370-16(a) Box Volume Calculations

Last month's In Focus introduced box fill, with emphasis on Section 370-16(a)'s box volume calculations. Table 370-16(a) is used to determine the maximum number of conductors (for sizes No. 18 through No. 6) permitted within common-size metal boxes. The number listed under each conductor size is the maximum allowed within a box containing no fittings or devices, such as fixture studs, cable clamps, hickeys, switches, or receptacles. Two factors, not relevant to Section 370-16, are the conductor material and the type of insulation.

370-16(b) Box Fill Calculations

It is necessary to reduce the maximum number of conductors permitted in boxes containing fittings or devices. Detailed box fill calculation requirements are provided in Section 370-16(b)(1) through (5). Proper calculation depends upon understanding which items are counted and how much volume is required for those items. Fill is computed for the following items: conductors, clamps, support fittings, devices or equipment, and equipment grounding conductors. Locknuts, bushings, wirenuts, and other small fittings are not counted in box fill calculations.

370-16(b)(1) Conductor Fill

Each conductor originating outside the box, which is terminated or spliced inside the box (the most common installation occurrence within junction, outlet, and device boxes), counts as one conductor. Conductors entering boxes are typically cut and joined (spliced) to other conductor(s), device(s), equipment, lighting fixture(s), or any combination thereof. Once a conductor is cut, regardless of whether it is spliced or terminated, it counts as one conductor.

A second common situation exists, which does not involve conductor splicing. In these instances, conductors run through a box uncut, in the same manner that larger conductors are installed in pull boxes. A conductor passing through the box without splice or termination (unbroken) counts as one conductor. Caution is advised when cutting and splicing previously unbroken conductors within a box. This procedure could, depending upon the box size and the number of conductors, violate box fill requirements. For example, a 4-inch square box with a depth of 1-1/2 inches contains four unbroken No. 12 conductors (black, red, blue, and white).

Another red, blue, and white No. 12 conductor is added. The existing red, blue, and white conductors are cut and spliced with the new conductors. This now exceeds the maximum number of conductors permitted for this size box. The remaining unbroken black conductor still counts as only one. The three new conductors count as three conductors. Since the existing red, blue, and white conductors have been cut, they now count as six, bringing the total number of conductors within this box to 10. Table 370-16(a) allows only nine No. 12 conductors within a 4-inch-by-1.5-inch square box. The example just described, therefore, represents a Code violation. A possible solution, space permitting, is the addition of an extension ring.

Conductors that do not exit the box, such as equipment bonding jumpers and pigtails, are not counted.

Equipment grounding conductor(s) and four or fewer fixture wires (No. 16 and smaller), entering a box from a domed fixture (or similar canopy) and terminating within that box, can be omitted from box fill calculations. [370-16(b)(1) Exception]

When all conductors within the box are the same size, simply count the number of conductors. If the box is a metal box listed in Table 370-16(a) and all the conductors are the same size, the maximum number of conductors is furnished in the appropriate column.

Example-a metal junction box is needed for four conduits. Three No. 12 unbroken conductors exit one conduit, and with sufficient slack, enter another conduit. The other two conduits contain three No. 12 conductors, each of which will be spliced together. What is the minimum depth 4-in. square box required? (The box contains no fittings or devices.) Three No. 12 conductors running through the box without splice (unbroken) count simply as three conductors. Since three of the No. 12 conductors are to be spliced to three other No. 12 conductors, they are counted as six. The total number of No. 12 conductors then is nine. Per Table 370-16(a), the minimum depth required for a 4-inch square box enclosing nine conductors is 1.5 inches.

When the box contains different-size conductors, or the box is not listed in Table 370-16(a), each conductor's cubic-inch volume is needed. (Note: Some nonmetallic boxes are marked with the maximum number of conductors permitted within the box, according to conductor size. For example: 14/9, 12/8, and 10/7, which means nine No. 14's, eight No. 12's, or seven No. 10's.) Table 370-16(b) provides the cubic-inch volume for No. 18 through No. 6 size conductors. The total cubic-inch volume for conductors, fittings, devices, etc. must not exceed the cubic-inch capacity of the box (or assembled sections).

Example-a 4-inch square metal box with a depth of 1.5 inches presently contains four No. 12 conductors. How many No. 14 conductors can be added? (There are no fittings or devices in the box.) First, find the box's cubic-inch capacity from Table 370-16(a); in this case, 21 cubic inches. Next, find the cubic-inch volume for each No. 14 and No. 12 conductor. No. 14 = 2 cubic inches and No. 12 = 2.25 cubic inches (Remember, conductor insulation is not a factor.) The total volume for four No. 12 conductors is 9 cubic inches (4 x 2.25 = 9). Now, deduct the total volume of these No. 12 conductors. The space remaining for the No. 14 conductors is 12 cubic inches (21 - 9 = 12). Finally, divide the available space by the No. 14's cubic-inch volume (12/2 = 6). As many as six No. 14 conductors can be added within this box.

300-14: Length of Free Conductors at Outlets, Junctions, and Switch Points

At least 6 inches of free conductor must be left at each outlet, junction, and switch point to facilitate splices, or the connection of fixtures or devices. Although the 6 inches of free conductor has been a National Electrical Code (NEC) requirement for many years, prior to the 1999 edition no explanation was offered as to how the measurement should be made. The new wording states, specifically and clearly, that the measurement starts from the place in the box where the conductor comes out of the raceway or cable sheath.

Where the opening to an outlet, junction, or switch point is less than 8 inches in any dimension, conductors must be of sufficient length to extend a minimum of 3 inches outside of the opening. Satisfying this requirement eliminates the occurrence of very short conductors within deep boxes.

No minimum length is specified for unbroken conductors within an outlet, junction, or switch point. [300-14 Exception] Although no minimum is specified, it is a good idea to leave adequate length, so that an unbroken conductor can be spliced or terminated, and still comply with Section 300-14.

Next month's issue of In Focus will conclude the procedures for calculating box fill. Specific items will include clamp fill, support fittings fill, device or equipment fill, equipment grounding conductor fill, and conduit bodies.

MILLER, owner of Lighthouse Educational Services, teaches custom-tailored classed and conducts seminars covering various aspects of the electrical industry. He is the author of Illustrated Guide to the National Electrical Code. For more information, visit his Web site at www.charlesRmiller.com. He can be reached by phone at (615) 333-3336, or via e-mail at charles@charlesRmiller.com.