Electrical Boxes
Electrical & Wiring · 10 specs
Direct Answer
Standard electrical outlet and junction boxes are specified by trade dimensions and minimum internal cubic inch volume, including 4-inch square boxes (1900/1920 boxes, 21.0–30.3 cu in), 4-inch octagon ceiling boxes (15.5–21.5 cu in), and single-gang device boxes (18.0 cu in).
Source: NEMA OS 1-2013 / NEC Section 314.16 Box Fill Calculations · Last reviewed Aug 28, 2026
Specs
| Governing Standards | NEMA OS 1 / NEC Article 314.16 |
| 4" Square Box (1-1/2" Deep–1900 Box) | 4.0 × 4.0 × 1.5 in (102 × 102 × 38 mm) | 21.0 cu in (344 cm³) |
| 4" Square Box (2-1/8" Deep–1920 Box) | 4.0 × 4.0 × 2.125 in (102 × 102 × 54 mm) | 30.3 cu in (496 cm³) |
| 4" Octagon / Round Ceiling Box | 4.0 × 4.0 × 1.5 in (102 × 102 × 38 mm) | 15.5 to 21.5 cu in |
| Handy / Utility Box (Surface 2×4") | 4.0 × 2.125 × 1.875 in | 13.0 to 14.5 cu in |
| Single-Gang Plastic Device Box | Standard 18.0 to 22.5 cu in (supports up to 9 × 14 AWG conductors) |
| Double-Gang Plastic Device Box | Standard 32.0 to 35.0 cu in |
| Volume Allowance per 14 AWG Conductor | 2.00 cu in (32.8 cm³) per wire |
| Volume Allowance per 12 AWG Conductor | 2.25 cu in (36.9 cm³) per wire |
| Volume Allowance per 10 AWG Conductor | 2.50 cu in (41.0 cm³) per wire |
Standard Electrical Box Sizing & Volume Matrix
| Box Trade Description | Dimensions (H × W × D) | Internal Volume | Max 14 AWG Wires (2.0 cu in) | Max 12 AWG Wires (2.25 cu in) | Primary Construction Application |
|---|---|---|---|---|---|
| 4" Square (1-1/2" Deep) | 4" × 4" × 1.5" | 21.0 cu in | 10 conductors | 9 conductors | Commercial conduit junctions, double devices |
| 4" Square (2-1/8" Deep) | 4" × 4" × 2.125" | 30.3 cu in | 15 conductors | 13 conductors | Heavy commercial junction box with mud ring |
| 4" Octagon (1-1/2" Deep) | 4" × 4" × 1.5" | 15.5 cu in | 7 conductors | 6 conductors | Ceiling light fixture / junction box |
| 4" Octagon (2-1/8" Deep) | 4" × 4" × 2.125" | 21.5 cu in | 10 conductors | 9 conductors | Deep ceiling box / ceiling fan rated |
| Handy / Utility Box | 4" × 2-1/8" × 1-7/8" | 13.0 cu in | 6 conductors | 5 conductors | Exposed surface-mount garage/basement conduit |
| 1-Gang New Work Plastic | 3.75" × 2.25" × 3.0" | 18.0–22.5 cu in | 9–11 conductors | 8–10 conductors | Standard residential drywall outlet / switch |
| 2-Gang New Work Plastic | 3.75" × 4.0" × 3.5" | 32.0–35.0 cu in | 16–17 conductors | 14–15 conductors | Side-by-side switches, dimmers, duplex outlets |
NEC 314.16 Box Fill Volume Calculation Rules
The National Electrical Code mandates that total device and wire volume cannot exceed the box rating:
- Conductors Originating Outside: Count each hot, neutral, and traveler wire as 1 volume unit based on gauge (2.0 cu in for 14 AWG; 2.25 cu in for 12 AWG).
- Internal Clamps: Count all factory cable clamps collectively as 1 conductor allowance (based on largest wire in box).
- Yoke Devices (Receptacle / Switch): Count each strap/device as 2 conductor allowances.
- Equipment Grounding Conductors: Count all ground wires collectively as 1 allowance (plus 1/4 allowance for each ground beyond 4).