How to Count Wires in an Electrical Box

If you're doing electrical work for your home remodel, it's important to know about the National Electrical Code for wiring. We're giving you the basics, below.

Placing too many wires into an outlet box is a common mistake with electrical work. Not only is it difficult and dangerous, but it's also against the National Electrical Code. To avoid possible shorts and overheating due to tightly packed wires, the code limits the number of wires you can put into an electrical box, which is also known as box fill.

However, wires aren't the only things that count during box fill calculations. You also need to take into account the cable clamps, outlets, switches, and other items you intend to put in the box. Here's how to count the wires and other components to ensure you're filling your electrical box safely. Find tips and information on counting wires in electrical boxes, the number of conductors allowed in a wiring box, and more.

Close up of hands testing wires in an electrical box
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How to Count Wires in Electrical Boxes

To figure out how many wires you can put into the electrical box, first, you need to count up all the components. Each item in an electrical box counts as a different number of wires. Each insulated wire, all cable clamps combined, all uninsulated wires combined, and each support for the light or another fixture count as one wire. Each switch, outlet, or other device counts as two wires. Jumper wires (which originate inside and don't leave the box), however, count as zero wires.

chart of number of conductors allowed in wiring box

Number of Conductors Allowed in a Wiring Box

Boxes of different shapes and sizes can safely accommodate a varying number of wires, depending on their size. The total volume of the box is the determining factor. This chart designates the number of conductors allowed in a wiring box based on its size, the type of box, and the wire size. After checking the total volume of your box, use this chart to determine the number of "wires" allowed based on the wire size you're using.

wires in switch box

How to Calculate Box Fill

To figure out the fill of your electrical box, use the following method. Count one for each of the four insulated wires, two for the switch, one for all the bare ground wires, and one for the cable clamps. For example, the switch box shown here has a total of eight "wires." The jumper wire does not count as a conductor. According to the chart above, you'd need a 3x2x3½-inch device box if you are wiring with 12-gauge wire.

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