How to Calculate the Number of Wires You Can Safely Put 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. Find tips and information on counting wires in electrical boxes, number of conductors allowed in a wiring box, and more.
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. Wires aren't the only things that count during box fill calculations, however. 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.
Counting 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 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.
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.
How to Calculate Box Fill
For example, the switch box shown here has a total of eight "wires." You 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. The jumper wire does not count as a conductor. According to the chart above, you'd need a 3x2x3-1/2-inch device box if you were wiring with 12-gauge wire.