Box Fill
Conductor volume allowances, standard box sizes, and box fill calculation examples.
3 reference items · 3 sections
Volume Allowance Per Conductor
Conductor Volume Allowances
NEC 314.16(B)
Volume Required per Conductor
| AWG | Volume (in³) |
|---|---|
| 18 | 1.50 |
| 16 | 1.75 |
| 14 | 2.00 |
| 12 | 2.25 |
| 10 | 2.50 |
| 8 | 3.00 |
| 6 | 5.00 |
Counting Rules
• Each conductor entering the box = 1 volume allowance
• Each conductor passing through (not spliced/terminated) = 1 volume allowance
• Conductor looped without splice and unbroken = 1 volume allowance
• Equipment grounding conductors (all combined) = 1 volume allowance (largest EGC size)
• Each device (switch/receptacle) on yoke = 2 volume allowances (largest conductor connected)
• All internal cable clamps combined = 1 volume allowance (largest conductor)
• Pigtails and splices originating in the box = 0 additional volume
When different wire sizes enter the box, use the volume for each wire's size individually. The clamp and device allowances use the volume of the LARGEST conductor in the box.
Standard Metal Box Volumes
Standard Metal Box Sizes & Volumes
NEC 314.16(A)
Metal Box Volumes
| Box Type | Dimensions | Volume (in³) | Max #14 | Max #12 |
|---|---|---|---|---|
| 4" Square | 1¼" deep | 18.0 | 9 | 8 |
| 4" Square | 1½" deep | 21.0 | 10 | 9 |
| 4" Square | 2⅛" deep | 30.3 | 15 | 13 |
| 4-11/16" Square | 1¼" deep | 25.5 | 12 | 11 |
| 4-11/16" Square | 1½" deep | 29.5 | 14 | 13 |
| 4-11/16" Square | 2⅛" deep | 42.0 | 21 | 18 |
| Handy Box | 1½" deep | 10.5 | 5 | 4 |
| Handy Box | 1⅞" deep | 13.0 | 6 | 5 |
| 3" × 2" Device | 2" deep | 10.0 | 5 | 4 |
| 3" × 2" Device | 2½" deep | 12.5 | 6 | 5 |
| 3" × 2" Device | 2¾" deep | 14.0 | 7 | 6 |
| 3" × 2" Device | 3½" deep | 18.0 | 9 | 8 |
| Octagon 4" | 1½" deep | 15.5 | 7 | 6 |
| Octagon 4" | 2⅛" deep | 21.5 | 10 | 9 |
1.Max conductor count assumes conductors only — no devices, clamps, or grounds
2.Actual count will be lower with devices and grounds in the box
Box Fill Calculation Example
Box Fill Calculation — Step by Step
NEC 314.16 Box Fill Calculator
Example: Single-gang box with one receptacle
Contents: 2 cables with 14/2 NM (4 × #14 conductors), 2 equipment grounds (#14), 1 receptacle on yoke, internal cable clamps
Count:
• Hot conductors: 2 × #14 = 2 volumes
• Neutral conductors: 2 × #14 = 2 volumes
• Equipment grounds (all count as 1): 1 × #14 = 1 volume
• Receptacle on yoke (counts as 2): 2 × #14 = 2 volumes
• Internal clamps (all count as 1): 1 × #14 = 1 volume
Total:
8 × 2.00 in³ = 16.0 in³ minimum box volume required
A standard 3×2×2½" device box (12.5 in³) would NOT be sufficient. Need a 3×2×3½" box (18.0 in³) or 4" square box.
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