods - Part 3 of 3

Understanding the wiring method requirements of NEC Article 314, including guidelines for conductor box fill

Article 314 contains installation requirements for outlet and pull boxes, junction boxes, conduit bodies, and handhole enclosures. These rules can seem arbitrary, but there really is logic behind them.

For example, nonmetallic boxes can be used with nonmetallic cables and raceways [314.3]. However, to use them with metallic cables and raceways, an internal bonding means must be provided between all metal parts. This rule is essential in order to maintain the electrical continuity of the effective ground-fault current path required for metallic cables and raceways [250.2 and 250.4(A)(3)].

For this same reason, metal boxes must be connected to an equipment grounding conductor [314.4] of a type listed in 250.118, if the circuit they enclose operates at 50V or more.


Boxes and conduit bodies in damp or wet locations must prevent moisture from entering or accumulating within the enclosure [314.15]. Parts installed in damp or wet locations must be listed for such locations. Conductors, splices, and terminations installed inside handhole enclosures must be listed as suitable for wet locations [314.30(C)].

Box volume

The volume of a box includes the total volume of its assembled parts. That includes plaster rings, extension rings, and domed covers that are either marked with their volume in cubic inches (cu in.) or made from boxes listed in Table 314.16(A).

If boxes contain 6 AWG and smaller conductors, size them per 314.16 to provide sufficient free space for their contents. The volume of the box as calculated in 314.16(A) must not be less than the volume requirement calculated in 314.16(B). Size conduit bodies per 314.16(C) and handhole enclosures per 314.30(A).

Box fill calculations

To determine box fill:

  1. Count the number of conductor volumes.
  2. Add them up.

This will give you the total volume of the conductors, devices, and fittings. When counting conductor volumes, calculate each one per Table 314.16(B).

Count as a single conductor volume:

  • Any conductor that runs through, or terminates in, a box [314.16(B)(1)].
  • One or more internal cable clamps, based on the largest conductor that enters the box [314.16(B)(2)].
  • Each luminaire stud or hickey, based on the largest conductor that enters the box [314.16(B)(3)].
  • Equipment grounding conductors, based on the largest equipment grounding conductor that enters the box [314.16(B)(5)].

Count as two conductor volumes, each:

  • Loop or coil of unbroken conductor having a length at least twice the minimum length required for free conductors in 300.14 [314.16(B)(1)] (Fig. 1).
  • Single-gang device yoke (regardless of the ampere rating), based on the largest conductor that terminates on the device [314.16(B)(4)].
  • Multigang device yoke (for each gang required for mounting), based on the largest conductor that terminates on the device [314.16(B)(4)] (Fig. 2).

But don't count:

  • Conductors that originate and terminate within the box, such as pigtails. [314.16(B)(1)]
  • Equipment grounding conductors, and up to four 16 AWG or smaller fixture wires that enter the box from a domed luminaire or similar canopy such as a ceiling paddle fan canopy. [314.16(B)(1) Ex]
  • Cable connectors that have their clamping mechanism outside of the box [314.16(B)(2)].
  • Luminaire stems.
  • Raceway and cable fittings, including locknuts and bushings [314.16(B)].

Table 314.16(B) also limits the number of splices permitted in a conduit body. Splices are only allowed in a conduit body when the manufacturer has stamped the allowable volume marking on the conduit body [314.16(C)(1)].

Openings and fastenings

Openings through which conductors enter must be adequately closed [314.17(A)]. The means of closing these openings must provide protection that is substantially equivalent to the equipment wall [110.12(A)].

Fasten raceways and cables to boxes or conduit bodies by fittings designed for the wiring method [300.15].

Recessed boxes

Depth limits apply when you install boxes with flush-type covers into recesses of walls or ceilings. If the wall or ceiling materials are:

  • Noncombustible, ensure the front edge of the box, plaster ring, extension ring, or listed extender is no more than 1⁄4 in. from the finished surface [314.20].
  • Combustible, ensure the front edge is flush with, or sticks out from, the finished surface.

Plaster rings and extension rings are available in a variety of depths to meet these requirements.

