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Electrical Code for Electrical Box Fill Capacity

electrical-box-wiring Summary: One of the mistakes often made is over loading an electrical box with too many wires. This will cause switches and outlets to not fit correctly and could even cause wires to become damaged. This information will help you plan your electrical box size properly.
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The Minimum Size Junction Box for Electrical Wiring


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Part (1) of Section 370-16(a) describes in detail the method of counting wires, as well as clamps, fittings, or devices (i.e., switches, receptacles, combination devices) - by establishing an equivalent conductor-value for each.

These values are added together to get a total number of conductors.

The minimum size box is the smallest one in the Box Fill Table (shown) that can accommodate that number of conductors.

BOX FILL TABLE
DEDUCTIONS 6 7 8 9 10 11 12 13 14
Volume Required per Conductor Size per Deductions #18
(1.50) 		9.00 10.50 12.00 13.50 15.00 16.50 18.00 19.50 21.00
#16
(1.75)		 10.50 12.25 14.00 15.75 17.50 19.25 21.00 22.75 24.50
#14
(2.00)		 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00
#12
(2.25)		 13.50 15.75
 18.00 20.25 22.50 24.75 27.00 29.25 31.50
#10
(2.50)		 15.00 17.50 20.00 22.50 25.00 27.50 30.00 32.50 35.00
#8
(3.00)		 18.00 21.00 24.00 27.00 30.00 33.00 36.00 39.00 42.00
#6
(5.00)		 30.00 35.00 40.00 45.00 50.00 55.00 60.00 65.00 70.00

Electrical Box Fill Table Information


Box Fill Guidelines:

  • No matter how many ground wires, they only count as one conductor in the box.
  • A wire running through the box counts as one wire.
  • Each wire coming into a splice connector is counted as one wire.
  • Each wire connecting to a device counts as one wire of that size.
  • When devices are mounted in the box, the total conductor count must be increased by two for each mounting strap.

NEC Table 370-16 Electrical Box Fill



NEC Table 370-16(a) for Metal Boxes

Number of Conductors in Outlet, Device,
and Junction Boxes

Box Dimension
in Inches
Trade Size
or Type

Minimum
Capacity
(in.3)

Maximum Number of Conductors

No. 18

No. 16

No. 14

No. 12

No. 10

No. 8

No. 6

4 X 1-1/4 round or octagonal
4 X 1-1/2 round or octagonal
4 X 2-1/8 round or octagonal

12.5
15.5
21.5

8
10
14

7
8
12

6
7
10

5
6
9

5
6
8

4
5
7

2
3
4

4 X 1-1/4 square
4 X 1-1/2 square
4 X 2-1/8 square

18.0
21.0
30.3

12
14
20

10
12
17

9
10
15

8
9
13

7
8
12

6
7
10

3
4
6

4-11/16 X 1-1/4 square
4-11/16 X 1-1/2 square
4-11/16 X 2-1/8 square

25.5
29.5
42.0

17
19
28

14
16
24

12
14
21

11
13
18

10
11
16

8
9
14

5
5
8

3 X 2 X 1-1/2 device
3 X 2 X 2 device
3 X 2 X 2-1/4 device
3 X 2 X 2-1/2 device
3 X 2 X 2-3/4 device
3 X 2 X 3-1/2 device

7.5
10.0
10.5
12.5
14.0
18.0

5
6
7
8
9
12

4
5
6
7
8
10

3
5
5
6
7
9

3
4
4
5
6
8

3
4
4
5
5
7

2
3
3
4
4
6

1
2
2
2
2
3

4 X 2-1/8 X 1-1/2 device
4 X 2-1/8 X 1-7/8 device
4 X 2-1/8 X 2-1/8 device

10.3
13.0
14.5

6
8
9

5
7
8

5
6
7

4
5
6

4
5
5

3
4
4

2
2
2

3-3/4 X 2 X 2-1/2 masonry
  box / gang
3-3/4 X 2 X 3-1/2 masonry
  box / gang

14.0

21.0

9

14

8

12

7

10

6

9

5

8

4

7

2

4
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