370-23(b)(2) Enclosures Mounted on Braces

Conduit bodies and boxes (outlet, device, pull, and junction) must be supported in accordance with one or more of the support provisions listed in Section 370-23(a) through (h). Last month’s In Focus discussed (a) surface mounting, (b) structural mounting, and (b)(1) nails. An enclosure mounted on a building, or other surface, must be rigidly and securely fastened in place, but if the mounting surface does not provide rigid and secure support, additional support must be provided. [370-23(a)]

A brace meeting the specifications of Section 370-23(b)(2) can serve as additional support. Section 370-23(b) also states that an enclosure must be rigidly supported. The enclosure can be supported either directly, or by using a brace (wood, metal, or polymeric). Where metal, wood, or polymeric braces are used, they must meet the requirements of Section 370-23(b)(2).

Two requirements must be met when using metal braces. First, the metal brace must be protected against corrosion. The metal must be suitable to withstand the conditions, therefore preventing corrosion. An enclosure’s support may be jeopardized if the metal brace supporting the box is deteriorating because of corrosion. Keep in mind also that moisture is not the only element that causes corrosion.

Second, the brace must have a thickness of at least 1/50 inch.

An enclosure’s support may also be jeopardized if the metal brace supporting the box is too thin. The minimum thickness must not include the thickness of any paint or coating.

If using a wood brace to support an enclosure, the brace’s cross section must measure at least one inch by two inches, nominal.

Also, where used in wet locations, wood braces must be treated to withstand the conditions. For example, a weatherproof, GFCI-protected receptacle is needed where there is no building or structure. One method of complying with this support requirement is to secure the receptacle’s enclosure to a 4-inch-by-4-inch pressure-treated wood brace.

Another method involves the use of rigid or intermediate metal conduit installed in accordance with Section 370-23(f). Caution is advised when mounting nonmetallic boxes. Screws used to support nonmetallic boxes must be mounted outside of the box, unless the box is constructed in a manner that prevents contact between the conductors and the supporting screws. [370-43]

The final type of brace mentioned in Section 370-23(b)(2) is a polymeric brace. Polymeric braces must be identified as being suitable for the use. Unlike wood braces, no minimum cross-section is stipulated for polymeric braces. Although no minimum size is stated, the brace must be strong enough to rigidly and securely support the enclosure. [370-23(b)]

370-23(c) Mounting in Finished Surfaces

An enclosure mounted in a finished surface must be rigidly secured to the surface by clamps, anchors, or fittings identified for the application. There are times when securing a box to a building’s structural member is impracticable. Sometimes a switch or receptacle must be installed after the drywall has already been set in place. This requires the use of a box and/or supports specifically designed for the purpose.

A number of boxes, both metal and nonmetallic, are available for a variety of installation applications. Cut-in, old-work, and wing boxes are just a few of the box types available. Accessories, such as old workbox support clips and switch box supports (battleships, F clips, box hold-its) are also available for use with switch boxes furnished with plaster ears.

The box chosen must contain the proper fittings or connectors for the particular wiring method selected. For example, metal-clad cable (Type MC) must not be installed in a box specifically designed for nonmetallic-sheathed cable. Also, installing nonmetallic-sheathed cable in a knockout opening where there is no cable connector is another violation. Conductors entering boxes, conduit bodies, or fittings must be protected from abrasion. [370-17]

Normally, cables must be supported within a specified distance from the box. Support measurements are listed in Section 333-7 for Type AC cable, Section 334-10(a) for Type MC cable, and Section 336-18 for nonmetallic-sheathed cable. Also, listed in these same sections is the maximum interval between cable supports or straps. While Type AC cable and nonmetallic-sheathed cable must be supported and secured at intervals not exceeding 4 1/2 feet, the interval for Type MC cable must not exceed 6 feet. Complying with these support provisions is not mandatory where cables are fished inside walls. Cables fished between access points, where concealed in finished buildings (or structures) and where support is not practical, can remain unsupported and unsecured. [333-7(b), 334-10(b), and 336-18 Exception No. 1] Where meeting the provisions of Section 348-13 Exception No. 2, electrical metallic tubing (EMT) can also be fished within finished walls.

370-23(d) Suspended Ceilings

An enclosure no greater than 100 cubic inches in size, securely fastened in place as per Section 370-23(d)(1) or 370-23(d)(2), can be mounted to structural or supporting elements of a suspended (drop) ceiling. While Section 370-23(d)(1) pertains to the framing members of a suspended ceiling, Section 370-23(d)(2) refers to support wires.

370-23(d)(1) Framing Members

An enclosure mounted in a suspended ceiling system must be fastened to the framing members mechanically (with bolts, screws, or rivets), or with clips or other securing means identified for use with the enclosure(s) and ceiling framing member(s) employed. Boxes can be attached to framing members or ceiling grids of suspended (drop) ceilings where certain specifications are met.

The first stipulation requires that the box be fastened to the framing member by mechanical means, such as bolts, screws, or rivets. Clips or other securing means, identified for use with the type of ceiling framing member(s) and enclosure(s), are also permitted.

Numerous fasteners are available that meet a variety of applications, such as the T-bar hanger. (See Figure 5.) This fastener attaches to an acoustical “Tee Bar” of a suspended ceiling system. An enclosure (four-square box, octagon box, etc.) is fastened to the hanger, between the acoustical “Tee Bars,” by means of a mounting clip.

Normally, as stated in Section 410-16(a), an outlet box can support a fixture weighing no more than 50 pounds. Don’t automatically assume that, just because the box can support a weight of 50 pounds, the ceiling grid fastener or hanger can also. Fasteners must be installed in accordance with their installation instructions. For example, certain T-bar hangers limit the maximum load to 20 pounds, unless independent support is provided. Therefore, if no additional support is provided, the manufacturer’s installation instructions limit the load to 20, not 50 pounds. Where additional support is provided, the load can exceed the 20-pound weight limit.

Be sure to read the installation instructions (contained within the packages) before using or applying any hangers or brackets.

The second stipulation requires that the framing members (ceiling grids) be adequately supported and securely fastened to one another as well as to the building structure. Although both of these stipulations are usually accomplished during the acoustical ceiling grid installation, the electrician must confirm this before securing enclosures to framing members. Unlike enclosures, cables and raceways cannot be supported by ceiling grids or ceiling grid support wires. See Section 300-11(a).

Next month’s In Focus, beginning with Section 370-23(d)(2), will continue discussion of Article 370, Part B, Installation. Section 370-23(d)(2) covers requirements for enclosures supported by suspended-ceiling support wires.

MILLER is owner of Lighthouse Educational Services. He can be reached by phone at (615) 333-3336, or via e-mail at charles@charlesRmiller.com.