Electrical boxes for outlets, light switches, and other devices are expected to contain both the back of the device and the wires leading up to it. That means that space is always limited.
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.
The plastic coating of wires that are too tightly packed can fray along stress points, potentially shorting out, arcing, or creating any number of safety hazards.
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
|Box Size (Inches)||14 Gauge||12 Gauge|
|4 x 1.25||9||8|
|4 x 1.50||10||9|
|4 x 2||21||18|
|3 x 2 x 1.5||3||3|
|3 x 2 x 2||5||4|
|3 x 2 x 2.25||5||4|
|3 x 2 x 2.50||6||5|
How to Insert Electrical Wires in a Box
With the wire cutter, cut off the ends of the wires. To better fit wires into boxes, it sometimes helps to use more wire. While this may seem counter-intuitive, it can help in many cases. By providing longer sections of wire, the wire easier to handle and to fold.
With the wire ripper, rip off the plastic cable sheathing. Sheathing must always extend inside of the box, not outside. At the same time, the more sheathing you have, the less space you have for the wires. So, keep the sheathing to a minimum.
Strip the ends of the wires, then attach the wires to the device (such as a light switch or outlet).
Fold Wires (Instead of Stuffing)
With 6 inches of wire protruding from the box, it is possible to make one or two folds in the wire, accordion-style. Fold the wires as gently as possible. Do not make sharp folds, as this may weaken the wires.
Push Into Box
Push the device into the electrical box. Use two hands: one hand pushes the device inward, while the other hand controls the wires.
Tips For Inserting Electrical Wires in a Box
Bring Down Your Wire Gauge If Appropriate
Do you really need that 12 gauge wire for that circuit? Dedicated lighting circuits, for example, only need 14 gauge wire and 15 amp circuit breakers. The 14 gauge wire allows more wires per box and is far easier to fold than 12 gauge. Remember that you can only reduce wire gauge if allowable by code.
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.