Pull- and Junction-Box Calculations, Part VII

314.28 Pull and Junction Boxes

Chapter 3 of the National Electrical Code (NEC) covers wiring methods and materials. Article 314 covers the installation and use of all boxes and conduit bodies used as outlet, device, junction or pull boxes, depending on their use. Requirements pertaining to conduit bodies, handhole enclosures, and installation requirements for fittings used to join and connect raceways and cables to boxes and conduit bodies, are also in Article 314.

This article is divided into four parts: I. Scope and General, II. Installation, III. Construction Specifications, and IV. Pull and Junction Boxes for Use on Systems Over 600 Volts, Nominal. Boxes and conduit bodies containing conductors 18 AWG through 6 AWG must meet the installation requirements in 314.16. Boxes and conduit bodies enclosing conductors 4 AWG or larger (under 600 volts) must be installed in accordance with the requirements in 314.28.

When determining the minimum size box for conductors 18 through 6 AWG, the sizes and numbers of conductors are needed to calculate the minimum size box. The sizes and numbers of raceways must be known before sizing boxes containing conductors of 4 AWG or larger, under 600 volts.

Where pull and junction boxes are used on systems over 600 volts, the installation must comply with the specifications in 314.70. Last month’s column discussed boxes containing both straight and angle pulls together in one box. This month, the discussion continues with pull- and junction-box calculations.

314.28(A) Minimum-Size Pull and Junction Boxes

Before the 1999 edition of the NEC, splices were not a factor when sizing pull and junction boxes. Starting with the 1999 edition, 314.28(A)(2) became applicable with not only angle pulls and U pulls, but also with boxes containing splices. A box containing only straight pulls that is calculated by the straight-pull method could be too small if there are spliced conductors in the junction box. A junction box with only straight pulls containing spliced 4 AWG or larger conductors must comply with the minimum-size requirements of 314.28(A)(1) and (A)(2). Therefore, where splices are made in a junction box having straight pulls only, follow these steps: 1. Calculate by both methods, 2. Compare the dimensions and 3. Select the largest size.

For example, a junction box is needed for eight 2-inch conduits. Four raceways will enter the left side and four will enter the right. No raceways will enter the top or bottom of the box. Each raceway will contain four 2/0 AWG conductors. The conductors entering the right side will be spliced to the conductors entering the left side. Because of the spliced conductors, calculate by the straight- and the angle-pull method.

The minimum horizontal (left/right) dimension, calculated by the straight-pull method, is 16 inches (2 x 8 = 16). If the conductors were pulled through the box without being spliced, the minimum horizontal dimension would be 16 inches. Since each side contains the same size and number of raceways, either can be used for the angle-pull calculation. The largest conduit on the left side is 2 inches, therefore multiply two by six (2 x 6 = 12). Next, add to that number the trade size of the other raceways on the same wall of the box (12 + 2 + 2 + 2 = 18). The minimum length required for the horizontal (left/right) dimension is 18 inches (see Figure 1).

Up to this point, all raceway entries were on the sides (right, left, top or bottom) of the box. There has been no discussion pertaining to raceways entering the back of a box on the opposite side of a removable cover. An exception in 314.28(A)(2) specifies the minimum distance from the back wall to the removable cover.

Where a raceway or cable entry is in the wall of a box or conduit body opposite a removable cover, the distance from that wall to the cover shall be permitted to comply with the distance required for one wire per terminal in Table 312.6(A) (see Figure 2). Article 312 covers the installation and construction specifications of cabinets, cutout boxes and meter socket enclosures. Although there is no mention of pull or junction boxes in the scope of Article 312 or in 312.6, the exception in 314.28(A)(2) permits the use of Table 312.6(A).

The distances specified in Table 312.6(A) are based on the conductor size (AWG or Kcmil) within the raceway and not the trade size (or metric designator) of the raceway. Although there are columns for up to five wires per terminal, select the distance specified for one wire per terminal. For example, a pull box is needed for two 3-inch raceways containing 500 Kcmil conductors. While one raceway enters the bottom of the box, the other enters the back opposite a removable cover. The trade size of the raceway is not a factor in determining the minimum distance from the back of the box to the cover.

Find the wire size (AWG or Kcmil) 400–500 in the left-hand column of Table 312.6(A). Follow the row across, and select the distance specified in the “1” column. The distance from the back of the box to the cover must be at least 6 inches (see Figure 3). Without this exception, the minimum depth of this box would be 18 inches as determined by the angle-pull method.

Unless all raceway entries are in the back of the box, at least one angle-pull calculation will be necessary to determine other dimensions for the junction- or pull-box. For example, a pull box is needed for two raceway entries. While one 2-inch raceway enters the bottom of the box, the other raceway entry is in the back of the box. The raceways will enclose 3/0 AWG conductors.

First, determine the minimum depth of the box. Find the wire size 3/0 AWG and follow the row across to the “1” column. The minimum depth required for this box is four inches. Next, use the angle-pull method to calculate the minimum distance for the vertical (top/bottom) dimension. Multiply the 2-inch raceway by six, and since no other raceways are entering the bottom, nothing is added to that number (2 x 6 = 12). The minimum length required for the vertical dimension is 12 inches (see Figure 4).

Compliance with one other dimension must also be observed. Since the two 2-inch raceways in Figure 4 enclose the same conductors, the distance between raceway entries must not be less than six times the trade diameter of the largest raceway. Therefore, the conduit entries must be at least twelve inches apart (see Figure 5).

Next month, the discussion of pull- and junction-box calculations continues. 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.


About the Author

Charles R. Miller

Code Contributor

Charles R. Miller, owner of Lighthouse Educational Services, teaches custom-tailored seminars on the National Electrical Code and NFPA 70E. He is the author of “Illustrated Guide to the National Electrical Code” and “Electrician's Exam Prep Manual.”...

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