314.28 Pull and Junction Boxes

The National Electrical Code covers outlet, device, pull and junction boxes; conduit bodies; fittings; and handhole enclosures in Article 314. As stated in the scope (314.1), this article covers the installation and use of all boxes and conduit bodies used as outlet, device, junction or pull boxes, depending on their use, and handhole enclosures.

Specifications for sizing boxes and conduit bodies containing conductors 18 AWG through 6 AWG are in 314.16. The sizes and numbers of conductors must be known before sizing a box in accordance with this section. In 314.16, conduits or raceways are not factored into the calculation. Boxes and conduit bodies enclosing conductors larger than 6 AWG, under 600 volts, are calculated from the sizes and numbers of raceways. Besides knowing the sizes and numbers of raceways, the types of pulls must be known before calculating in accordance with 314.28. While straight-pull specifications are in 314.28(A)(1), the procedures for calculating angle and U pulls are in 314.28(A)(2). Part IV of Article 314 contains requirements for pull and junction boxes used on systems over 600 volts. Last month’s column discussed angle pulls. This month, the discussion continues with angle-pull calculations.

314.28(A)(2) Angle or U Pulls

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 shall be used. In a box containing angle or U pulls, it is not necessary to include all the raceways in one calculation if there is more than one row. Where a box has more than one row, calculate each row individually. Think of each row as a separate box (see Figure 1).

If there are two rows, perform two box calculations. The row requiring the maximum distance is the minimum length for the dimension being calculated. For example, the front row for the right side of a box contains a 3-inch and a 2-inch raceway. The back row on the right side has a 4-inch and 1-inch raceway entering the box. The front row for the top of the box contains a 3-inch and a 2-inch raceway. The back row on top has two 3-inch raceway entries. All conductors are larger than 6 AWG and all the pulls are angle pulls. 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.

The minimum length required for the right side of the front box is 20 inches (3 x 6 + 2). The minimum length required for the right side of the back box is 25 inches (4 x 6 + 1).

The minimum length required for the top of the front box is 20 inches (3 x 6 + 2). The minimum length required for the top of the back box is 21 inches (3 x 6 + 3). The minimum length for the X (right to left) dimension is 25 inches, and the minimum length for the Y (top to bottom) dimension is 21 inches (see Figure 2).

Increase the distance for additional entries for angle or U pulls in boxes with more than two raceways in each row. In the last example, the minimum length was determined by the back row, but that may not always be the case. For example, the front row for the right side of a box contains four 3-inch conduits. The back row on the right side contains three 4-inch conduits.

The front row for the top of the box contains one 2-inch, one 4-inch and two 3-inch conduits. The back row for the top contains five 3-inch conduits. All conductors are larger than 6 AWG and all the pulls are angle pulls. The left/right (horizontal) dimension will be the X dimension, and the top/bottom (vertical) dimension will be the Y dimension. Since there are two rows of conduit entries for both the right side and top of this box, calculate each row separately.

The largest conduit for the front row on the right side is 3 inches, therefore multiply three by six (3 x 6 = 18). Add to that number the trade size (or metric designator) of the other raceways in the same row (18 + 3 + 3 + 3 = 27). Since the largest conduit for the back row on the right side is 4 inches, multiply four by six (4 x 6 = 24). Add to that number the other conduits in the same row, on the same wall of the box (24 + 4 + 4 = 32). The minimum length required for the X (right to left) dimension is the larger of the two individually calculated rows: 32 inches. The largest conduit entering the top for the front row is 4 inches, therefore multiply four by six (4 x 6 = 24). Add to that number the trade size of the other raceways in the same row (24 + 3 + 3 + 2 = 32). Since the largest conduit for the back row on top is 3 inches, multiply three by six (3 x 6 = 18).

Add to that number the other conduits in the same row, on the same side of the box (18 + 3 + 3 + 3 + 3 = 30). The minimum length required for the Y (top to bottom) dimension is 32 inches (see Figure 3). Note the largest number for the X (right to left) dimension is from the back row, but the largest number for the Y (top to bottom) dimension is from the front row.

A U pull is where two raceways enclosing the same conductors are located on the same wall of a box (see Figure 4). Boxes can contain U pull(s), angle pull(s), straight pull(s) or any combination thereof. Calculate U pulls using the same method as angle pulls. A box with conduit entries only on one wall will have a minimum distance to the opposite wall.

Multiply the largest raceway by six and add the sum of the other raceways entering the same wall. For example, the bottom of a box contains two 2-inch conduits that will enclose the same 3/0 copper conductors. This box will contain no other conduit entries. Since both conduits are trade size 2 inches, multiply two by six (2 x 6 = 12). Add to that number the trade size of the other raceway (12 + 2 = 14). The minimum length required for the bottom to top dimension is 14 inches (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.