Electrical Box Fill Calculator
Professional NEC 314.16 compliant box fill calculator for electrical installations. Ensure proper box sizing for switches, outlets, and junction boxes with accurate conductor volume calculations.
NEC 314.16
Code compliant sizing
Volume Calculator
Cubic inch capacity
Code Compliance
Pass inspections
NEC Table 314.16(A) - Metal Box Volumes
Standard Device Boxes
| Box Type | Size (inches) | Volume (cu.in) | Max #12 | Max #14 |
|---|---|---|---|---|
| 3×2×1½ | Device | 7.5 | 3 | 3 |
| 3×2×2 | Device | 10.0 | 4 | 5 |
| 3×2×2¼ | Device | 10.5 | 4 | 5 |
| 3×2×2½ | Device | 12.5 | 5 | 6 |
| 3×2×2¾ | Device | 14.0 | 6 | 7 |
| 3×2×3½ | Device | 18.0 | 8 | 9 |
| 4×2⅛×1⅞ | Device | 10.3 | 4 | 5 |
| 4×2⅛×2⅛ | Device | 14.5 | 6 | 7 |
Square & Octagonal Boxes
| Box Type | Depth | Volume (cu.in) | Max #12 | Max #14 |
|---|---|---|---|---|
| 4×1¼ Oct | 1¼" | 12.5 | 5 | 6 |
| 4×1½ Oct | 1½" | 15.5 | 6 | 7 |
| 4×2⅛ Oct | 2⅛" | 21.5 | 9 | 10 |
| 4×1¼ Square | 1¼" | 18.0 | 8 | 9 |
| 4×1½ Square | 1½" | 21.0 | 9 | 10 |
| 4×2⅛ Square | 2⅛" | 30.3 | 13 | 15 |
| 4¹¹⁄₁₆×1¼ | 1¼" | 25.5 | 11 | 12 |
| 4¹¹⁄₁₆×1½ | 1½" | 29.5 | 13 | 14 |
| 4¹¹⁄₁₆×2⅛ | 2⅛" | 42.0 | 18 | 21 |
NEC Table 314.16(B) - Conductor Volume Allowances
Volume per Conductor
| Wire Size (AWG) | Volume (cu.in) | Metric (cm³) |
|---|---|---|
| 18 AWG | 1.50 | 24.6 |
| 16 AWG | 1.75 | 28.7 |
| 14 AWG | 2.00 | 32.8 |
| 12 AWG | 2.25 | 36.9 |
| 10 AWG | 2.50 | 41.0 |
| 8 AWG | 3.00 | 49.2 |
| 6 AWG | 5.00 | 81.9 |
Counting Rules
Conductors
Each conductor originating outside the box = 1
Equipment Grounds
All grounds together = 1 of largest size
Devices
Each device yoke = 2 conductors
Clamps
All cable clamps = 1 conductor
Fittings
Each stud/hickey = 1 conductor
Don't Count
• Pigtails under 6"
• Pass-through conductors
• Wire nuts
Real-World Box Fill Examples
Single Switch Box
Setup: Switch controlling one light
Wiring: 14-2 NM in, 14-2 NM out
Count Breakdown:
| Hot in: | 1 |
| Neutral in: | 1 |
| Hot out: | 1 |
| Neutral out: | 1 |
| Grounds (all): | 1 |
| Switch device: | 2 |
| Cable clamps: | 1 |
| Total: | 8 |
Volume: 8 × 2.00 = 16 cu.in
Box Required: 3×2×2¾" (18 cu.in)
3-Way Switch Location
Setup: 3-way switch with traveler
Wiring: 14-2 power, 14-3 traveler
Count Breakdown:
| Power hot: | 1 |
| Power neutral: | 1 |
| Traveler 1: | 1 |
| Traveler 2: | 1 |
| Common: | 1 |
| Neutral (14-3): | 1 |
| Grounds: | 1 |
| 3-way switch: | 2 |
| Cable clamps: | 1 |
| Total: | 10 |
Volume: 10 × 2.00 = 20 cu.in
Box Required: 3×2×3½" (18 cu.in) tight!
Better: 4" square (21 cu.in)
GFCI Outlet Protection
Setup: GFCI protecting downstream
Wiring: 12-2 in, 12-2 load out
Count Breakdown:
| Line hot: | 1 |
| Line neutral: | 1 |
| Load hot: | 1 |
| Load neutral: | 1 |
| Grounds: | 1 |
| GFCI device: | 2 |
| Cable clamps: | 1 |
| Total: | 8 |
Volume: 8 × 2.25 = 18 cu.in
Box Required: 3×2×3½" (18 cu.in)
Note: GFCIs are bulky!
4-Circuit Junction Box
Setup: Splicing 4 circuits
Wiring: 4× 12-2 NM cables
Count Breakdown:
| Circuit 1 (2 cond): | 2 |
| Circuit 2 (2 cond): | 2 |
| Circuit 3 (2 cond): | 2 |
| Circuit 4 (2 cond): | 2 |
| All grounds: | 1 |
| Cable clamps: | 1 |
| Total: | 10 |
Volume: 10 × 2.25 = 22.5 cu.in
Box: 4" square × 1½" (21 cu.in) - Too small!
