When you need to select an electrical enclosure for a building, one of the primary concerns is a concept known as box fill. This is a way to determine how many wires and devices can safely reside inside of a given electrical box.
How do you know the appropriate box fill for a given enclosure? For residential enclosures, there are standard sizes with standard fill limits. If you have a custom box, then there is a reliable way to calculate the maximum safe box fill.
Single Gang Boxes
Single-gang electrical enclosures are those with a single rectangular space. Everything inside of the box is in that single space, meaning there is less overall room for wires and equipment. Single-gang boxes also tend to collect heat faster than other designs.
A standard single-gang box has 18 cubic inches of space. That equates to enough room to hold 9 wires that are #14 gauge, 8 #12 gauge, or 7 #10 gauge. These numbers are for boxes that only house wires. If you add any devices to the equation, subtract two wires per device.
Double Gang Boxes
Since double-gang boxes have two separate regions within, they tend to be bigger overall and hold more equipment. A standard double-gang box holds a volume of 34.3 cubic inches. That’s enough space to store the following:
- 16 wires that are #14 gauge
- 15 wires that are #12 gauge
- 13 wires that are #10 gauge
Once again, devices take up space, but the scaling is a little different for double-gang boxes. In this case, subtract three wires for every two devices. That will tell you how many items can fit in your standard electrical enclosure.
Rules for Custom Sizes
Knowing the standards is nice, but not all electrical boxes are built to the same dimensions and specifications. You can get custom boxes, and if that’s the case, then you have different rules for calculating what the box can hold.
The volume of the box is what really matters. For any box volume, you can fit a specific number of conductors, depending on the gauge you are using. But there’s a little more to it. Depending on the conductor, it might count as one, two, or zero wires.
Insulated wires, cable clamps, uninsulated wires, and fixture supports each count as one conductor. Switches, outlets, and other devices count as two conductors each. Jumper wires that are housed entirely inside of the box do not count as a conductor.
With all of that in mind, here is the scale for box sizes. For every 12 cubic inches of box space, you can hold the following number of conductors:
- 5 at #14 gauge
- 4 at #12 gauge
- 4 at #10 gauge
- 3 at #8 gauge
You can proportionally factor the number of conductors that fit in your enclosure based on the total volume of the custom device.
That sums up the essentials of box fill. If you ever have questions or uncertainty regarding the capacity of a specific box, you can consult the manufacturer. Especially when it comes to custom designs, manufacturers build enclosures with precision and can help you better determine the heat and safety limits of any particular design.