NEC box fill calculation (314.16)
Box fill questions appear on every journeyman exam. You count each item in the box as a number of conductor volumes, multiply by the volume for the largest conductor involved, and compare the total with the box volume from Table 314.16(A) or the volume marked on the box.
A timed 80-question exam in the ICC R17 format on the 2023 NEC, or your state’s exact format. Every answer has a worked explanation.
Or start with the free box and conduit fill drillWatch worked examples, step by step
The 314.16 box fill and Chapter 9 conduit fill methods, worked through 11 questions.
Volume per conductor: Table 314.16(B)
| Conductor size (AWG) | Volume (in³) |
|---|---|
| 18 | 1.50 |
| 16 | 1.75 |
| 14 | 2.00 |
| 12 | 2.25 |
| 10 | 2.50 |
| 8 | 3.00 |
| 6 | 5.00 |
What counts, and how many volumes
- Each conductor that starts outside the box and ends or is spliced inside it counts as one volume. A conductor that passes through without a splice counts as one.
- An unbroken loop or coil at least twice the 300.14 minimum free length counts as two volumes.
- Conductors that never leave the box, such as pigtails and jumpers, count as zero.
- Equipment grounding conductors and up to four fixture wires smaller than 14 AWG that enter from a domed luminaire canopy and end in the box count as zero.
- One or more internal cable clamps count as one volume, based on the largest conductor in the box.
- Each type of luminaire stud or hickey counts as one volume, based on the largest conductor in the box.
- Each device yoke or strap counts as two volumes, based on the largest conductor connected to that device. A device wider than a single 2 in. box counts two volumes per gang.
- All equipment grounding conductors together count as one volume, based on the largest one. An additional isolated set under 250.146(D) adds one more.
Common box volumes: Table 314.16(A)
| Box | Volume (in³) |
|---|---|
| 3 × 2 × 2-1/2 in. device box | 12.5 |
| 3 × 2 × 3-1/2 in. device box | 18.0 |
| 4 × 1-1/2 in. octagonal box | 15.5 |
| 4 × 2-1/8 in. octagonal box | 21.5 |
| 4 × 1-1/2 in. square box | 21.0 |
| 4 × 2-1/8 in. square box | 30.3 |
| 4-11/16 × 1-1/2 in. square box | 29.5 |
| 4-11/16 × 2-1/8 in. square box | 42.0 |
Practice box fill on a timed exam
A timed 80-question exam in the ICC R17 format on the 2023 NEC, or your state’s exact format. Every answer has a worked explanation.
Or start with the free box and conduit fill drillWorked example
Two 12/2 NM cables and a receptacle in a 4 × 1-1/2 in. square box with internal clamps
- Insulated conductors: 2 cables × 2 = 4 volumes.
- Equipment grounding conductors: all together = 1 volume.
- Receptacle yoke: 2 volumes. Internal clamps: 1 volume.
- Total: 4 + 1 + 2 + 1 = 8 volumes × 2.25 in³ (12 AWG) = 18.0 in³.
- The box holds 21.0 in³ (Table 314.16(A)), so the fill is compliant.
Exam traps
- Use the volume of the largest conductor for clamps, studs and grounds, but the largest conductor connected to the device for a yoke.
- Add the volume of plaster rings, extension rings and domed covers when they are marked with their volume or made from a box listed in Table 314.16(A).
- For conductors 4 AWG and larger, use the pull box sizing rules in 314.28, not 314.16.
Common questions
Do grounding conductors count in box fill?
Yes. All equipment grounding conductors in the box together count as one volume allowance, based on the largest one (314.16(B)(5)).
How many volumes does a switch or receptacle count?
Two, based on the largest conductor connected to the device (314.16(B)(4)).
What is the volume for 14 AWG wire in box fill?
2.00 cubic inches per conductor (Table 314.16(B)). 12 AWG is 2.25 in³.
Practice box fill on a timed exam
A timed 80-question exam in the ICC R17 format on the 2023 NEC, or your state’s exact format. Every answer has a worked explanation.
Or start with the free box and conduit fill drillMore journeyman exam guides
Section numbers follow the 2023 National Electrical Code (NFPA 70). Independent preparation, not affiliated with NFPA, ICC, PSI, Prometric, Pearson VUE or any state board.