Use: 4" × 2⅛" (30.3 cu.in)
3-Gang Switch Box
Setup: 3 switches, one location
Wiring: 14-2 feed, 3× 14-2 switch legs
Count Breakdown:
| Feed hot: | 1 |
| Feed neutral: | 1 |
| 3 switch legs: | 3 |
| 3 neutrals: | 3 |
| All grounds: | 1 |
| 3 switches: | 6 |
| Cable clamps: | 1 |
| Total: | 16 |
Volume: 16 × 2.00 = 32 cu.in
3-gang box typically 45-50 cu.in ✓
Ceiling Fan/Light Box
Setup: Fan with light kit
Wiring: 14-3 from switch, 14-2 power
Count Breakdown:
| Power hot: | 1 |
| Power neutral: | 1 |
| Fan hot: | 1 |
| Light hot: | 1 |
| Switch neutral: | 1 |
| All grounds: | 1 |
| Support fitting: | 1 |
| Cable clamps: | 1 |
| Total: | 8 |
Volume: 8 × 2.00 = 16 cu.in
Use fan-rated 4" octagon (21.5 cu.in)
Common Electrical Box Types & Applications
Device Boxes
Single Gang
Switches, outlets, dimmers
18-22 cu.in typical
2-Gang
Double switches/outlets
30-34 cu.in typical
3+ Gang
Multiple devices
45+ cu.in typical
Old Work
Retrofit installations
Swing clamps/wings
Junction Boxes
4" Square
Most common junction
21-42 cu.in range
4-11/16" Square
Larger junctions
29-42 cu.in range
Octagonal
Ceiling fixtures
12.5-21.5 cu.in
Round/Pan
Shallow ceiling boxes
6-10 cu.in typical
Specialty Boxes
Ceiling Fan
Rated for 35-70 lbs
Reinforced mounting
Weatherproof
Outdoor installations
NEMA 3R/4X rated
Floor Boxes
In-floor receptacles
Adjustable depth
Arlington Boxes
Siding/exterior
Built-in mounting
Quick Box Fill Reference Chart
| Installation Type | #14 Count | Volume Req'd | Min Box Size | Recommended |
|---|---|---|---|---|
| Single switch | 6-8 | 12-16 cu.in | 3×2×2½" | 3×2×2¾" |
| Single outlet | 6-8 | 12-16 cu.in | 3×2×2½" | 3×2×2¾" |
| GFCI outlet | 8-10 | 16-20 cu.in | 3×2×2¾" | 3×2×3½" |
| 3-way switch | 9-11 | 18-22 cu.in | 3×2×3½" | 4" square |
| Dimmer switch | 6-8 | 14-18 cu.in | 3×2×2¾" | 3×2×3½" |
| 2 switches | 10-12 | 20-24 cu.in | 2-gang | 2-gang deep |
| 3 switches | 14-16 | 28-32 cu.in | 3-gang | 3-gang deep |
| Junction 3 cables | 7-8 | 14-16 cu.in | 4×1¼" | 4×1½" |
| Junction 4 cables | 9-10 | 18-20 cu.in | 4×1½" | 4×2⅛" |
| Ceiling fan/light | 8-10 | 16-20 cu.in | 4" oct deep | 4" oct 2⅛" |
Common Box Fill Violations
Counting Errors
- ✗Forgetting device count: Each device counts as 2 conductors
- ✗Counting grounds separately: All grounds = 1 conductor total
- ✗Missing cable clamps: Internal clamps count as 1 conductor
- ✗Counting pigtails: Pigtails under 6" don't count
- ✗Wrong wire size volume: Using #14 volume for #12 wire
Installation Issues
- ⚠Box too shallow: GFCIs and dimmers need deeper boxes
- ⚠Overstuffing: Forcing wires damages insulation
- ⚠Wrong box type: Using device boxes for junctions
- ⚠Multiple circuits: Not accounting for all conductors
- ⚠Plastic box limits: Exceeding manufacturer ratings
Related Electrical Calculators
Industry Standards & Resources
NFPA 70 - NEC
Article 314.16 box fill calculations and installation requirements
Legrand/Pass & Seymour
Electrical box specifications and installation guides
RACO/Hubbell
Metal electrical boxes and box fill capacity charts
Carlon/Thomas & Betts
Non-metallic boxes and volume specifications
Arlington Industries
Specialty boxes and innovative mounting solutions
EC&M Magazine
Box fill calculation guides and code interpretations
Code Compliance Required
Proper box fill calculations are mandatory per NEC 314.16 and critical for electrical safety. Overcrowded boxes cause overheating, damaged wire insulation, and poor connections that can lead to fires. Undersized boxes make device installation difficult and may fail inspection. Always verify calculations and use boxes with adequate volume ratings. Local codes may have additional requirements. Consult with licensed electricians for complex installations.