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.
Boxes and conduit bodies containing conductors 18 AWG through 6 AWG must be installed in accordance with the provisions in 314.16. Boxes enclosing these conductors are calculated from the sizes and numbers of conductors. Boxes containing conductors of 4 AWG or larger, under 600 volts, are calculated from the sizes and numbers of raceways.
Where pull and junction boxes are used on systems over 600V, the installation must comply with the provisions in 314.70. Calculation procedures are provided for three types of pulls: straight, angle and U.
Last month’s Code in Focus started discussing angle pulls. This month, the discussion continues with angle-pull calculations.
314.28(A)(2) Angle or U Pulls
Boxes and conduit bodies containing conductors larger than 6 AWG (under 600V), and containing angle or U pulls, must be sized in accordance with the specifications in 314.28(A)(2).
Where splices or where angle or U pulls are made, the distance between each raceway entry inside the box and the opposite wall of the box must not be less than six times the trade size (metric designator) of the largest raceway in a row.
This distance must be increased for additional entries by the amount of the sum of the diameters of all other raceway entries in the same row on the same wall of the box. Each row shall be calculated individually, and the single row that provides the maximum distance must be used.
This dimension starts on the wall where the raceway enters and ends on the opposite wall, not where the conductors actually go. In a box with angle pull(s), if there is only one raceway entry, multiply the trade size (metric designator) of the raceway by six.
This is a simple calculation if there is only one angle pull and two raceways. Where there are two angle pulls and four raceways (one on each side of the box), select the largest raceway for the left/right (horizontal) dimension and the largest raceway for the top/bottom (vertical) dimension.
For example, a pull box is needed for four raceways that will contain two angle pulls. A 2-inch conduit will enter the top and left side of the box. A 3-inch conduit will enter the bottom and right side of the box. The largest raceway for the horizontal dimension is a 3-inch conduit. Therefore, the minimum length for this dimension is 18 inches (3 × 6).
Since the largest raceway for the top/bottom dimension is also a 3-inch conduit, the minimum length for the vertical dimension is 18 inches (3 × 6). The minimum size required for this box is 18 inches by 18 inches (see Figure 1).
In the last example, each wall had only one raceway entry. If more than one raceway entry is on a wall, the calculation requires an additional step. First, select one wall of the box and multiply the trade size (metric designator) of the largest raceway by six.
Next, add to that number the trade size of all other raceways in the same row, on the same wall of the box. By using a simple formula, this calculation is made easy (see Figure 2).
Where all the raceways are the same trade size, select one and multiply by six. For example, the right side of a box contains two 2-inch conduits, and the top also contains two 2-inch conduits.
For the purpose of these illustrations, the left/right (horizontal) dimension is referred to as the “X” dimension, and the top/bottom (vertical) dimension is the “Y” dimension. Since both conduits on the right side are 2-inch, multiply two by six (2 × 6 = 12). Next, add to that number the trade size of the other raceway on the same wall of the box.
The minimum length required for the “X” dimension is 14 inches (2 × 6 + 2). Since the top of the box contains the same size and number of conduits, this calculation is the same as the left/right (horizontal) dimension. Therefore, the minimum “Y” dimension is also 14 inches (see Figure 3).
Where different sizes of raceways enter boxes and the pulls are angle pulls, select the largest trade size (metric designator) of the largest raceway and multiply by six. Add to that number the trade size of all other raceways in the same row, on the same wall of the box.
For example, a pull box is needed for six raceways. One 3-inch and two 2-inch conduits will enter the right side. One 4-inch, one 3-inch and one 2-inch conduit will enter the top of the box. To find the minimum length for the “X” (horizontal) dimension, start by multiplying the trade size of the largest conduit on the right side by six (3 × 6 = 18).
Add to that number the other conduits in the same row, on the same side of the box (18 + 2 + 2 = 22). The minimum length required for the “X” dimension is 22 inches. Because the raceways entering the top are not the same as the side of the box, a separate calculation is required.
Since the largest trade size conduit is 4 inches, multiply four by six (4 × 6 = 24). Add the other conduits in the same row, on the same side of the box, to that number (24 + 3 + 2 = 29). The minimum length required for the “Y” dimension is 29 inches (see Figure 4).
Raceways can be installed in more than one row in junction or pull boxes. In this type of installation, it is not necessary to include all the raceways into one calculation. Where two or more rows of raceways enter a box, calculate each row individually. The single row that requires the maximum distance is the minimum length for dimension being calculated.
For example, a junction box has two rows of raceways on the right side and two rows on top. All the pulls in the box will be angle pulls. At the top of the box, the front row has four conduits and the back row has five conduits. On the right side of the box, the front row has four conduits and the back row has three conduits.
Although the top has nine raceways, all nine are not included in one calculation. The same can be said for the right side. It is not necessary to include all eight raceways into one calculation (see Figure 5).
Next month’s Code in Focus continues the discussion of pull and junction box calculations. 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.