NEC 314.16 Code Reference
Quick reference guide for electrical box fill requirements per National Electrical Code Article 314.16.
Box Fill Formula Summary
Total Fill = =
- Conductors (each x Table 314.16(B))
- + Clamps (1 x largest conductor)
- + Support fittings (1 x largest)
- + Devices (2 x largest per yoke)
- + Grounds (1 x largest ground)
- + Terminal blocks (NEC 2023)
Compliance Check::
Total Fill = ≤ Box Volume = PASS
Total Fill = > Box Volume = FAIL
Fill-in-the-blank answers for 314.16(B)
Exam and study questions often phrase 314.16(B) as a blank to fill in. This table gives the answer in plain words, with the allowance each item takes for a 14 AWG circuit. The wording of the code itself should be checked in your adopted edition.
| Question | Answer | 14 AWG example |
|---|---|---|
| Each conductor entering and spliced or terminated in the box counts as | One conductor volume (B)(1) | 1 x 2.00 = 2.00 cu. in. |
| Internal cable clamps (one or several) count as | A single allowance, largest conductor (B)(2) | 1 x 2.00 = 2.00 cu. in. |
| Each luminaire stud or hickey type counts as | A single allowance per type of fitting, largest conductor (B)(3) | 1 x 2.00 = 2.00 cu. in. per type |
| Each strap or yoke holding one or more devices counts as | A double allowance, largest conductor on a device (B)(4) | 2 x 2.00 = 4.00 cu. in. per yoke |
| All equipment grounding conductors together count as | A single allowance, largest grounding conductor (B)(5) | 1 x 2.00 = 2.00 cu. in. |
| The 3 x 2 x 2 inch device box holds | 10.0 cu. in. (Table 314.16(A)) | 5 counted 14 AWG conductors |
Where a question says “single” or “double”, it means one or two times the volume of the largest conductor the rule applies to, taken from Table 314.16(B).
NEC 314.16 Section Breakdown
Box Volume Calculations
Boxes shall be of sufficient size to provide free space for all enclosed conductors.
Key Points:
- • Box volume is based on dimensions or manufacturer markings
- • Standard box volumes are listed in Table 314.16(A)
- • Custom boxes must have volume marked by manufacturer
Conductor Fill
Each conductor that originates outside the box and terminates or is spliced within the box shall be counted once.
Key Points:
- • Each conductor entering the box counts as one
- • Conductors passing through without splice count once per conductor
- • Volume per conductor is based on Table 314.16(B)
Clamp Fill
Where one or more internal cable clamps are present, a single volume allowance based on the largest conductor present shall be made.
Key Points:
- • All internal clamps = 1 conductor volume
- • Based on largest conductor in the box
- • External clamps do not require allowance
Support Fittings Fill
Where one or more luminaire studs or hickeys are present, a single volume allowance shall be made for each type of fitting, based on the largest conductor present in the box.
Key Points:
- • One allowance for each type of fitting present: a stud is one type and a hickey is another
- • Several studs of the same type still take one allowance
- • Sized from the largest conductor in the box
Device/Equipment Fill
For each yoke or strap containing one or more devices or equipment, a double volume allowance shall be made for each yoke based on the largest conductor connected to a device.
Key Points:
- • Each device yoke = 2x largest connected conductor
- • Applies to switches, receptacles, dimmers
- • Multiple devices on same yoke still count as 2x (single gang)
Equipment Grounding Conductor Fill
Where one or more equipment grounding conductors or bonding jumpers enter a box, a single volume allowance based on the largest equipment grounding conductor shall be made.
Key Points:
- • ALL ground wires = 1 conductor volume total
- • Based on largest ground entering box
- • Includes bonding jumpers
Terminal Block Fill (NEC 2023)
NEC 2023 added a volume allowance for terminal blocks installed in a box: one allowance for each terminal block assembly, based on the largest conductor terminated on it.
Key Points:
- • Added in the 2023 edition of the NEC (check which edition your jurisdiction has adopted)
- • One allowance per terminal block assembly, sized from the largest conductor terminated on it
- • Not one allowance per connected wire; confirm the exact wording in your adopted edition
Table 314.16(B) - Volume Allowances
| Wire Size (AWG) (AWG) | Volume (cu. in.) | Volume (cm³) |
|---|---|---|
| 18 | 1.50 | 24.6 |
| 16 | 1.75 | 28.7 |
| 14 | 2.00 | 32.8 |
| 12 | 2.25 | 36.9 |
| 10 | 2.50 | 41.0 |
| 8 | 3.00 | 49.2 |
| 6 | 5.00 | 82.0 |
Table 314.16(A): standard box volumes and counted-conductor limits
The maximum number of counted conductors is the box volume divided by the per-conductor allowance, rounded down. It assumes every counted conductor is the same gauge and no devices, clamps, or grounds are present; deduct those first when they apply. For the full box-by-gauge matrix see the box fill chart.
