NEC Table 314.16
Box Fill Volume Chart (NEC 314.16)
Cubic-inch volume allowance per conductor from NEC Table 314.16(B), the standard box volumes from 314.16(A), and the counting rules for devices, clamps, and grounds.
NEC 314.16(B) Volume Per Conductor
| Conductor size (AWG) | Volume each (cu in) |
|---|---|
| #6 | 5.00 |
| #8 | 3.00 |
| #10 | 2.50 |
| #12 | 2.25 |
| #14 | 2.00 |
| #16 | 1.75 |
| #18 | 1.50 |
NEC 314.16(A) Standard Box Volumes
| Box (in) | Volume (cu in) |
|---|---|
| 4x1-1/4 square | 18.0 |
| 4x1-1/2 square | 21.0 |
| 4x2-1/8 square | 30.3 |
| 4-11/16x1-1/4 square | 25.5 |
| 4-11/16x1-1/2 square | 29.5 |
| 4-11/16x2-1/8 square | 42.0 |
| 3x2x2 device | 10.0 |
| 3x2x2-1/4 device | 10.5 |
| 3x2x2-1/2 device | 12.5 |
| 3x2x2-3/4 device | 14.0 |
| 3x2x3-1/2 device | 18.0 |
| 4x2-1/8x1-1/2 | 10.3 |
| 4x2-1/8x1-7/8 | 13.0 |
| 4x2-1/8x2-1/8 | 14.5 |
| masonry 3-3/4x2x2-1/2 | 14.0 |
| masonry 3-3/4x2x3-1/2 | 21.0 |
| FS single | 13.5 |
| FD single | 18.0 |
| FS double | 18.0 |
| FD double | 24.0 |
The Five Things That Count
Box fill is the sum of five categories, each from NEC 314.16(B), and the total must stay at or below the box volume from 314.16(A):
- Conductors (B)(1): each conductor that runs through or terminates counts as one volume at its own size. A conductor that only passes through counts once.
- Clamps (B)(2): all internal cable clamps together count as one volume at the largest conductor size.
- Support fittings (B)(3): each fixture stud or hickey counts as one volume at the largest conductor.
- Devices (B)(4): each yoke or strap (receptacle, switch) counts as two volumes at the largest conductor connected to it.
- Grounds (B)(5): all equipment grounding conductors together count as one volume at the largest ground. A second isolated-ground system adds one more.
Worked Example
Device box with two 12-2 cables, one receptacle, internal clamps
4 #12 conductors x 2.25 = 9.00
2 grounds count as 1 x 2.25 = 2.25
clamps count as 1 x 2.25 = 2.25
receptacle (1 yoke) counts as 2 x 2.25 = 4.50
Total = 9.00 + 2.25 + 2.25 + 4.50 = 18.00 cu in
Result: the box must be at least 18.0 cubic inches. A 3 x 2 x 3-1/2 inch device box (18.0 cu in) meets it exactly; a 4 x 1-1/2 square box (21.0 cu in) gives room to spare.
Frequently Asked Questions
What is the box fill volume for #12 wire?
Each #12 conductor counts as 2.25 cubic inches for box fill per NEC Table 314.16(B). A #14 is 2.00, #10 is 2.50, #8 is 3.00, and #6 is 5.00 cubic inches.
What is the box fill volume for #14 wire?
Each #14 conductor counts as 2.00 cubic inches per NEC Table 314.16(B). That is the most common branch-circuit conductor, so a box with six #14 conductors uses 12.0 cubic inches of conductor fill before devices and clamps.
How much box fill does a receptacle or switch take?
Each device yoke (receptacle or switch) counts as two conductor volumes based on the largest conductor connected to it, per NEC 314.16(B)(4). For a device on #12 that is 2 x 2.25 = 4.50 cubic inches.
How do you count grounds in box fill?
All equipment grounding conductors in the box together count as one single conductor volume, based on the largest ground, per NEC 314.16(B)(5). Four #12 grounds still count as just 2.25 cubic inches, not four.
Do cable clamps count toward box fill?
Yes. One or more internal cable clamps together count as one conductor volume based on the largest conductor in the box, per NEC 314.16(B)(2). Cable connectors on the outside of the box do not count.
How many #12 conductors fit in a 4-square box?
A 4 x 1-1/2 inch square box is 21.0 cubic inches (NEC Table 314.16(A)). At 2.25 cubic inches each, it holds up to 9 #12 conductors (21.0 / 2.25 = 9.3) if there are no devices, clamps, or grounds. Each of those subtracts from the count.
Related Calculators & Charts
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Know the volumes. Let the calculator do the count.
Box fill is easy to get wrong by one device or ground. The box fill calculator applies all five 314.16(B) rules automatically and tells you the minimum box size in cubic inches.