ARTICLE 314 C OUTLET, DEVICE, PULL AND JUNCTION BOXES; CONDUIT BODIES; FITTINGS; AND MANHOLES


314.16(B) Box-Fill Calculations

Article 314 of the National Electrical Code contains requirements pertaining to the installation and use of all boxes and conduit bodies used as outlet, device, junction or pull boxes. Article 314 also covers manholes and other electric enclosures intended for personnel entry. Provisions for calculating the minimum size outlet, device or junction box (or conduit body) or the maximum number and sizes of conductors permitted in a box (or conduit body) are in 314.16. While box volume calculations are in 314.16(A), box fill calculations are in 314.16(B). Last month's Code In Focus covered 314.16(B) and 314.16(B)(1). This month, the discussion continues with box fill calculations in 314.16(B)(1).

When all conductors within the box are the same size, simply count the number of conductors. (Equipment grounding conductors will be discussed at a later time.) If the box is a metal box listed in Table 314.16(A) and all the conductors are the same size, the maximum number of conductors provided in the appropriate column can be installed. For example, a metal junction box is needed for four electrical metallic tubing raceways. Three 12 AWG unbroken conductors enter the box from one conduit, and with sufficient slack, exit the box through another conduit. The unbroken conductors are not more than two times the minimum length required for free conductors as specified in 300.14. The other two conduits contain three 12 AWG conductors, each of which will be spliced together. What is the minimum depth four-inch square box required? (The box contains no fittings or devices.) Three 12 AWG conductors running through the box without a splice (unbroken) count simply as three conductors. Since three of the 12 AWG conductors are to be spliced to three other 12 AWG conductors, they are counted as six. Therefore the total number of 12 AWG conductors is nine. In accordance with Table 314.16(A), the minimum depth required for a four-inch square box enclosing nine conductors is 1 1/2 inches. (See Figure 1.) The maximum number of conductors in Table 314.16(A) cannot be installed if the box contains any of the following items: internal cable clamp, luminaire (fixture) stud, hickey, device or equipment.

The 2005 edition of the National Electrical Code added a new provision pertaining to conductor fill in 314.16(B)(1). A looped, unbroken conductor not less than twice the minimum length required for free conductors in 300.14 must be counted twice. An unbroken conductor, passing through the box without splice or termination, can be counted as one conductor if it is less than two times the minimum length required for free conductor in 300.14. For example, two conduits are entering opposite sides of a four-inch square metal box that has a depth of 1 1/2 inches. One black and one blue conductor enter the box from one conduit and exit the box through the other conduit. The conductors are looped through the box without splice or termination. Each conductor measures 11 inches in length from the point where it leaves one conduit to the point where it enters the other conduit. Since the conductors are less than twice the minimum length required for free conductor in 300.14, each conductor can be counted as one conductor. (See Figure 2.)

Until the 2005 edition, the length of an unbroken conductor was not a factor. Regardless of the length, it only counted as one conductor. Now, in the 2005 NEC, if the unbroken conductor is two times or more the minimum length required for free conductors in 300.14, it must be counted as two conductors. For example, two conduits are entering opposite sides of a four-inch square metal box that has a depth of 1 1/2 inches. One black and one blue conductor enter the box from one conduit and exit the box through the other conduit. The conductors are looped through the box without splice or termination. Each conductor measures 12 inches in length from the point where it leaves one conduit to the point where it enters the other conduit. Since the minimum length required for free conductors in 300.14 is six inches (150 mm), this conductor is twice the minimum length. Therefore in accordance with the new provision, each conductor must be counted as two conductors. (See Figure 3.)

The specification for the length of free conductors at outlets, junctions and switch points is in the first article of Chapter 3, Wiring Methods and Materials. At least six inches (150 mm) of free conductor, measured from the point in the box where it emerges from its raceway or cable sheath, must be left at each outlet, junction and switch point for splices or the connection of luminaires (fixtures) or devices. [300.14] For many years, there was no explanation of how to measure the length of free conductor. In the 1999 edition of the NEC, the wording was specifically and clearly stated. The measurement starts from the point in the box where the conductor emerges from its raceway or cable sheath. (See Figure 4.)

Where the opening to an outlet, junction or switch point is eight inches (200 mm) or more at the least dimension, the conductors are not required to extend beyond the opening of the box. Although they are not required to extend beyond the box's opening, each conductor must still have at least six inches (150 mm) of free conductor. For example, two conduits are entering a 10-inch square metal box that is six inches deep. Because of the lip that holds the cover plate, the vertical and horizontal dimension of the box's opening is nine inches by nine inches. Since the opening is not less than eight inches (200 mm) in any dimension, the conductors are not required to extend beyond the box's opening. (See Figure 5.)

Where the opening to an outlet, junction, or switch point is less than eight inches (200 mm) in any dimension, each conductor must be of sufficient length to extend at least three inches (75 mm) outside of the opening. Satisfying this requirement eliminates the occurrence of very short conductors within deep boxes. (See Figure 6.)

Conductors that are not spliced or terminated at the outlet, junction or switch point are not required to comply with the minimum length for free conductors in 300.14 [300.14 Exception]. Although no minimum length is specified, it is a good idea to leave adequate length, so that unbroken (looped) conductors can be spliced or terminated and still comply with 300.14.

Next month's Code in Focus will continue the discussion of box-fill 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.