sizing junction boxes

SIZING JUNCTION BOXES

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 (NEC), 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 straight pull situation.

Fig. 1. Use 314.28(A) to size pull boxes, junction boxes, and conduit bodies when using conductor sizes 4 AWG and larger.

Suppose you have a 2-inch raceway coming into a 10-inch square box, and a 2-inch raceway leaves it on the opposite side. If you're pulling several 1/0 AWG conductors through the box, how are you going to pull them through one side and into the other without bending them so much you damage the insulation? Answer: You're not.

Now, suppose you have those same two raceways mounted on a 4-foot box. Obviously, you have plenty of space, and you won't need to make severe bends in the conductors. Because you don't have the luxury of installing 4-foot boxes for every wire pull you do, you need to calculate the box size you really need.

The scenario we just discussed is a straight pull, one in which the conductor enters the box on one side and exits on the other — but not at an angle and not with any splices. It's a “straight-through” installation. The NEC addresses these separately from other types of pulls.

Straight pulls

Fig. 2. When there is more than one row of conduit entries on the same wall, each row must be calculated separately and the larger answer used.

Why would you install a pull box for a straight pull? Why break the raceway with a box at all? One reason is a very long conductor run; another is to restart the 360° bend limitation of the applicable raceway article. The pull box provides another lubrication point, plus it allows you to reduce the force needed to pull that run in the first place.

For straight pulls, 314.28(A)(1) requires that the length of the box be at least eight times the trade size of the largest raceway. To correctly apply 314.28(A)(1), just multiply the largest raceway size by eight. The box must be at least as long as that number.

Angle pulls, u-pulls, and splices

Of course, junction boxes are used for reasons other than simply making a straight pull easier. You may use a junction box to splice conductors, combine runs from multiple raceways into one raceway, split runs from one raceway into multiple raceways, or simply to change the direction of the raceways. In such situations, junction box sizing requirements are a bit more complicated. Start with the largest raceway size like you did in the straight pull, but also do the following:

1. Multiply the raceway size by six, rather than eight.2. Add together the sizes of any other raceways on the same wall and row.

3. Add the results of step 1 and step 2.

4. The length of the box must be at least the number you came up with in step 3.

Fig. 3. Example of how to calculate the horizontal dimension of this box.

Where there are multiple rows of raceways, calculate each row individually. Then, use the row with the longest distance to determine your minimum box size (Fig. 2).

When conductors come into a junction box in one raceway and exit the box in another raceway (not a straight pull) of a different size, how much distance do you need between those raceways? You need six times the trade size of the larger raceway. Measure that distance from the nearest edge of one to the nearest edge the other — not center to center.

Removable covers

When conductors enter an enclosure with a removable cover (e.g., conduit body, handhole enclosure, or wireway), the distance from the removable cover to where the conductors enter must not be less than the bending distance listed in Table 312.6(A), based on one conductor per terminal.

Pull box sizing (Example 1)

Let's say a junction box contains two trade size 3 raceways on the left side and one trade size 3 raceway on the right side. The conductors from one of the trade size 3 raceways on the left wall are pulled through a trade size 3 raceway on the right wall (straight pull). The conductors from the other trade size 3 raceways on the left wall are pulled through a single trade size 3 raceway at the bottom of the pull box (angle pull). If it sounds confusing, draw it on paper.

Fig. 4. Example of how to calculate the vertical dimension of this box.

You need to determine the correct horizontal dimension, vertical dimension, and spacing between the raceways containing the same conductors. If you follow the steps laid out in Sizing Steps, you'll find this easy to do.

What is the horizontal dimension of this box? See Fig. 3 on page 47.

Left to right straight pull 8 × 3 inches = 24 inches

Right to left straight pull 8 × 3 inches = 24 inches

Left to right angle pull (6 × 3 inches) + 3 inches = 21 inches

Right to left angle pull No calculation

Answer: 24 inches [314.28]

Fig. 5. Example demonstrating how to calculate minimum distance between raceways.

What is the vertical dimension of this box? See Fig. 4.

Top to bottom straight No calculation

Bottom to top straight No calculation

Top to bottom angle No calculation

Bottom to top angle 6 × 3 inches = 18 inches


What is the minimum distance between the two trade size 3 raceways that contain the same conductors? See Fig. 5.

6 × 3 in. = 18 inches


Pull box sizing (Example 2)

Let's say a pull box contains a trade size 2 and trade size 3 raceway on the left side, a trade size 3 raceway on the top, and a trade size 2 raceway on the right side. The trade size 2 raceways are a straight pull, and the trade size 3 raceways

are an angle pull. Again, you have to determine three numbers, so you'll probably want to draw this on paper.

What is the horizontal dimension of the box?

Left to right straight pull 8 × 2 inches = 16 inches

Right to left straight pull 8 × 2 inches = 16 inches

Left to right angle pull (6 × 3 inches) + 2 inches = 20 inches

Right to left angle pull No calculation

Answer: 20 inches [314.28(A)(2)]

What is the vertical dimension of the box?

Top to bottom straight No calculation

Bottom to top straight No calculation

Top to bottom angle 6 × 3 inches = 18 inches

Bottom to top angle No calculation


If the two trade size 3 raceways contain the same conductors, what is the minimum distance between these raceways?

6 × 3 inches = 18 inches


Avoiding errors

How can you prevent junction box and handhole enclosure sizing errors on your electrical projects? One of the most common mistakes is to calculate for the wrong wall of the box. To avoid that particular mistake, draw out the problem so you can see what you're doing instead of having to juggle things around in your head.

Before your next project arrives with a deadline that's ridiculous — but must be met anyhow — become a junction box sizing whiz by practicing. If you don't have a previous job to use as practice, then create your own practice exercises. Use Examples 1 and 2 again, but just change the numbers. Each time you do one,

you'll get a little faster. Then when you need to quickly and correctly size your junction boxes, you'll be completely up to speed.

