314.28 Pull and Junction Boxes
Requirements pertaining to the installation and use of all boxes and conduit bodies used as outlet, device, junction or pull boxes are in Article 314 of the National Electrical Code (NEC). This article also covers conduit bodies, handhole enclosures, and installation requirements for fittings used to join raceways and to connect raceways and cables to boxes and conduit bodies. Provisions for sizing boxes and conduit bodies are in 314.16 and 314.28.
Boxes and conduit bodies containing conductors 18 through 6 AWG must be installed in accordance with the requirements in 314.16. Boxes containing these conductors are calculated from the sizes and numbers of conductors. Boxes and conduit bodies enclosing conductors larger than 6 AWG, under 600 volts, must meet the installation requirements in 314.28.
Boxes containing conductors 4 AWG or larger, under 600 volts, are calculated from the sizes and numbers of raceways. The actual sizes and numbers of conductors are not needed when calculating a box in accordance with 314.28. Last month’s column discussed boxes containing splices and boxes with raceways entering the back of a box opposite a removable cover. This month concludes the series on pull- and junction-box calculations.
314.28(A) Minimum Size Pull and Junction Boxes
One example in Part VII of pull- and junction-box calculations illustrated a junction box with one raceway entering the bottom and one raceway entering the back of the box. As specified in the exception to 314.28(A)(2), the distance from a wall to a removable cover shall be permitted to comply with distance required for one wire per terminal in Table 312.6(A). Where more than one raceway enters the back of a box (opposite of a removable cover), select the largest size conductor entering the back.
For example, a pull box is needed for six raceways. Three raceways will enter the bottom, and three raceways will enter the back opposite a removable cover. One 3-inch, one 2-inch and one 1-inch raceways will enter the back and one 3-inch, one 2-inch and one 1-inch raceways will enter the bottom of the box.
The 3-inch raceways will enclose 500 Kcmil conductors, the 2-inch raceways will enclose 3/0 AWG conductors, and the 1-inch raceways will enclose 4 AWG conductors. In determining the minimum distance from the back of the box to the cover, the trade sizes of the raceways and the number of conductors are not factors.
Select the largest conductors entering the back of the box to determine the minimum depth of the box. The column for one wire per terminal in Table 312.6(A) specifies 6 inches for 500 Kcmil conductors. Although three raceways enter the back of the box, the minimum distance from the back to the removable cover is just 6 inches (see Figure 1).
The minimum vertical (top/bottom) dimension for the box in Figure 1 is determined by the angle pull method. First, multiply the trade size (metric designator) of the largest raceway by six (3 x 6 = 18). Next, add to that number the trade size of the other raceways in the same row on the same wall of the box (18 + 2 + 1 = 21). The minimum length required for the vertical dimension is 21 inches (see Figure 2).
One more consideration is necessary to make sure the box dimensions in Figure 1 are Code-compliant. The distance between raceway entries enclosing the same conductors must be at least six times the trade size (metric designator) of the larger raceway. Because the 3-inch raceways will enclose the same conductors, the distance between them must be at least 18 inches (3 x 6 = 18). The distance between the 2-inch raceways must be at least 12 inches (2 x 6 = 12). The distance between the 1-inch raceways must be at least 6 inches (1 x 6 = 6). Because of where the raceways enter the box, this installation is Code compliant (see Figure 3).
Where all raceway entries are in the back of the box, there are only two things to consider: 1) the depth of the box and 2) the distance between raceway entries enclosing the same conductors. For example, a pull box is needed for two 3-inch raceways. Both raceways enter the back of the box opposite a removable cover. The raceways will contain 500 Kcmil conductors.
As per Table 312.6(A), the minimum depth of the box is 6 inches. Because the raceways will enclose the same conductors, the minimum distance between raceway entries is 18 inches. Since there are only two raceways, the pull box could be long and narrow. In this installation, there is no minimum vertical (top/bottom) dimension. The vertical dimension must be large enough to provide proper installation of the raceway, including locknuts and bushings (see Figure 4).
Pull or junction boxes enclosing cables containing conductors of 4 AWG or larger must also comply with the requirements of 314.28(A)(1) and (A)(2). When transposing cable size into raceway size in 314.28(A)(1) and (A)(2), the minimum trade size (metric designator) required for the number and size of conductors in the cable shall be used. [314.28(A)(2)] The overall dimension of the cable is not a factor. The first step for sizing boxes containing cables is to convert each cable size into a raceway size. To transpose cable size into raceway size, simply find the minimum trade size (metric designator) raceway required for the size and number of individual conductors within the cable. Use Tables 4 and 5 of Chapter or Appendix C to find the raceway size.
For example, a junction box is needed for two cables. One cable will enter the left side and one will enter the right. Each cable contains three 2 AWG XHHW conductors. First, convert the cable size into a raceway size. Appendix C, Table C1, specifies the minimum size required for three 2 AWG XHHW conductors is a 1-inch raceway. Next, size the junction box.
Since the cables are on opposite walls, calculate with the straight-pull method to find the minimum horizontal (left/right) dimension. Because the conductors within the cable would require a 1-inch raceway, use 1 inch as the cable’s trade size. Multiply the 1-inch trade size by eight (1 x 8 = 8). The minimum length required for the horizontal (left/right) dimension is eight inches (see Figure 5). EC
MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and NFPA’s “Electrical Reference.” He can be reached at 615.333-3336, charles@charlesRmiller.com or www.charlesRmiller.com.