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
Nail Through Box Mounting
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.