5.3 Boxes and Enclosures: Box Fill and Pull Box Sizing
Key Takeaways
- Table 314.16(B) assigns free space per conductor: 2.00 cubic inches for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG, 3.00 for 8 AWG, and 5.00 for 6 AWG.
- Box fill counts one volume per conductor entering and terminating or passing through, one volume total for all internal cable clamps, two volumes per yoke or strap for each device, and one volume total for all equipment grounding conductors.
- A conductor that both originates and terminates within the box, such as a pigtail, is not counted at all.
- NEC 314.28(A)(1) requires straight pulls to have a length not less than eight times the trade size of the largest raceway; 314.28(A)(2) requires angle and U pulls to be six times the largest raceway plus the sum of the other raceway diameters in the same row.
- NEC 314.29 requires boxes and conduit bodies to be installed so that the wiring inside is accessible without removing any part of the building or, in underground installations, without excavating.
Why Box Fill Exists
Conductors crammed into a box are hard to fold without damaging insulation, they trap heat, and they push against devices when the cover goes on. 314.16 sets a maximum number of conductors based on the free volume the box provides.
314.16(A) Box Volume Calculations. The volume of a wiring enclosure is the total volume of its assembled sections and any listed extension rings, plaster rings, domed covers, and mud rings marked with their volume. Standard box volumes are listed in Table 314.16(A) for metal boxes; nonmetallic boxes must be durably and legibly marked with their volume by the manufacturer.
Table 314.16(B) — Volume Required per Conductor
| Conductor size | Free space required |
|---|---|
| 18 AWG | 1.50 cubic inches |
| 16 AWG | 1.75 cubic inches |
| 14 AWG | 2.00 cubic inches |
| 12 AWG | 2.25 cubic inches |
| 10 AWG | 2.50 cubic inches |
| 8 AWG | 3.00 cubic inches |
| 6 AWG | 5.00 cubic inches |
The Five Counting Rules
(1) Conductor fill — 314.16(B)(1). Each conductor that originates outside the box and terminates or is spliced within the box counts as one. Each conductor that passes through the box without splice or termination counts as one. A conductor that originates and terminates within the box — a pigtail or jumper — is not counted.
(2) Clamp fill — 314.16(B)(2). Where one or more internal cable clamps are present, a single volume allowance based on the largest conductor in the box shall be made. Cable connectors with their clamping mechanism outside the box do not count. Note this is one allowance total, not one per clamp.
(3) Support fittings fill — 314.16(B)(3). Where one or more luminaire studs or hickeys are present, a single volume allowance for each type shall be made, based on the largest conductor present.
(4) Device or equipment fill — 314.16(B)(4). For each yoke or strap containing one or more devices or equipment, a double volume allowance based on the largest conductor connected to that yoke shall be made. A device that is too large for a single gang box, occupying two gang positions, requires double volume for each gang it occupies.
(5) Equipment grounding conductor fill — 314.16(B)(5). Where one or more equipment grounding conductors enter the box, a single volume allowance based on the largest equipment grounding conductor present shall be made. Where an additional set of isolated equipment grounding conductors is present per 250.146(D), an additional one-quarter volume allowance based on the largest isolated EGC shall be made.
Worked Example 1 — Switch Box
A device box contains two 14/2 NM cables with ground, internal cable clamps, and one single-pole switch. What minimum box volume is required?
Insulated conductors: 2 cables x 2 conductors = 4 x 2.00 = 8.00 cu in
Equipment grounding conductors: all count as ONE x 2.00 = 2.00 cu in
Internal clamps: ONE allowance, largest conductor x 2.00 = 2.00 cu in
Device yoke (switch): TWO allowances 2 x 2.00 = 4.00 cu in
-----------------
Total required 16.00 cu in
16.00 cubic inches. A standard 3 x 2 x 3.5-inch device box is 18.0 cubic inches, so it is acceptable; a 3 x 2 x 2.5-inch box at 12.5 cubic inches is not.
Worked Example 2 — Junction Box with Mixed Sizes
A 4 x 4 x 1.5-inch square box (21.0 cubic inches) contains: three 12/2 NM cables with ground, one 10/2 NM cable with ground, internal clamps, and no devices. Does it comply?
12 AWG insulated: 3 cables x 2 = 6 x 2.25 = 13.50 cu in
10 AWG insulated: 1 cable x 2 = 2 x 2.50 = 5.00 cu in
EGCs: one allowance at the LARGEST EGC (10 AWG) = 2.50 cu in
Clamps: one allowance at the LARGEST conductor (10 AWG) = 2.50 cu in
----------------
Total required 23.50 cu in
23.50 cubic inches required against 21.0 available — it does not comply. Two points to notice: the equipment grounding conductors get one allowance total, sized on the largest of them, and the clamp allowance is also sized on the largest conductor in the box, not on the conductor the clamp happens to grip.
