NEC 314.16 Code Reference
Quick reference guide for electrical box fill requirements per National Electrical Code Article 314.16.
Box Fill Formula Summary
Total Fill =
- Conductors (each × Table 314.16(B))
- + Clamps (1 × largest conductor)
- + Support fittings (1 × largest)
- + Devices (2 × largest per yoke)
- + Grounds (1 × largest ground)
- + Terminal blocks (NEC 2023)
Compliance Check:
Total Fill Volume ≤ Box Volume = PASS
Total Fill Volume > Box Volume = FAIL
NEC 314.16 Section Breakdown
Box Volume Calculations
Boxes shall be of sufficient size to provide free space for all enclosed conductors. The minimum volume shall be determined by the largest conductor entering the box.
Key Points:
- • Box volume is based on dimensions or manufacturer markings
- • Standard box volumes are listed in Table 314.16(A)
- • Custom boxes must have volume marked by manufacturer
Conductor Fill
Each conductor that originates outside the box and terminates or is spliced within the box shall be counted once.
Key Points:
- • Each conductor entering the box counts as one
- • Conductors passing through without splice count once per conductor
- • Volume per conductor is based on Table 314.16(B)
Clamp Fill
Where one or more internal cable clamps are present, a single volume allowance based on the largest conductor present shall be made.
Key Points:
- • All internal clamps = 1 conductor volume
- • Based on largest conductor in the box
- • External clamps do not require allowance
Support Fittings Fill
Where one or more fixture studs or hickeys are present, a single volume allowance based on the largest conductor shall be made.
Key Points:
- • Fixture studs/hickeys = 1 conductor volume
- • Based on largest conductor present
- • Includes luminaire support fittings
Device/Equipment Fill
For each yoke or strap containing one or more devices or equipment, a double volume allowance shall be made for each yoke based on the largest conductor connected to a device.
Key Points:
- • Each device yoke = 2× largest connected conductor
- • Applies to switches, receptacles, dimmers
- • Multiple devices on same yoke still count as 2× (single gang)
Equipment Grounding Conductor Fill
Where one or more equipment grounding conductors or bonding jumpers enter a box, a single volume allowance based on the largest equipment grounding conductor shall be made.
Key Points:
- • ALL ground wires = 1 conductor volume total
- • Based on largest ground entering box
- • Includes bonding jumpers
Terminal Block Fill (NEC 2023)
Where one or more terminal blocks are installed in a box, volume allowances shall be calculated based on conductors connected to the terminal block.
Key Points:
- • New requirement in NEC 2023
- • Based on conductors connected to terminal block
- • Volume calculated per connected conductor
Table 314.16(B) - Volume Allowances
| Wire Size (AWG) | Volume (cu. in.) | Volume (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 |
About NEC 314.16
NEC 314.16 establishes the requirements for electrical box fill calculations. Proper box sizing is critical for safe electrical installations, preventing overheating and ensuring adequate space for conductors and devices.
Why Box Fill Matters
Overcrowded electrical boxes create serious safety hazards:
- Heat buildup can damage conductor insulation
- Difficult to make proper connections
- Increased risk of short circuits and fires
- May damage devices and equipment
- Code violations can result in failed inspections
Use Our Free Calculator
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