Box Fill Calculations for the Electrician Exam (NEC 314.16)
Box fill shows up on almost every journeyman and master exam, and on the job it's the difference between a clean install and a box you can't close. The math is simple once you count the right things. Most wrong answers come from counting something twice or forgetting something that counts.
Section and table numbers here follow NEC 314.16 as it has read for the last few editions. Always confirm against the edition your exam or your inspector uses. In Massachusetts that means the NEC as amended by 527 CMR 12.00.
Step 1: Find the box volume
For standard metal boxes, use the volume in Table 314.16(A). For boxes not in the table (plastic boxes and many device boxes), use the volume marked on the box. Add the volume of plaster rings, extension rings and domed covers if they're marked with a volume.
Step 2: Count the allowances
| Item | Counts as |
|---|---|
| Each conductor that starts outside the box and ends or splices inside | 1 allowance, sized by that conductor |
| Each conductor that passes through without a splice | 1 allowance |
| Conductors that start and end inside the box (pigtails) | 0 |
| Internal cable clamps (one or more) | 1 allowance, sized by the largest conductor in the box |
| Each luminaire stud or hickey | 1 allowance, sized by the largest conductor |
| Each device yoke or strap (receptacle, switch) | 2 allowances, sized by the largest conductor connected to that device |
| All equipment grounding conductors | 1 allowance for up to four, sized by the largest EGC, plus 1/4 allowance for each one beyond four |
A fixture's own wires that are 16 AWG or smaller (up to four of them, plus the fixture ground) don't count when a domed canopy covers the box, per the exception in 314.16(B)(1).
Step 3: Multiply by the volume per conductor
Table 314.16(B) gives the volume each conductor needs:
| Size (AWG) | Cubic inches each |
|---|---|
| 18 | 1.50 |
| 16 | 1.75 |
| 14 | 2.00 |
| 12 | 2.25 |
| 10 | 2.50 |
| 8 | 3.00 |
| 6 | 5.00 |
Worked example
A single-gang box has one duplex receptacle, two 12/2 NM cables (with ground) coming in, and internal cable clamps.
- Insulated conductors: 2 hots + 2 neutrals = 4
- Grounds: 2 grounds = 1 allowance
- Cable clamps: 1
- Receptacle yoke: 2
Total: 8 allowances × 2.25 cu in = 18.0 cubic inches. A box marked 18 cu in or more works. A smaller one doesn't.
Common traps
- Counting each ground separately. All of them together count as one, until you have more than four.
- Forgetting the yoke counts double.
- Sizing the yoke allowance by the largest conductor in the box instead of the largest conductor connected to that device.
- Counting pigtails.
- Mixing wire sizes and using one volume for everything. Do each size on its own line.
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More guides: How to navigate the NEC · 2026 NEC changes for exam takers · Massachusetts exam: 527 CMR and the NEC · How to tab and mark your NEC · 30-day journeyman study plan · Box fill calculations (314.16) · Conductor ampacity and derating · Grounding conductor sizing (250.66 vs 250.122) · Voltage drop calculations