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 x Table 314.16(B))
- + Clamps (1 x largest conductor)
- + Support fittings (1 x largest)
- + Devices (2 x largest per yoke)
- + Grounds (1 x largest ground)
- + Terminal blocks (NEC 2023)
Compliance Check::
Total Fill = \u2264 Box Volume = PASS
Total Fill = > 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.
Key Points:
- \u2022 Box volume is based on dimensions or manufacturer markings
- \u2022 Standard box volumes are listed in Table 314.16(A)
- \u2022 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:
- \u2022 Each conductor entering the box counts as one
- \u2022 Conductors passing through without splice count once per conductor
- \u2022 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:
- \u2022 All internal clamps = 1 conductor volume
- \u2022 Based on largest conductor in the box
- \u2022 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:
- \u2022 Fixture studs/hickeys = 1 conductor volume
- \u2022 Based on largest conductor present
- \u2022 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:
- \u2022 Each device yoke = 2x largest connected conductor
- \u2022 Applies to switches, receptacles, dimmers
- \u2022 Multiple devices on same yoke still count as 2x (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:
- \u2022 ALL ground wires = 1 conductor volume total
- \u2022 Based on largest ground entering box
- \u2022 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:
- \u2022 New requirement in NEC 2023
- \u2022 Based on conductors connected to terminal block
- \u2022 Volume calculated per connected conductor
Table 314.16(B) - Volume Allowances
| Wire Size (AWG) (AWG) | Volume (cu. in.) | Volume (cm\u00B3) |
|---|---|---|
| 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 |