at the table for metal boxes in the NEC, we do not find a listing given for seven No. 12 conductors in a device box. There are a couple of listings for eight conductors; one indicates a device box 3 by 2 by 3 1/2 inches is required. Since there will be an equivalent of just five conductors in the device box for the second outlet, the table shows a 3 by 2 by 2 1/2-inch box to be adequate.
The table does not cover all the requirements for conductor space in boxes. Boxes of 100 cubic inches or less, not covered by the table, and nonmetallic boxes are marked with their cubic inch capacity. When these boxes are used or when conductors of different sizes are installed in the same box, the number of conductors allowed in a box is based on the free air space requirement for each conductor. The free air space needed is given in table 370-16(b) in the NEC. According to the table, the volume of space needed in cubic inches per conductor is 2 for No. 14, .2 1/4 for No. 12, 2 1/2 for No. 10, and so on. As an example, if a box is to contain four No. 10 conductors and two No. 12 conductors, multiply 4 times 2 1/2 and 2 times 2 1/4. This equals 14 1/2 cubic inches, the minimum sized box that can be installed.
Figure 5-10. - Box installed with nails.
Outlet and junction boxes are installed in a number of ways in either new construction or an old building. Article 370 of the NEC gives the installation rules for outlet, switch, and junction boxes. In most cases, boxes in new construction are fastened with nails or screws. Usually, nails are preferred because they are cheaper and quicker to use. Unless the box has a bracket on it, the side of the box must be removed to use screws for mounting. Some of the newer box mounting brackets have prepunched and preformed devices that are driven into wood framing to support the box in the place of nails.
One of the simpler boxes to mount is a device box. Boxes without brackets are mounted by putting two sixteen penny nails (3 1/2 inches long) through the holes in both sides of the box and then driving them into the wood framing member (stud). Nails that pass through the inside of a box must not be more than a quarter inch from the bottom (or back) of the box, as shown in figure 5-10. Also, note the markings on the side of the box. These are depth markings which let you easily install the box to project the proper distance from the edge of the stud to offset the thickness of the wall material that will be installed. Another way to mount device boxes with sixteen penny nails is shown in figure 5-11. In this case, the nails are outside the box, eliminating the possibility of wiring interference inside. The extension of the box sides, as is done here to provide for nailing, is often referred to as an S bracket mount. The bracket is made so that the nails can be driven in straight or on a slant, depending on whichever is easier. Being able to drive the nails at a slant is especially useful when the stud spacing is less than normal. The notches on the front outer ends of the bracket serve as a depth guide for mounting, the same as the markings mentioned before.
Figure 5-11. - S bracket box.Continue Reading