Article 370 - Boxes, Conduit Bodies, and Fittings

370-23(d)(2) Enclosures Fastened to Support Wires An enclosure may be fastened to a suspended (drop) ceiling support wire while meeting the requirements of Sections 300-11(a) and 370-23(d)(2). As stated in Section 370-23(d), the enclosure’s size cannot exceed 100 cubic inches. The enclosure must be securely fastened to the support wire with a fastening device specifically identified for the purpose. As mentioned in last month’s In Focus, fasteners are available which meet a variety of installation applications. Some hangers are designed for securing boxes to support wires. Support wires, not providing secure support, are not permitted as the sole support. Boxes can also be supported by installing independent (additional) support wires. Wires used for enclosure support must be secured or fastened at each end, so as to be taut (tight) within the ceiling cavity.Besides the fact that only attaching one end of the support wire is a violation, unless the ceiling grid support wire is tight, the fastening clip will not function properly. Section 300-11(a) provides requirements for securing and supporting raceways, cable assemblies, cabinets, and fittings. Using drop ceiling support wires to support raceways or cables is a violation. Therefore, independent support wires must be installed. Depending upon the type of ceiling cavity (fire-rated or nonfire-rated), the independent support wires may require a distinguishable attribute form those that support the ceiling assembly. These wires can be distinguishable by color, tagging, or other effective means. Support wires within the cavity of a nonfire-rated floor-ceiling or roof-ceiling assembly do not require a different appearance than the ceiling support wires. [300-11(a)(2)] On the other hand, support wires within the cavity of a fire-rated floor-ceiling or roof-ceiling assembly must have an appearance different from those that are part of the fire-rated design. [300-11(a)(1)] Although an enclosure is supported by a support wire, raceway support requirements remain the same. For example, electrical metallic tubing (EMT) must be securely fastened within 3 feet of each outlet box, junction box, device box, cabinet, conduit body, or other tubing termination. [348-13] Where structural members do not readily permit fastening within 3 feet, unbroken lengths of EMT can be fastened within 5 feet. [348-13 Exception No. 1] As mentioned in last month’s In Focus, cables and raceways cannot be supported by ceiling grids or ceiling grid support wires, unless meeting an exception in Section 300-11(a)(1) or (2). 370-23(e) Enclosures (with No Devices or Fixtures) Supported by Raceways An enclosure containing no devices, or supporting no fixtures (or other equipment), can be supported by the entering raceways if all four conditions in Section 370-23(e) have been met. Normally, enclosures themselves must be rigidly and securely fastened in place, as stipulated in Section 300-11(a). Enclosures are generally fastened in place with screws, nails, brackets, etc. Where meeting all four requirements in Section 370-23(e), no additional support is required for the enclosure. With this type of installation, the enclosure is supported by the attached raceways. First, the enclosure must not exceed 100 cubic inches in size. To find the cubic inch size of a square (or rectangular) box, simply multiply the height times the width times the depth. For example, a single-gang weatherproof box, with dimensions of 41/2 inches in height, 3 inches in width, and 21/4 inches in depth, is 302/5 cubic inches in size (41/2h ¥ 3w ¥ 21/4d = 303/8). This cubic-inch size should not be confused with the minimum cubic-inch capacity for metal boxes, as listed in Table 370-16(a). The cubic-inch capacity is the area within the box available for conductors, not the size of the box. A round box can be sized by simply multiplying the radius times the radius times the depth times 3.1416. The radius of a circle is the distance from the center to the outside edge of that circle. In other words, it is half the diameter’s length. The diameter is simply the distance across a circle through the center. Therefore, the radius of a 41/2-inch round box is 21/4 inches. A 41/2-inch round box with a depth of 2 inches is 314/5 cubic inches in size (21/4r ¥ 21/4r ¥ 2d ¥ 3.1416 = 314/5). An enclosure, supported by raceways, must also have threaded entries or hubs identified for the purpose. Standard metal boxes with conduit knockouts (KOs) do not meet this qualification, unless conduit hubs identified for the purpose are employed. Weatherproof boxes, containing threaded entries, comply with this provision. Another stipulation requires the enclosure to be supported by two or more conduits. A single conduit cannot support an enclosure, except for a conduit body constructed with only one conduit entry. Each supporting conduit (at least two) must be threaded wrench-tight into the enclosure or conduit hub. Since only two types of conduit can be threaded, the supporting conduits are restricted to either rigid metal conduit or intermediate metal conduit. Unless all conduit entries are on the same side, each conduit must be secured within 3 feet of the enclosure. Having met all four requirements, no additional support is required for the enclosure. Remember that these requirements are for an enclosure that does not contain a device(s) or support a fixture(s) or other equipment. Where the support conduits enter on the same side of an enclosure, the 3-foot distance for securing the conduit is no longer permissible. The first three qualifications, as previously stated, remain the same, they are: 1) the enclosure must not exceed 100 cubic inches in size; 2) the enclosure must have threaded entries or hubs identified for the purpose; and 3) the enclosure must be supported by two or more conduits threaded wrench-tight into the enclosure or hubs. When all of the conduits supporting the enclosure (usually two) are on the same side, each conduit must be secured within 18 inches of the enclosure. The only difference is that each conduit must be secured within 18 inches of the enclosure, not 36. Conduit Bodies (Condulets) Conduit bodies (condulets) of any size can be supported by the attached raceways as stated in the exception to Section 370-23(e). Unless the conduit body is independently supported, two raceways must be installed for support. The only conduit body not requiring two support raceways is a conduit body constructed with only one conduit entry. A Type E conduit body, which has only one conduit entry, can be supported with one raceway. Unlike enclosures that cannot exceed 100 cubic inches in size, conduit bodies of any size can be supported with the attached raceways. Two additional raceway types are permitted as support, provided the conduit body’s trade size is not larger than the largest trade size of the supporting raceway. Besides rigid metal and intermediate metal conduit, rigid nonmetallic conduit and EMT are permitted as means of support. Using rigid nonmetallic conduit or EMT as support is only permitted if the raceway’s trade size matches, or exceeds, the conduit body’s trade size. At times, additional space is needed within the conduit body, but not within the attached raceways. Therefore, a larger conduit body is installed without increasing the trade size of the raceways. Since the trade size of the conduit body is larger than that of the entering raceways, neither rigid nonmetallic conduit nor EMT is permitted to support the conduit body. [347-2(h) and 348-5(5)] This does not mean that all oversized conduit bodies must be independently supported. A conduit body, not exceeding 100 cubic inches in size, can be supported by either rigid metal or intermediate metal conduit, when the installation complies with Section 370-23(e). A 1-inch, Type C conduit body, less than 100 cubic inches in size, is installed between two block walls. After reducing bushings are installed, two 1/2-inch rigid metal conduits are threaded wrench-tight into the conduit body. The raceways are secured within 3 feet of the conduit body. This installation is Code compliant, and therefore acceptable. (See Figure 7.) For this installation, substituting either rigid nonmetallic conduit or EMT is a violation. Next month’s In Focus, beginning with Section 370-23(f), will continue discussion of Article 370, Part B, Installation. Section 370-23(f) covers requirements for enclosures, containing devices or fixtures, supported by raceways. MILLER, owner of Lighthouse Educational Services and author of Illustrated Guide to the National Electrical Code, can be reached by telephone at (615) 333-3336, or via e-mail at

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