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Before doing any home electrical work, it's important to know the National Electrical Code for wiring.
ByTimothy Dale
Updated on May 10, 2023
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Electrical boxes or junction boxes are designed to enclose wire connections in order to help protect against short circuits and prevent fires. However, placing too many wires into an electrical box is against the National Electrical Code. The reason for limiting the number of wires and other components is to avoid possible shorts and overheating due to tightly packed wires. Box fill is the term used to describe the total volume of the conductors, devices, and fittings within the electrical box.
Counting the number of wires seems like a simple task, but there are many factors that can affect the box fill, including the gauge of the wires, the volume of the electrical box, and the presence of any additional electrical components, like cable clamps, outlets, switches, and other items you intend to put in the box. Use this guide to learn how to count wires in an electrical box to ensure your electrical upgrade is safe and successful.
Calculating Electrical Box Volume
The volume of an electrical box determines the number and size of conductors and wiring devices that can be contained within the box. For an electrical box to be considered safe, the volume of the wiring enclosure must equal or exceed the total volume of the assembled components, including conductors, clamps, support fittings, devices, equipment, and grounding conductors.
Typically, manufacturers will list the volume of the electrical box on the box itself. Additional wiring enclosure sections, like plaster rings, domed covers, or extension rings, should also be marked with the volume. If the volume is not listed, measure the electrical box, and use the table below to find the minimum box volume.
| Electrical Box Size (in.) | Electrical Box Shape | Minimum Box Volume (in.³) | Maximum Number of Conductors (Arranged by AWG Size) | ||||||
|---|---|---|---|---|---|---|---|---|---|
| 18 | 16 | 14 | 12 | 10 | 8 | 6 | |||
| 4 × 1 ¼ | Round/Octagonal | 12.5 | 8 | 7 | 6 | 5 | 5 | 5 | 2 |
| 4 x 1 ½ | Round/Octagonal | 15.5 | 10 | 8 | 7 | 6 | 6 | 5 | 3 |
| 4 x 2 ⅛ | Round/Octagonal | 21.5 | 14 | 12 | 10 | 9 | 8 | 7 | 4 |
| 4 x 1 ¼ | Square | 18 | 12 | 10 | 9 | 8 | 7 | 6 | 3 |
| 4 x 1 ½ | Square | 21 | 14 | 12 | 10 | 9 | 8 | 7 | 4 |
| 4 x 2 ⅛ | Square | 30.3 | 20 | 17 | 15 | 13 | 12 | 10 | 6 |
| 4 11/16 x 1 ¼ | Square | 25.5 | 17 | 14 | 12 | 11 | 10 | 8 | 5 |
| 4 11/16 x 1 ½ | Square | 29.5 | 19 | 16 | 14 | 13 | 11 | 9 | 5 |
| 4 11/16 x 2 ⅛ | Square | 42 | 28 | 24 | 21 | 18 | 16 | 14 | 8 |
| 3 x 2 x 1 ½ | Square | 7.5 | 5 | 4 | 3 | 3 | 3 | 2 | 1 |
| 3 x 2 x 2 | Device | 10 | 6 | 5 | 5 | 4 | 4 | 3 | 2 |
| 3 x 2 x 2 ¼ | Device | 10.5 | 7 | 6 | 5 | 4 | 4 | 3 | 2 |
| 3 x 2 x 2 ½ | Device | 12.5 | 8 | 7 | 6 | 5 | 5 | 4 | 2 |
| 3 x 2 x 2 ¾ | Device | 14 | 9 | 8 | 7 | 6 | 5 | 4 | 2 |
| 3 x 2 x 3 ½ | Device | 18 | 12 | 10 | 9 | 8 | 7 | 6 | 3 |
| 4 x 2 ⅛ x 1 ½ | Device | 10.3 | 6 | 5 | 5 | 4 | 4 | 3 | 2 |
| 4 x 2 ⅛ x 1 ⅞ | Device | 13 | 8 | 7 | 6 | 5 | 5 | 4 | 2 |
| 4 x 2 ⅛ x 2 ⅛ | Device | 14.5 | 9 | 8 | 7 | 6 | 5 | 4 | 2 |
| 3 ¾ x 2 x 2 ½ | Masonry Box/Gang | 14 | 9 | 8 | 7 | 6 | 5 | 4 | 2 |
| 3 ¾ x 2 x 3 ½ | Masonry Box/Gang | 21 | 14 | 12 | 10 | 9 | 8 | 7 | 4 |
| Min. 1 ¾ depth | Single Cover/Gang | 13.5 | 9 | 7 | 6 | 6 | 5 | 4 | 2 |
| Min. 2 ⅜ depth | Single Cover/Gang | 18 | 12 | 10 | 9 | 8 | 7 | 6 | 3 |
| Min. 1 ¾ depth | Multiple Cover/Gang | 18 | 12 | 10 | 9 | 8 | 7 | 6 | 3 |
| Min. 2 ⅜ depth | Multiple Cover/Gang | 24 | 16 | 13 | 12 | 10 | 9 | 8 | 4 |
Calculating Volume Allowance
Wires aren't the only components that you need to worry about when calculating box fill. You also need to consider the volume of any conductors, clamps, support fittings, devices, equipment, and grounding conductors. Each component is given a specific conductor count, which can be multiplied by the volume allowance required per conductor to find the total box fill. Code 314.16 of the NEC provides specific details for calculating volume allowance based on the size and type of components.
| Type of Component | Conductor Count |
| Each conductor passing through unbroken or that originates outside the box and terminates or is spliced within the box. | 1 |
| Conductors that do not leave the box. | 0 |
| Each loop or coil of unbroken conductor that is at least 12 inches in length. | 2 |
| One or more internal cable clamps. | 1 |
| Cable connectors with clamping mechanisms located outside the box. | 0 |
| Each type of support fitting, like luminaire studs or hickeys. | 1 |
| Each device or equipment yoke, like switches, receptacles, or pilot lights. | 2 |
| Each gang required to mount a device or utilization equipment wider than a single 2-inch box. | 2 |
| Up to four equipment grounding conductors or equipment boding jumpers. | 1 |
| Each additional equipment grounding conductor or equipment bonding jumper over four. | ¼ |
How to Count Wires in an Electrical Box
Count the wires and other components within an electrical box to determine if the current setup or planned additions will fit the electrical box. Start by finding the electrical box volume. The volume will dictate the number and size of the conductors that can be added to the box while remaining within the minimum box volume.
Next, count the number of conductors based on the size and type of components. Keep in mind that some conductors may add nothing, such as conductors that do not leave the box, while other conductors may count as two, like each device or equipment yoke.
The volume allowance required per conductor is determined by the largest conductor present in the box or connected to a device, so if the largest conductor is 12 gauge, then the volume allowance required per conductor is 2.25 inches cubed. To find the total box fill, multiply the total conductor count by the volume allowance required per conductor as listed in the table below.
| Size of Conductor (AWG) | Volume Allowance Required Per Conductor (in.³) |
| 18 | 1.5 |
| 16 | 1.75 |
| 14 | 2 |
| 12 | 2.25 |
| 10 | 2.5 |
| 8 | 3 |
| 6 | 5 |
Once you have calculated the box fill, compare it against the minimum electrical box volume. The box volume should equal or exceed the box fill. If the box fill exceeds the electrical box volume, then you will need a larger electrical box or may have to add additional sections to increase the box volume, like plaster rings, domed covers, or extension rings.
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