Junction Box Sizing Calculator

This electrical junction box sizing calculator will be your companion when deciding what size of electrical boxes to get for your pull boxes or junction boxes while, at the same time, complying with the National Electrical Code®.

Keep on reading to learn:

• Why finding the right electrical junction box size is essential;
• How to size junction boxes; and
• How to use this electrical junction box sizing calculator.

Until now, the easiest way to transport electricity from a power source to any equipment throughout a structure is through electrical wires. Connecting various devices like lighting fixtures and outlets to the power source means lots of cables to manage.

Since we don't want our cables lying around the floor or within the walls and ceiling, we use conduits like tubings and pipes to keep the wires together organized. We then connect these conduits using connectors and boxes where we can reorganize the wires to direct them to the locations they need to be.

We call these boxes our junction boxes or pull boxes (as we pull wires to them before inserting them back into other conduits). Just like when choosing the correct wire sizes or wire gauges suitable for our electrical needs, these boxes should also have the appropriate dimensions to accommodate the wires that enter them. We'll also have enough room for the necessary bending of cables so we don't end up folding the wires sharply, which could damage their insulation.

For our safety and protection, the NFPA® has set out guidelines we can follow to avoid fire and damage to our devices. In the next section of this text, we will discuss the minimum requirements when choosing the proper junction box sizes.

When discussing how to size junction boxes, it is best to consider some situational examples. For our first example, let's say we need to attach two 1½" conduits to the left side of our junction box, where we'll pull wires horizontally to the right side of the box with one 2½" conduit.

According to the 2020 NEC®, our box should have a size of at least eight times the largest conduit when considering straight pulls. Since we're dealing with horizontal straight pulls, we can determine the horizontal dimension of our junction box using this formula:

where:

• $L_\text{min}$ – Minimum length our junction box should be; and
• $t_\text{max}$ – Largest conduit's trade size used in straight pulls.

Since our largest conduit for horizontal pulls is 2½ or 2.5 inches, we use that in our formula to get:

We use the same concept to determine the minimum height of our junction box if we're going to have vertical straight pulls in our junction box.

However, our junction boxes may not only have straight pulls, but our junction boxes can also have angle pulls, U-pulls, and splices. We have angle pulls when we need to pull our wire from one side of the box and insert it back at a 90° angle and a U-pull when we have to insert the wire back at the same side of the box but to another conduit. In other words, U-pulls are also angle pulls at 180° angles.

For angle pulls, we use this formula to determine the dimension of the box parallel to the direction of the wire's path of entry:

where:

• $t_1$, $t_2$, ..., $t_n$ – Conduit sizes of the smaller conduits on the same row as the considered largest conduit.

For example, if we consider a wire entering from the box's left side, we calculate another minimum length measurement of our junction box. We say "another" because this is another specification our box must follow.

Suppose a junction box has horizontal straight pulls and angle pulls from its left or right side. In that case, we should calculate the minimum length due to the straight pulls (using our first equation), find the minimum size due to the angle pulls (using the second equation), and use whichever is larger between the two.

Let's say we're adding two more conduits on the left side of our junction box from our earlier example, and we're adding one 4-inch conduit and one 1-inch conduit for angle pulls towards the bottom of the box where will have three 2-inch conduits.

Since our largest conduit for angle pulls from the left side is 4 inches, we multiply that by 6 and add the sizes of the other pipes that fall in the same row as this conduit, as shown in the junction box sizing calculation below:

Because we obtained a larger $L_\text{min}$ than earlier, we'll use this larger measurement of 28 inches for the length of our junction box.

To find the minimum height of our junction box (which we can denote as $H_\text{min}$, we'll use the same concept and consider the angle pulls from the bottom of the junction box (which are the same ones from the left side of the box).

Since the largest conduit size that we have at the bottom of our junction box is 2 inches, we multiply that by 6 and add the two other 2 inches of conduits to get:

Since we don't have another minimum height to compare 16 inches from, we can already consider this measurement for the minimum height of our junction box.

That said, we need to get a box with minimum dimensions of 28 × 16 inches for the junction box in our example.

Speaking of bending wires, you may be interested to learn something about "bend allowance," which is essential in forming metal sheets into different shapes such as a junction box. You can check out our bend allowance calculator for that.

Using our junction box sizing calculator is very easy and convenient. However, before using it, please inspect the electrical plan of the junction box you wish to design.

After that, you can now follow these steps when using our junction box or pull box sizing calculator:

1. Select the size of the largest conduit attached to the junction box where you will pull wires straight horizontally. Select None if there aren't any.

2. Choose the size of the largest conduit attached to the junction box where you will pull wires straight vertically.

3. If your junction box is going to have angle pulls (i.e., from the left side of the box to the top or bottom of the box), select Yes to the Any angle pulls from the box's left side? question.

4. Pick the number of rows of conduits on the left side of the box. Our tool can handle up to three rows of conduits.

5. Select the size of the largest conduit on the first row, then select the sizes of the other conduits on that row, if applicable. Our junction box calculator can handle up to 10 additional conduits per row.

6. Repeat step 5 for all the rows of conduits on the left side of the box.

7. Repeat steps 3 to 6, but this time, for the angle pulls from the right, top, and bottom sides of your junction box, if applicable.

At this point, our junction box calculator will already have the recommended minimum length and height of the junction box for your specified sizes and numbers of conduits.

To size a pull box:

1. Determine the size of the largest conduit for straight pulls and multiply that by 8. Horizontal and vertical straight pulls give us our box's minimum horizontal and vertical measurements, respectively.

2. Multiply the largest conduit for angle pulls by 6 and add the sizes of any other conduits in the same row as that conduit. Angle pulls from left/right, and top/bottom provide us with horizontal and vertical box measurements, respectively.

3. The largest measurements for vertical and horizontal measurements will be our box's recommended minimum dimensions.

The junction box's standard size starts from 4×4 inches. Imagine a cross-junction with horizontal and vertical straight pulls of wires from ½-inch conduits to ½-inch conduits.

According to the 2020 National Electrical Code®, the length of a junction box must be at least eight times the largest conduit for straight pulls. Because of that, we get ½ inches × 8 =4 inches for our box's minimum horizontal and vertical dimensions.

From their names, a junction box is where two or more conduits join together, while a utility box is where we install electrical devices like convenience outlets, light switches, and lighting fixtures, to name some.

We use junction boxes to:

• Safely house our electrical wires;

• Help us organize our cables;

• Manage the distribution of electrical wires throughout an establishment; and

• Protect us from potential electrical shock and fire in case of damaged wire insulations or open splices.

The results in this tool are for informational purposes only and do not intend to replace professional guidance from a certified electrician. Please consult a qualified electrician before proceeding with any electrical installation.