370-28 Pull and Junction Boxes
Article 370 covers the installation and use of all boxes (and conduit bodies) used as outlet, junction, or pull boxes, depending on their use. [370-1] Boxes containing No. 18 through No. 6 conductors must be sized in accordance with the specifications in 370-16. These boxes are calculated from the sizes and numbers of conductors. Unlike those boxes, pull and junction boxes containing conductors of No. 4 or larger, under 600 volts, are calculated from the sizes and numbers of raceways.
Specific provisions for calculating pull or junction boxes containing these larger-size conductors are covered in Section 370-28. These provisions apply to both metallic and nonmetallic boxes. Because of various types of raceway arrangements, two calculation methods are provided-straight pulls and angle or U pulls. For raceways containing conductors of No. 4 or larger, and for cables containing conductors of No. 4 or larger, the minimum dimensions of pull or junction boxes installed in a raceway or cable run must comply with 370-28(a)(1) and (2). [370-28(a)]
370-28(a)(1) Straight Pulls
Boxes containing straight pulls are sized according to the largest raceway entering the box. The length of the box must not be less than eight times the trade diameter of the largest raceway. The first step is to determine the type of pull. Installing a raceway in each end of a wireway (gutter) is one example of a straight pull.
The ends of this pull box provide only enough room for one raceway each. Although these raceways are straight across from each other, this is only one type of installation. A straight pull does not necessarily mean the raceways are directly opposite one another. Boxes containing raceways on opposite walls, regardless of the offset, qualify as straight pulls.
Computing the minimum dimensions of a box containing straight pull(s) is an easy calculation—simply multiply the largest raceway (trade diameter) by eight. For example, calculate the minimum dimension for a pull box with two 2-inch conduits. The conduits are directly across from each other on opposite walls.
Since the conduits are the same size, multiply either trade diameter (2 inches) by eight. The minimum length required for this box is 16 inches. What about the width and depth of this pull box? Unless a raceway enters the back of the box, no requirement specifies the width or depth. The box’s width and depth must be large enough to provide proper installation of the raceway (or cable), including locknuts and bushings.
Use the largest raceway to size pull boxes containing different-size raceways. For example, a pull box contains two conduits located on opposite walls. While one conduit has a trade diameter of 2 inches, the other conduit is 3 inches. What is the minimum length for this pull box?
Simply select the larger raceway (3 inches) and multiply by eight. This pull box’s length must be at least 24 inches.
Boxes having more than two raceway entries are calculated exactly the same, provided the box only contains straight pulls. For example, the left side of a box contains three conduits, one 3-inch, and two 2-inch conduits. The right side contains one 4-inch, one 3-inch, and one 2-inch conduit. What is the minimum length required for this box?
Since the largest single raceway is 4 inches, multiply four by eight. The minimum length for this box is 32 inches. No extra space is required for additional raceways when calculating the minimum length of a box containing straight pulls. Again, the width and depth of the box must be large enough to provide proper installation of the raceway (or cable), including locknuts and bushings.
Just because a box has raceway entries on all four sides does not automatically mean it contains angle pulls—the conductors could pass straight through the box.
Where raceways enter on all four sides and all the pulls are straight, two separate calculations are needed. First, find the largest-size raceway (trade diameter) for the left/right (horizontal) dimension and multiply by eight. Next, find the largest-size raceway for the top/bottom (vertical) dimension and multiply by eight. For example, the left and right sides of a box contain one 3-inch conduit each. The top and bottom of the same box contain one 2-inch conduit each. What are the minimum length and height dimensions required for this box?
The largest-size raceway for the left/right (horizontal) dimension is a 3-inch conduit. Therefore, the minimum length for this dimension is 24 inches. Since the largest-size raceway for the top/bottom (vertical) dimension is a 2-inch conduit, the minimum height of the box is 16 inches.
A 24- by 16-inch box is required for the configuration shown in Figure 8. What if, upon installation, this box is rotated 90 degrees? Of course, one dimension would be correct, but the other would not. A solution, which eliminates this possibility, is to buy a square box equal to the largest dimension.
Sometimes, confusion arises concerning these large boxes enclosing smaller conductors. For example, a 16-inch square box has one 2-inch conduit entering each side. Each conduit is filled to capacity with No. 12 and No. 10 conductors. Unless the cover is removed, the box appears to be the correct size. However, sizing this box per Section 370-28(a) is incorrect. Boxes enclosing conductors smaller than No. 4 must comply with Section 370-16. Another major concern pertains to the number of current-carrying conductors within each conduit. When more than three current-carrying conductors are in a raceway or cable, the allowable ampacity of each conductor must be reduced (derated) as shown in Table 310-15(b)(2)(a).
Next month’s In Focus, beginning with Section 370-28(a)(2), will continue discussion of pull and junction box sizing. This section covers calculation procedures for boxes containing angle or U pulls.
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.