| Box (metal) | Volume (cu. in.) | 14 AWG | 12 AWG | 10 AWG |
|---|---|---|---|---|
| 4" x 1-1/4" Round/Octagonal | 12.5 | 6 | 5 | 5 |
| 4" x 1-1/2" Round/Octagonal | 15.5 | 7 | 6 | 6 |
| 4" x 2-1/8" Round/Octagonal | 21.5 | 10 | 9 | 8 |
| 4" x 1-1/4" Square | 18.0 | 9 | 8 | 7 |
| 4" x 1-1/2" Square | 21.0 | 10 | 9 | 8 |
| 4" x 2-1/8" Square | 30.3 | 15 | 13 | 12 |
| 4-11/16" x 1-1/4" Square | 25.5 | 12 | 11 | 10 |
| 4-11/16" x 1-1/2" Square | 29.5 | 14 | 13 | 11 |
| 4-11/16" x 2-1/8" Square | 42.0 | 21 | 18 | 16 |
| 3" x 2" x 1-1/2" Device Box | 7.5 | 3 | 3 | 3 |
| 3" x 2" x 2" Device Box | 10.0 | 5 | 4 | 4 |
| 3" x 2" x 2-1/4" Device Box | 10.5 | 5 | 4 | 4 |
| 3" x 2" x 2-1/2" Device Box | 12.5 | 6 | 5 | 5 |
| 3" x 2" x 2-3/4" Device Box | 14.0 | 7 | 6 | 5 |
| 3" x 2" x 3-1/2" Device Box | 18.0 | 9 | 8 | 7 |
Boxes that are not in the table (plastic boxes, extension rings, and listed special boxes) use the volume marked by the manufacturer.
The counting rules in practice
| Rule | What is counted | Allowance | Common mistake |
|---|---|---|---|
| (B)(1) | Each conductor that enters from outside and is spliced or terminated in the box; each unbroken conductor passing through | 1 x its own gauge | Counting a pigtail or jumper that starts and ends inside the box |
| (B)(2) | One or more internal cable clamps | 1 x largest conductor in the box, total | Adding one allowance per clamp |
| (B)(3) | Luminaire studs or hickeys | 1 x largest conductor, per type of fitting | Forgetting the fitting entirely |
| (B)(4) | Each device yoke or strap | 2 x largest conductor connected to that yoke | Counting a switch or receptacle as one wire |
| (B)(5) | All equipment grounding conductors and bonding jumpers | 1 x largest grounding conductor, total | Counting every ground wire separately |
| (B)(6) | Each terminal block assembly (NEC 2023) | 1 x largest conductor terminated on it | Applying a 2023 rule where an older edition is adopted |
The code also contains exceptions, for example for certain small luminaire fixture wires, so read the text of 314.16(B) for unusual cases. Related guides: device yokes, equipment grounding conductors, terminal blocks, and wire connectors.
Worked examples
Example 1: a receptacle in an 18.0 cu. in. device box (14 AWG)
Two 14/2 cables enter (4 current-carrying conductors and 2 grounds), one receptacle yoke, and internal clamps are present. The 3 x 2 x 3-1/2 inch device box is 18.0 cu. in.
- Conductors: 4 x 2.00 = 8.00 cu. in.
- Device yoke: 2 x 2.00 = 4.00 cu. in.
- Internal clamps: 1 x 2.00 = 2.00 cu. in.
- Grounds (all together): 1 x 2.00 = 2.00 cu. in.
Total 16.00 cu. in. against 18.00 cu. in.: passes with 2.00 cu. in. to spare.
Adding a third 14/2 cable (6 current-carrying conductors, 3 grounds) raises the total to 20.00 cu. in., which is over 18.00: fails. Use a deeper or larger box.
Example 2: a splice box with 12 AWG cables
Three 12/2 cables are spliced in a 4-inch square, 1-1/2 inch deep box (21.0 cu. in.) with internal clamps and no devices: 6 current-carrying conductors and 3 grounds.
- Conductors: 6 x 2.25 = 13.50 cu. in.
- Internal clamps: 1 x 2.25 = 2.25 cu. in.
- Grounds (all together): 1 x 2.25 = 2.25 cu. in.
