Box Fill Calculator — NEC 314.16(B)
Calculate required electrical box volume based on NEC Table 314.16(B) standards
How Box Fill is Calculated
The National Electrical Code (NEC) Section 314.16(B) establishes minimum box volume requirements based on the number and size of conductors, devices, and fittings. The calculation follows these steps:
- Count conductor allowances: Each insulated current-carrying conductor counts as one allowance based on its wire size.
- Count device allowances: Each device (switch, receptacle, etc.) counts as two allowances based on the largest conductor connected to it.
- Count equipment grounding conductors: If 1-4 grounds are present, count as one allowance. If more than 4, add 0.25 allowances for each additional ground beyond 4.
- Count clamps and fittings: Each internal cable clamp or fixture stud/hickey counts as one allowance based on the largest conductor.
- Calculate total volume: Multiply total allowances by the volume per conductor from NEC Table 314.16(B).
Example Calculation
Scenario: A single-gang box with four 14 AWG conductors, one receptacle, and two equipment grounding conductors.
Calculation:
- • Conductors: 4 × 1 = 4 allowances
- • Devices: 1 × 2 = 2 allowances
- • Equipment grounds: 2 grounds = 1 allowance
- • Total allowances: 4 + 2 + 1 = 7
- • Volume per 14 AWG conductor: 2.00 in³
- • Required volume: 7 × 2.00 = 14.00 in³
This box would need a minimum volume of 14 cubic inches, which could be satisfied by a standard 2×4×1½” box (10.5 in³ is too small) or a 2×4×2⅛” box (15.5 in³).
NEC Table 314.16(B) — Free Space Within Box for Each Conductor
| Wire Size (AWG) | Volume Allowance (in³) | Volume Allowance (cm³) |
|---|---|---|
| 18 | 1.50 | 24.6 |
| 16 | 1.75 | 28.7 |
| 14 | 2.00 | 32.8 |
| 12 | 2.25 | 36.9 |
| 10 | 2.50 | 41.0 |
| 8 | 3.00 | 49.2 |
| 6 | 5.00 | 82.0 |
| 4 | 5.00 | 82.0 |
| 3 | 5.00 | 82.0 |
| 2 | 5.00 | 82.0 |
| 1 | 5.00 | 82.0 |
| 1/0 | 5.00 | 82.0 |
| 2/0 | 5.00 | 82.0 |
| 3/0 | 5.00 | 82.0 |
| 4/0 | 5.00 | 82.0 |
Source: NFPA 70, National Electrical Code (NEC), Table 314.16(B)
Frequently Asked Questions
Yes, but differently than other conductors. If you have 1-4 equipment grounding conductors in the box, count them collectively as one allowance. If you have more than 4, add 0.25 allowances for each ground beyond the fourth.
Each device (switch, receptacle, combination device) counts as two conductor allowances. Use the size of the largest conductor connected to that device.
Each set of internal cable clamps counts as one allowance based on the largest conductor entering the box. If clamps are external to the box, they do not count.
Yes, pigtails are conductors and must be counted in the box fill calculation. Each pigtail that originates in the box counts as one conductor.
Count each conductor size separately, then multiply each count by its corresponding volume allowance. Use the largest conductor size for device and clamp allowances.