370-20 In Wall or Ceiling
Section 370-20 can easily be overlooked during the box installation procedure. At times, the box’s front edge ends up more than 1/4 inch behind the finished surface. This is a violation, regardless of the type of material surrounding the box. The National Electrical Code (NEC) contains minimum setback requirements for boxes installed in combustible, as well as noncombustible types of material.
In walls or ceilings constructed of noncombustible material (concrete, tile, etc.), boxes must be installed so that the front edge will not be set back more than 1/4 inch from the finished surface.
Boxes in walls or ceilings constructed of combustible material, such as wood, must be flush with, or project beyond, the finished surface.
Although these rules are easily understood, they are often ignored. To reiterate, within combustible material, boxes must either be flush or extend beyond the finished surface. In noncombustible material, the front edge of the box must be within 1/4 inch of the finished surface.
370-21 Repairing Drywall (or Plasterboard) and Plaster
Usually, electricians are not responsible for cutting holes in drywall, plasterboard, or plaster. But, where the drywall is existing or has already been installed, the electrician usually cuts the holes. Whoever performs the cutting, compliance with Section 370-21 is mandatory. Broken (or incomplete) drywall, plasterboard, or plaster surfaces must be repaired so that no gap or open space surrounding the box or fitting exceeds 1/8 inch.
Hiding damaged or incomplete drywall (plaster, etc.) by installing an oversize (or jumbo) cover plate does not satisfy the requirement of this section. Greater-than-1/8-inch gaps or open spaces must be eliminated regardless of cover plate size.
370-22 Exposed Surface Extensions
A surface-mounted box installed over a concealed box, for the purpose of extending the electrical wiring system, is called a surface extension. A surface extension is simply a transition point between concealed wiring and exposed wiring. A box or extension ring, covering a box from a concealed wiring system, must be mechanically secured over the concealed box.
Section 370-22 also states that equipment grounding, where required, must be in accordance with Article 250. While Article 250, Part F (Sections 250-110 through 126) covers equipment grounding and equipment grounding conductors, Part G (Sections 250-130 through 148) covers methods of equipment grounding. Although the stipulations in Parts F and G are too numerous to mention, two sections are notable.
Section 250-118 discusses types of equipment grounding conductors. Types include: a copper or other corrosion-resistant conductor, metal conduit (rigid and intermediate), electrical metallic tubing, and 10 other specific items. Section 250-148(a) requires that a connection be made between the equipment grounding conductor(s) and a metal box by means of a listed grounding device or a grounding screw that serves no other purpose. [Section 250-148(a)] Where an equipment grounding conductor enters a metal box, the box must be grounded or bonded to the grounding conductor. This connection must be accomplished by means of a listed grounding device, or a grounding screw used for no other purpose. Section 250-8 prohibits using sheet metal screws to connect the grounding conductor to the enclosure.
An exception to 370-22 permits another type of transition point between concealed wiring and exposed wiring. Where all the requirements within the exception are met, the cover of a concealed box is permitted as the surface extension.
A surface extension, made from the cover of a concealed box, must be designed so it is not likely to fall off, or be removed if its securing means becomes loose. Also, the wiring method must be flexible, and configured so that any required grounding continuity is independent of the connection between the box and cover. Installing nonflexible conduit or tubing, such as electrical metallic tubing, onto the cover of a concealed box violates this requirement. Flexible metal conduit, liquid-tight flexible metal conduit, and liquid-tight flexible nonmetallic conduit are some wiring methods permitted. This exception’s final requirement states that the continuity of the grounding system cannot be dependent upon the connection between the box and cover. Therefore, to ensure a safe return path to ground, an equipment grounding conductor must be installed. Table 250-122 provides the minimum-size equipment grounding conductors for grounding raceways and equipment.
Enclosures within the scope of Article 370 must comply with one or more of the support specifications listed in Section 370-23(a) through (h). These include: (a) surface mounting; (b) structural mounting; (c) mounting in finished surfaces; (d) suspended ceilings; (e) raceway supported enclosure, without devices or fixtures; (f) raceway-supported enclosure with devices or fixtures; (g) enclosures in concrete or masonry; and (h) pendant boxes.
370-23(a) Surface Mounting
An enclosure mounted on a building or other surface must be rigidly and securely fastened in place. A similar requirement in Section 110-13 states that electrical equipment must be firmly secured to the surface on which it is mounted. Although the Code does not provide acceptable fastening procedures, it does prohibit one method. Wooden plugs driven into holes in masonry, concrete, plaster, or similar materials shall not be used. [Section 110-13] If the mounting surface does not provide rigid and secure support, additional support must not only be provided, it must also comply with Section 370-23(a) through (h).
A solid building surface, capable of supporting an enclosure, is not always available. For example, a weatherproof, ground fault circuit interrupter (GFCI)-protected receptacle is needed in a landscaped area without buildings or structures. One permitted method to support the box is to install a brace as per Section 370-23(b)(2). Another method involves the use of rigid or intermediate metal conduit installed in accordance with Section 370-23(f).
370-23(b) Structural Mounting
An enclosure supported from a structural member of a building or from grade must be rigidly supported either directly, or by use of a brace (wood, metal, or polymeric). The importance of properly mounting electrical equipment cannot be emphasized enough. How many times do electricians see boxes, enclosures, or conduit mounted improperly? As required by Section 300-11(a) raceways, cable assemblies, boxes, cabinets, and fittings must be securely fastened in place.
Nails are not permitted as fasteners unless they are used to attach brackets on the outside of the enclosure, or unless they extend through the interior within 1/4 inch of the back or ends of the enclosure. [370-23(b)(1)]
This requirement somewhat limits the use of nails as an approved fastening method. Where brackets are located on the outside of the enclosure, nails are permitted. Nails, passing through the interior, must be within 1/4 inch of the back or ends of the enclosure. The design of some metal device boxes (containing no exterior brackets) provides mounting holes meeting this specification.
Next month’s In Focus, beginning with Section 370-23(b)(2), will continue discussing Article 370, Part B, Installation. Section 370-23(b)(2) covers requirements for braces (wood, metal, or polymeric) used to support enclosures.
MILLER, owner of Lighthouse Educational Services and author of Illustrated Guide to the National Electrical Code, can be reached by phone at (615) 333-3336, or via e-mail at charles@charlesRmiller.com.