# Electrical Code for Electrical Box Fill Capacity

 By Dave Rongey 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 10.5 12 13.5 15 16.5 18 19.5 21.00 #16 (1.75) 10.50 12.25 14 15.75 17.5 19.25 21 22.75 24.5 #14 (2.00) 12.00 14 16 18 20 22 24 26 28 #12 (2.25) 13.50 15.75 18 20.25 22.5 24.75 27 29.25 31.5 #10 (2.50) 15.00 17.5 20 22.5 25 27.5 30 32.5 35 #8 (3.00) 18.00 21 24 27 30 33 36 39 42 #6 (5.00) 30.00 35 40 45 50 55 60 65 70

## 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). 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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