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
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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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.
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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.
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wire electrical box - 1907
Electrical Wiring Codes
Electrical Project Skill Level:
Intermediate to Advanced - Best performed by a Certified Electrician or Licensed Electrical Contractor.
Electrical Tools Required:
Basic Electricians Pouch of Hand Tools, Battery Tools, Electric Power Tools, Electrical Meters and Voltage Testers.
Estimated Time:
Depends on personal level of working capabilities and electrical experience, electrical certification or contractors license, ability to work with tools, install electrical circuit wiring, and the available access to the project area. Electrical Safety:
Identify the electric panel circuits found in the project area, turn them OFF and Tag them with a Note before working with the electrical wiring.
Electrical Wiring Parts and Materials:
Electrical parts and materials for all electrical projects should be approved for the specific project and compliant with local and national electrical codes.
Electrical Codes and Inspections:
Installing additional electrical circuits or wiring should be done according to local and national electrical codes with a permit and be inspected.
This is a testing tool that 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.
This is great to troubleshoot a problem with outlet circuit wiring, 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.
A wire stripping tool used 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.
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