Total 18.00 cu. in. against 21.00 cu. in.: passes with 3.00 cu. in. to spare.
Example 3: a ceiling box with a luminaire stud
Two 14/2 cables enter a 4" x 1-1/2" Round/Octagonal box (15.5 cu. in.) through connectors on the outside, so there are no internal clamps. They are spliced to a luminaire on a stud: 4 current-carrying conductors and 2 grounds.
- Conductors: 4 x 2.00 = 8.00 cu. in.
- Luminaire stud (one type): 1 x 2.00 = 2.00 cu. in.
- Grounds (all together): 1 x 2.00 = 2.00 cu. in.
Total 12.00 cu. in. against 15.50 cu. in.: passes with 3.50 cu. in. to spare.
If the fixture also uses a hickey, that is a second type of fitting and adds another 2.00 cu. in., for a total of 14.00 cu. in. against 15.50: still passes with 1.50 cu. in. to spare. A second stud of the same type would add nothing.
When conductors are different gauges, count each at its own allowance; the clamp, device, and grounding allowances use the largest conductor they apply to. Check your own layout with the box fill calculator, or read the step-by-step box fill how-to.
Editions and local adoption
Jurisdictions adopt the NEC on their own schedule, so the edition your inspector enforces may be older or newer than the 2023 code. The terminal-block allowance in 314.16(B)(6) is the main difference this page covers; the other counting rules are long-standing. This page is a study aid: always confirm with the adopted code text and the authority having jurisdiction, and for conductors 4 AWG and larger use the pull and junction box rules in 314.28 instead of box fill.
Frequently asked questions
What does NEC 314.16 require?
The box must be large enough for everything in it. Add the volume allowances for conductors, internal clamps, support fittings, device yokes, equipment grounding conductors (and terminal blocks in the 2023 edition), then check that the total does not exceed the box volume from Table 314.16(A) or the manufacturer marking.
How many 12 AWG conductors fit in a 4-inch square, 1-1/2 inch deep box?
The box is 21.0 cu. in. and each 12 AWG conductor takes 2.25 cu. in., so 21.0 ÷ 2.25 = 9.33, which is 9 counted conductors before any deductions for devices, clamps, or grounds.
Do wire connectors (wire nuts) count toward box fill?
Wire connectors do not get their own volume allowance under 314.16(B). The conductors they join are counted as described in the rules above.
Does a pigtail count as a conductor?
A conductor that starts and ends inside the same box, such as a pigtail or jumper, is not counted. The conductors that enter the box from outside and are spliced to it are counted.
Does each luminaire stud or hickey count separately?
No. Under 314.16(B)(3) one allowance is made for each type of fitting present, sized from the largest conductor in the box. A stud is one type and a hickey is another, so a box with only studs takes one allowance however many studs there are, and a box with both a stud and a hickey takes two.
How much volume does a device yoke or strap count as?
Each yoke or strap that holds one or more devices counts as two allowances under 314.16(B)(4), based on the largest conductor connected to a device on it. A 14 AWG circuit therefore takes 2 x 2.00 = 4.00 cu. in. per yoke.
What is the volume of a 3 in x 2 in x 2 in device box?
Table 314.16(A) lists the 3 x 2 x 2 inch device box at 10.0 cu. in. At 2.00 cu. in. per 14 AWG conductor that is 5 counted conductors before any device, clamp or ground allowances.
Which NEC edition does this page follow?
The tables and rules here follow NEC Article 314.16 including the 2023 terminal-block allowance. Local jurisdictions adopt different editions, so confirm with your authority having jurisdiction, and use the code text itself for final decisions.
Definitions and cited references
The National Electrical Code is a widely adopted electrical safety code used in the United States, and Article 314.16 is the section that governs box volume and conductor-fill allowances. Box fill refers to the method of converting a wiring arrangement into cubic-inch requirements so an enclosure is not overcrowded.
A junction box is an enclosure for conductor splices, taps, or terminations, and NEC 314.16 is the rule set that determines whether that enclosure is large enough. The section matters because overheated conductors, damaged insulation, and difficult servicing often begin with boxes that were technically close but physically overcrowded.
Author: Jack Shi is the editor of Box Fill Calculator. He researches each guide against the published text of NEC Article 314.16 and cross-checks every conductor-count and box-volume figure so the guidance stays accurate and practical for electricians, inspectors, and DIY installers.
- National Electrical Code explains the governing code framework.
- Junction box provides the enclosure context for Article 314.16.
- American wire gauge provides the conductor-size system used by the allowance table.
- IEC 60364 offers an international installation-code reference point.