National Electrical Code: Number of Wires in a Box

Placing too many wires into an electrical box is difficult, dangerous, and 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. Wires aren't the only things that count as wires, however.

The switch box shown has a total of eight "wires" -- one for each of the four insulated wires, two for the switch, one for all of the bare ground wires, and one for the cable clamps. The jumper wire does not count as a conductor. You'd need a 3x2x3-1/2-inch device box if you were wiring with 12-gauge wire.