chapter 8 interior wiring and lighting

When gangable boxes are assembled together box volume is the total of all sections assembled together Also when a raised cover or a box extension with volume markings is added to a box as shown in Figure 8-16 its capacity is added to the volume

Select the box to be mounted in each location after you know the number and size of conductors it will contain In many cases the usual box for the intended purpose is not adequate To ensure proper air space for a conductor in a box consult Table 31416(A) for metal boxes in the NECreg This table lists dimensions for common boxes their capacity in cubic inches and the maximum permitted number of conductors in wire sizes Number 18 through Number 6 The number of conductors listed does not make allowance for fixture studs cable clamps grounding conductors switches or receptacles or straps for mounting these devices Deduct one conductor from those listed when the box contains one or more fixture studs or cable clamps Each strap containing one or more devices requires deduction of one conductor Also deduct one conductor for one or more grounding conductors that enter the box Count a conductor that runs through the box as one conductor Each conductor that terminates in the box counts as one Fixture wires and conductors that do not leave the box such as an internal grounding wire are not counted Let us use an example to see how the table works Two receptacle outlets are to be installed using Number 12 nonmetallic cable One is to be an extension to the other Device boxes with cable clamps are to be used if possible The first step is to determine the number of conductors that will be wired through the box Two conductors plus a grounding conductor enter this box Also if a second outlet is to be connected to this one then two conductors plus a grounding conductor must leave the box If the preceding rules are followed we have four conductors plus one for the grounding conductors the equivalent of one conductor for the cable clamps and the equivalent of one conductor for the receptacle outlet This method gives us an equivalent of seven conductors The table for metal boxes in the NECreg has no listing given for seven Number 12 conductors in a device box There are a couple of listings for eight conductors one indicates a device box (3 X 2 X 312 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 X 2 X 2 12 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 are 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 31416(B) in the NECreg According to the table the volume of space needed in cubic inches per conductor is 2 for Number 14 2frac14 for Number 12 2frac12for Number 10 and so on As an example if a box is to contain four Number 10 conductors and two Number 12 conductors multiply 4 times 2 12 and 2 times 2 14 This equals 14 12 cubic inches the minimum sized box that can be installed Outlet and junction boxes are installed in a number of ways in either new construction or an old building Article 314 of the NECreg 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 better 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

NAVEDTRA 14026A 8-26