Use this junction box sizing calculator to determine the recommended dimensions of a junction box depending on the number of straight and angle pulls entering it and meet the National Electrical Code®.
The sizing requirements for pull boxes, junction boxes, handhole enclosures, and conduit bodies exist to prevent conductor insulation damage. Those requirements are in 314.28, and they apply to all conductors 4 AWG and larger (Fig. 1). To illustrate how these requirements prevent conductor insulation damage, let''s consider two extremes in a
You must size pull boxes, junction boxes, and conduit bodies large enough so a crew can install the conductors without damaging them. For conductors 4 AWG and larger, you size pull boxes, junction boxes, and conduit bodies per Sec. 314.28. That means the minimum dimensions of boxes and conduit bodies must comply with the following:
These regulations describe in detail the required junction box size, depending on the number and size of conductors and fittings enclosed within the box. You''ll need to calculate both box volume and box fill when determining the appropriate size for a junction box .
Determine the number and type of conductors entering and exiting the box. Include all devices, such as switches, receptacles, or splices, that will be housed within the junction box. According to NEC 314.16 (B), calculate the box fill units based on the conductor sizes and types.
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®.
🙋 In this junction box calculator, we refer to the specifications provided by the National Fire Protection Association® (NFPA®) in the NFPA 70: National Electrical Code® 2020 (2020 NEC®) Article 314.28 Pull and Junction Boxes and Conduit Bodies.
💡 This junction box sizing calculator, just like our box fill calculator, is meant for estimation purposes and not to substitute professional expertise.
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.
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.
💡 You can learn more about how to choose the correct size of wires in our wire size calculator and our wire gauge calculator.
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:
Since our largest conduit for horizontal pulls is 2½ or 2.5 inches, we use that in our formula to get:
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 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.
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:
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.
🔎 Note that these junction box and pull box sizing calculations only apply when pulling 4 AWG wires or larger. But it is still a good practice to always keep in mind to carefully bend your wires when reinserting them into their respective conduits to avoid damaging their insulations.
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:
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.
Choose the size of the largest conduit attached to the junction box where you will pull wires straight vertically.
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.
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