Electrician Training Electrical Certification
»NEW eBOOK«
wiring recessed light fixtures Electrical Wiring Electrical Troubleshooting and Electrical Repairs
wire outdoor light fixture Home Electrical Wiring Diagrams
wiring home generator and transfer switch wiring a 220 volt range cord outlet Wiring for GFCI Outlets Wiring Outlets and a Switched Outlet Wiring and Installing Ceiling Fans and Remote Controls wire dimmer switch wiring diagrams for switches wiring a dryer cord and 220 outlet circuit breaker panel
Electrical Wire and Cable

Electrical Codes - Electrical Wiring - Wiring Diagrams - Ceiling Fans - Lighting - Switches - GFCI - Outlets - Circuit Breakers - Home Wiring - Generators


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.

Calculate 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

Prevent Electrical Box Over Fill By Using This Table and Information

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


Electrical Question or Comment

  Electrical Question
 
click here to ask a question
  Comment
 
click here to leave a comment



»Avoid Costly Mistakes«
Wire It Right with the help of my Illustrated Wiring Book

Great for any Home Wiring Project,
Perfect for Homeowners, Students and Electricians!
Complete Guide to Home Electrical Wiring
» Click here to learn more about Home Electrical Wiring «
Including:

  • Fully Illustrated
  • Home Electrical Wiring - Room by Room
  • Wiring 120 and 240 Volt Circuits
  • Wiring Main Panels
  • Underground and Overhead Services
  • Grounding and Bonding
  • 55 Electrical Code Topics
  • 6 Most Commonly Used Code Tables
  • 11 Home Wiring Plans
  • Over 370 Photos
  • 28 Wiring Diagrams and Illustrations
  • Illustrated Wiring Methods
  • Wiring Codes for Home Electrical Wiring
  • ....and much more.

Be Sure to Get Your Copy Today!
electrical wiring

Residential Electrician Book
 


Top of Page