Box-Fill Calculations, Part IV

By Charles R. Miller | Oct 15, 2004




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ARTICLE 314 C Outlet, Device, Pull and Junction Bozes, Conduit Bodies; Fittings; and Manholes

314.16(B) Box-Fill Calculations

The National Electrical Code contains requirements for the numbers and sizes of conductors that can be installed in boxes and conduit bodies. The provisions pertaining to the installation and use of all boxes and conduit bodies used as outlet, device, junction or pull boxes are in Article 314. Prior to the reorganization of article numbers in the 2002 edition, Article 314 was Article 370. Specific requirements pertaining to the number of conductors in boxes (outlet, device and junction) and conduit bodies are in 314.16. Last month’s “In Focus” covered 314.16(A)(1) and (A)(2). This month, the discussion continues with box-fill calculations in 314.16(B).

The maximum number of conductors listed in Table 314.16(A) can only be installed in boxes not containing any fittings or devices. Where boxes contain fittings or devices such as fixture studs, cable clamps, hickeys, receptacles or switches, the maximum number of conductors from Table 314.16(A) must be reduced. Detailed box-fill calculation requirements are provided in 314.16(B)(1) through (5). The volume from conductor fill, clamp fill, support-fittings fill, device or equipment fill, and equipment-grounding conductor fill, where applicable, must be added together. Proper calculation depends on understanding which items are counted and how much volume is required for those items. Locknuts, bushings, wire connectors and other small fittings are not counted in box-fill calculations. (See Figure 1.)

314.16(B)(1) Conductor Fill

Each conductor originating outside the box that is terminated or spliced inside the box must be counted once. This is the most common type of installation within junction, outlet and device boxes. Conductors entering boxes are typically cut and spliced to other conductors, devices, equipment, light fixtures or any combination thereof. Once a conductor is cut, regardless of whether it is spliced or terminated, it counts as one conductor. For example, two conduits are entering a 4-inch square metal box. Each conduit contains a red, blue and white conductor. The red, blue and white conductors are spliced inside the box. Since all six conductors are spliced inside the box, all six conductors are counted in a box-fill calculation. (See Figure 2.)

Sometimes conductors enter and exit a box without being cut or spliced. Each conductor that passes through a box without splice or termination counts as one conductor. [314.16(B)(1)] A conductor that runs through a box uncut, in the same manner that larger conductors are installed in pull boxes, counts as only one conductor. For example, six 12 AWG conductors are running through a 4-inch square metal box that is 1 1/2 inches deep. Since the conductors are without splice or termination, only six conductors are counted. Table 314.16(A) permits a maximum of nine 12 AWG conductors in a 4-inch square box that is 1 1/2 inches deep. This installation is Code compliant. (See Figure 3.)

Caution is advised when cutting and splicing previously unbroken conductors within a box. This procedure could, depending on the box size and the number of conductors, violate the box-fill requirements. For example, a 4-inch square metal box that is 11/2 inches deep contains six unbroken 12 AWG conductors. If all the conductors are cut and spliced, the number of conductors in this box would double. Since the maximum number of 12 AWG conductors permitted in a 4-inch square box that is 1 1/2 inches deep is nine, this installation now violates the box-fill requirements. (See Figure 4.) If the conductors must be cut and spliced, one possible solution, space permitting, is to add an extension ring. (See Figure 5.)

A conductor, no part of which leaves the box, shall not be counted. [314.16(B)(1)] Since pigtails do not leave the box, they are not counted in box-fill calculations. Likewise, equipment-bonding jumpers are not counted because they do not leave the box. (See Figure 6.) Note that equipment-bonding jumpers are not the same as equipment-grounding conductors. Equipment-grounding conductors enter the box through a raceway or cable and therefore must be counted in box-fill calculations.

There is an exception under the conductor-fill provision that pertains to luminaire (fixture) conductors. An equipment-grounding conductor or four or fewer luminaire (fixture) wires smaller than 14 AWG, or both, entering a box from a domed luminaire (fixture) or similar canopy and terminating within that box, can be omitted from box-fill calculations. Under certain conditions, up to four luminaire (fixture) conductors and one equipment-grounding conductor can be installed but not counted in the box-fill calculation. The conductors must be 16 AWG or smaller and must enter the box from a domed luminaire (fixture) or similar canopy. (See Figure 7.)

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, [email protected] or


About The Author

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.” He can be reached at 615.333.3336 and [email protected]. Connect with him on LinkedIn.

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