Understanding the wiring method requirements of 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)].
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).
To determine box fill:
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:
Count as two conductor volumes, each:
But don't count:
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 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].
Depth limits apply when you install boxes with flush-type covers into recesses of walls or ceilings. If the wall or ceiling materials are:
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]:
A box may be supported by two intermediate metal or rigid metal conduits, threaded wrenchtight into the enclosure, if the box:
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)].
While the rules for boxes seem overwhelming at first, they really aren't. These rules try to accomplish three things:
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.