The NEC requires that oversized, uneven holes around a flush box be repaired so that the gap around a flush-covered box is a maximum of 1⁄8 in. [314.21]. Correctly sized openings can be made by using templates or similar means to help reduce this type of repair work.

A surface extension can be made to a flush covered box by mounting an extension ring over it [314.22]. A surface extension can be made from a box cover only if the extension is a flexible wiring method that will permit the removal of the cover and provide access to the box interior. The continuity of the equipment grounding conductor must be maintained independently of the connection between the box and the cover so that this connection will not be interrupted when removing the box cover [314.22 Ex] (Fig. 3).


Securely support boxes by one of the following methods [314.23]:

  • Surface. Boxes may be fastened to a surface that provides adequate support [314.23(A)].
  • Structural mounting. Boxes may be fastened to structural members of a building. Boxes may be fastened using brackets along with nails or screws. Nails or screws can pass through the interior of the box if a means of preventing abrasion of conductors is provided [314.23(B)(1)].
  • Braces. When metal braces are used for structural mounting they must be at least 0.020 in. thick; wood braces must be at least a nominal 1 in. x 2 in. Plastic braces must be identified for the use [314.23(B)(2)].
  • Finished surface support. Boxes may be mounted by cutting them into a finished wall or ceiling surface such as drywall or plaster and securing the box to the surface by clamps, anchors, or fittings identified for the purpose [314.23(C)].
  • Suspended-ceiling framing members. Boxes may be secured to suspended-ceiling framing members by bolts, screws, rivets, clips, or other means identified for those framing member(s) [314.23(D)(1)].
  • Independent support wires. Boxes must not be attached to the ceiling-support wires of suspended ceilings, but additional independent support wires may be added for the support of boxes, raceways and cables. Boxes may be supported to the independent ceiling support wires with fittings identified for the purpose. When using independent support wires, ensure they are taut and secured at each end [300.11(A) and 314.23(D)(2)].

A box may be supported by two intermediate metal or rigid metal conduits, threaded wrenchtight into the enclosure, if the box:

  • Doesn't contain a device or luminaire, and all conduit entries are on the same side, each raceway must be supported within 36 in. of the box. When all conduit entries are on the same side, the conduit must be supported within 18 in. of the enclosure [314.23(E)].
  • Does contain a device or luminaire, each raceway must be supported within 18 in. of the box [314.23(F)] (Fig. 4 ).

Boxes can be supported from a cord connected to fittings that prevent tension from being transmitted to joints or terminals [314.23(H)(1) and 400.10].


Each box must be provided with a cover or faceplate, unless it's covered by a fixture canopy, lampholder, or similar device [314.25].

A nonmetallic cover may be installed on any box, but a metal cover or faceplate is allowed only if it is connected to an equipment grounding conductor per 250.110 [250.4(A)(3)].

A box installed in a ceiling for luminaire support must be able to hold a luminaire that weighs 50 lb [314.27].

A box installed in a wall for luminaire support must state on the box the maximum luminaire weight it can support. If the luminaire weighs less than 6 lb, you can support it to a device box or plaster ring secured to a box [314.27(A) Ex].

Any luminaire that weighs more than 50 lb must be supported independently of the lighting outlet box, unless that box is listed and marked for the maximum weight of the luminaire [314.27(B)].

Outlet boxes for ceiling paddle fans must be listed and marked as suitable for the purpose, and must not support a fan weighing more than 70 lb. A box for a paddle fan that weighs more than 35 lb must include the maximum weight in the required marking. If the fan weighs more than 70 lb — or the box doesn't show the maximum weight — support the fan independently of the box [314.27(D)]. A similar rule applies to utilization equipment, except the limit is 50 lb instead of 70 lb [314.27(E)].

Adding it up

While the rules for boxes seem overwhelming at first, they really aren't. These rules try to accomplish three things:

  1. Adequate support.
  2. Adequate internal space.
  3. Continuation of bonding path.

If you keep those three goals in mind, the rules for boxes will make more sense, and you'll better understand why it’s important to correctly apply them.

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