Worked Example 3 — A Pigtail Does Not Count
Add a 12 AWG pigtail from the neutral splice to a receptacle inside the box in Example 2.
No change. 314.16(B)(1) excludes conductors that both originate and terminate within the box. Pigtails are free.
Conduit Bodies — 314.16(C)
Conduit bodies enclosing 6 AWG or smaller conductors, other than short-radius bodies, shall have a cross-sectional area not less than twice the cross-sectional area of the largest conduit or tubing to which they are attached. The maximum number of conductors is the same as permitted for the entering raceway.
Conduit bodies shall not contain splices, taps, or devices unless they are durably and legibly marked by the manufacturer with their volume, are supported in a rigid and secure manner, and the conductor fill does not exceed that volume.
Short-radius conduit bodies — capped elbows, service-entrance elbows — shall not contain conductors larger than that for which the body is designed, and shall not contain splices, taps, or devices.
Pull and Junction Boxes — 314.28
These rules apply to boxes and conduit bodies containing conductors 4 AWG or larger that are required to be insulated.
314.28(A)(1) Straight Pulls. The length of the box or conduit body shall not be less than eight times the metric designator (trade size) of the largest raceway.
Example: a straight pull with a 3-inch raceway requires a box at least 8 x 3 = 24 inches long.
314.28(A)(2) Angle or U Pulls, or Splices. Where splices or where angle or U pulls are made, the distance between each raceway entry inside the box and the opposite wall shall not be less than six times the trade size of the largest raceway in a row. That distance shall be increased for additional entries by the sum of the diameters of all other raceway entries in the same row on the same wall.
Where a raceway enters the wall of a box opposite a removable cover, the distance from that wall to the cover shall not be less than the outside diameter of the largest raceway plus 6 times its trade size... in the 2020 text, the rule is that the distance shall not be less than the trade size of the largest raceway multiplied by six, plus the sum of the diameters of the other raceways.
U pull rule: the distance between raceway entries enclosing the same conductor shall not be less than six times the trade size of the larger raceway.
Worked Example 4 — Angle Pull
A junction box has three raceways entering the left wall: one 3-inch and two 2-inch. Conductors turn 90 degrees and exit the top wall. What minimum horizontal dimension is required?
6 x largest raceway trade size = 6 x 3 = 18 in
Plus sum of other raceway diameters in the same row = 2 + 2 = 4 in
Minimum dimension = 18 + 4 = 22 inches
314.28(A)(3) allows smaller dimensions where the box or conduit body is permanently marked with the maximum number and maximum size of conductors and the marking is based on calculations by a qualified person under engineering supervision.
314.28(B) Conductors in Pull or Junction Boxes. In boxes where the dimension of one side exceeds 6 feet (1.8 m), all conductors shall be cabled or racked up in an approved manner.
314.28(C) Covers. All pull boxes, junction boxes, and conduit bodies shall be provided with a cover compatible with the box or conduit body construction and suitable for the conditions of use. Metal covers shall comply with the grounding requirements of 250.110.
Accessibility and Support
314.29 Boxes, Conduit Bodies, and Handhole Enclosures to Be Accessible. Boxes, conduit bodies, and handhole enclosures shall be installed so that the wiring contained in them can be rendered accessible without removing any part of the building or structure, or in underground circuits, without excavating sidewalks, paving, earth, or other substance that is to be used to establish the finished grade.
314.23 covers supports: boxes shall be rigidly and securely fastened to the surface, or supported from structural members, or in specified cases supported by the entering raceways. A conduit body may be supported by threaded conduit entries where the entries are made up wrenchtight and the conduit is supported within 3 feet on two or more sides.
314.20 requires boxes in walls or ceilings of noncombustible material to be installed so that the front edge is set back not more than 1/4 inch (6 mm) from the finished surface; in walls of combustible material the box must be flush with or project from the finished surface.
314.21 requires plaster, drywall, or plasterboard surfaces broken or incomplete around boxes to be repaired so there are no gaps or open spaces greater than 1/8 inch (3 mm) at the edge of the box.
A device box contains three 12/2 NM cables with ground, internal cable clamps, and one duplex receptacle. What minimum box volume is required?
Two 14 AWG pigtails are added inside a box, each spliced to a wire nut at one end and landed on a device inside the same box at the other. How do they affect the box fill calculation?
A pull box is used for a straight pull with a 4-inch trade size raceway entering one end and leaving the other. What minimum length is required by NEC 314.28(A)(1)?
A junction box has a 3-inch raceway and two 1-inch raceways entering the same wall, with the conductors making a 90-degree turn. What minimum distance from that wall to the opposite wall is required?