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National Electric Codes
Wire in Electrical Box NEC-Table370-16a

electrical-wires

Summary: The National Electrical Code explains the Maximum Number of Wires that can be installed into a box, otherwise known as Box Fill. This code is based upon the type of box, wires, wire sizes, wire clamps and conduit fittings. Adjustments are made for the ground wire as you will see in the table below.

Calculating The Minimum Size Junction Box for Electrical Wiring


Wire in Electrical Box - Box Fill Calculations

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

Points to Remember:

  1. No matter how many ground wires come into a box, they only count as one conductor within the box.
  2. Any wire running unbroken through the box counts as one wire.
  3. Each wire coming into a splice device (crimp or twist-on type) is counted as one wire.
  4. Each wire coming into the box and connecting to a device counts as one wire of that size.
  5. Where devices are mounted in the box, the total conductor count must be increased by two for each mounting strap because the strap takes up space.

NEC Table 370-16(a). 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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