One frustrating aspect of electrical work is pushing wires into the box after you have connected the device. Boxes always seem to be too small, devices too big, and wires too numerous. But you can accurately gauge the types and quantities of wires that go into both metal and plastic boxes. Once this has been done, you’ll need to safely and effectively insert the wires into the box.
How Many Electrical Wires in a Box?
As determined by the electrical code, electrical boxes have limits as to the type and quantity of wires they can accommodate.
Plastic Box Fill Capacities
For plastic boxes, the box-fill capacity is printed in tiny letters on the inside of the box for different gauges of wire. Capacities for two common sizes:
Single Gang Boxes: A total capacity of 18 cubic inches allows for nine #14 gauge wires, eight #12 gauge wires, or seven #10 gauge wires. This is a wires-only capacity. With one device added to the box, subtract two wires in each category.Double Gang Boxes: A total capacity of 34.3 cubic inches allows for 16 #14 gauge wires, 15 #12 gauge wires, or 13 #10 gauge wires. When two devices are added to the box, subtract three wires in each category.
Metal Box Fill Capacities Without Devices
Use a Larger Electrical Box
With electrical boxes, it is difficult to go wider, but you sometimes can go deeper. One example is with the common plastic old-work or remodel box, the type of box that clips onto drywall rather than attaching to a stud. The commonly-found 14 cubic inch single gang box is suitable for most applications. But it’s also possible to purchase a single gang 20 cubic inch box that is deeper, allowing the wires to be pushed in more easily. As long as you have enough space in your wall cavity, there is no reason you should not use a deeper box.
Reduce Sheathing Ahead of Time
Sheathing is the thick plastic coating that binds multiple wires. Too much sheathing only steals room from your electrical box. You only need a very short section of sheathing protruding inside the box. If you rip sheathing with the wire in the box, it is difficult to cut it to down so far. Instead, insert the cable in your box, mark the entry point with a Sharpie, remove, and rip to the proper length. Then, insert the wires in the box again for installation.
