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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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Electrical Tips to Help You Wire it Right

The Safest Way to Test Electrical Devices and Identify Electric Wires!

The Non-Contact Electrical Tester
This is a testing tool that I have had in my personal electrical tool pouch for years, and is the first test tool I grab to help identify electrical wiring. It is a Non-contact tester that I use to easily Detect Voltage in Cables, Cords, Circuit Breakers, Lighting Fixtures, Switches, Outlets and Wires. Simply insert the end of the tester into an outlet, lamp socket, or hold the end of the tester against the wire you wish to test. Very handy and easy to use.

The Quickest Way to Check for Faulty Electrical Wiring!

The Plug-In Outlet Tester
This is the first tool I grab to troubleshoot a problem with outlet circuit wiring. This popular tester is also used by most inspectors to test for power and check the polarity of circuit wiring.
It detects probable improper wiring conditions in standard 110-125 VAC outlets Provides 6 probable wiring conditions that are quick and easy to read for ultimate efficiency Lights indicate if wiring is correct and indicator light chart is included Tests standard 3-wire outlets UL Listed Light indicates if wiring is incorrect Very handy and easy to use.

Strip Off Wire Insulation without Nicking and Damaging the Electric Wire!

The Wire Stripper and Wire Cutter
My absolute favorite wire stripping tool that I have had in my personal electrical tool pouch for years, and this is the tool I use to safely strip electrical wires.
This handy tool has multiple uses:
The wire gauges are shown on the side of the tool so you know which slot to use for stripping insulation.
The end of the tool can be used to grip and bend wire which is handy for attaching wire onto the screw terminals of switches and outlets..

The wire stripper will work on both solid and stranded wire. This tool is Very Handy and Easy to Use.
electrical parts

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