FUENTE: http://ecmweb.com/nec/code-basics/sizing_junction_boxes/

SIZE DOES MATTERS

The NEC requirements for sizing and installing boxes, conduit bodies, and fittings help prevent damage to conductor insulation during installation and subsequent maintenance. What are those requirements, and how will you be sure you meet them? To answer those questions, turn to Art. 314 of the NEC, which consists of the following five parts:

Part I — Scope and General. This section of the Code sets forth some basic rules on the installation and use of all boxes and conduit bodies used as outlet, device, junction, or pull boxes, as well as manholes and other electric enclosures intended for personnel entry.

Part II — Installation. Because of its extensive coverage of box and body fill calculations, Part II makes up the bulk of Art. 314. In addition to all the detailed text, it contains Tables 314.16(A) and (B), which you should refer to when calculating these box sizes.

Part III — Construction Specifications. Little of what appears in Part III concerns the typical electrician — if you use listed and labeled equipment, you're covered. Just be aware of the provisions, and don't modify listed or labeled equipment.

Part IV — Manholes and Other Electric Enclosures Intended for Personnel Entry. As with Part III, be aware of the provisions and don't modify listed and labeled equipment.

Part V — Systems Over 600V. The major concerns in this section are maintaining distances and clearances from other equipment.

When using conductors 4 AWG and larger, you must size pull boxes, junction boxes, and conduit bodies per 314.28. Let's dig right in and see what 314.28 requires, based on the necessary type of calculation.

Straight-pulls [314.28(A)(1)]. A straight-pull calculation applies when conductors enter one side of a box and leave through the opposite wall of the box. The minimum distance from where the raceway enters to the opposite wall must not be less than eight times the trade size of the largest raceway (Fig. 1).

Angle-pulls [314.28(A)(2)]. An angle-pull calculation applies when conductors enter one wall and leave the enclosure not opposite the wall of the conductor entry. The distance for angle-pull calculations from where the raceway enters to the opposite wall must not be less than six times the trade diameter of the largest raceway, plus the sum of the diameters of the remaining raceways on the same wall and row (Fig. 2).

That sounds a bit complicated, so let's put it another way. If you apply the following variables:

E=Entry point of raceway

O=Opposite wall

D=Trade diameter

S=Sum of the diameters of the remaining raceways on the same wall and row

Then the minimum distance from E to O=6ÛD+S

When there is more than one row or raceway, you must calculate each row separately. Use the largest calculated value for your minimum angle-pull dimension.

U-pulls [314.28(A)(2)]. A U-pull calculation applies when the conductors enter and leave from the same wall. The distance from where the raceways enter to the opposite wall must not be less than six times the trade diameter of the largest raceway, plus the sum of the diameters of the remaining raceways on the same wall. Fig. 3 above helps clarify this concept.

Distance between raceways containing the same conductor calculation [314.28(A)(2)]. After sizing the pull box, the raceways must be installed so the distance between raceways that enclose the same conductors is not less than six times the trade diameter of the largest raceway. You measure this distance from the nearest edge of one raceway to the nearest edge of the other raceway (Fig. 4).

Depth of box and conduit body sizing [314.28(A)(2), Ex.]. When conductors enter an enclosure opposite a removable cover, such as the back of a pull box or conduit body, the distance from where the conductors enter to the removable cover shall not be less than the distances listed in Table 312.6(A); one wire per terminal (Fig. 5).

Let's work through an example to try out the material covered so far.

Example No. 1: Two 2-inch conduits that contain 4/0 AWG conductors enter the back of a 24-inch by 24-inch pull box. What is the minimum depth of the box?

(a) 4 inches(b) 6 inches

(c) 8 inches

(d) 10 inches

Per Table 312.6(A), the correct answer is (a), 4 inches.

Junction and pull box sizing tips. When sizing pull and junction boxes, the following suggestions should be helpful:

Step 1: Draw out the problem on paper so you can visualize it. This reduces error and usually speeds up the process.

Step 2: Calculate the horizontal distance(s):

Left to right straight calculation

Left to right angle or U-pull calculation

Right to left straight calculation

Right to left angle or U-pull calculation

Step 3: Calculate the vertical distance(s):

Top to bottom straight calculation Top to bottom angle or U-pull calculation

Bottom to top straight calculation

Bottom to top angle or U-pull calculation

Let's now apply what we've covered so far so you can see how this process of correct calculation works. As you do some sample exercises, the knowledge you've gained should really begin to stick.

Example No. 2: A junction box contains two 3-inch raceways on the left side and one 3-inch raceway on the right side. The conductors from one of the 3-inch raceways on the left wall are pulled through a 3-inch raceway on the right wall. The conductors from the other 3-inch raceway on the left wall are pulled through a 3-inch raceway at the bottom of the pull box (Fig. 6).

What is the horizontal dimension of this box?


(c) 24 inches

(d) 30 inches

First, determine the left to right angle pull: (6×3 in.)+3 in.=21 in.

Then, calculate the left to right straight pull: 8×3 in.=24 in.

Next, check the right to left angle pull: No calculation required.

Finally, determine the right to left straight pull: 8×3 in.=24 in.

Based on these calculations, the answer is (c), 24 inches.

What is the vertical dimension of this box?


(c) 24 inches

(d) 30 inches

First, check the top to bottom angle pull: No calculation required.

Then, check the top to bottom straight pull: Again, no calculation required.

Next, calculate the bottom to top angle pull: 6×3 in.=18 in.

Finally, determine the bottom to top straight pull: Again, no calculation required.

The answer is (a), 18 inches.

What is the minimum distance between the two 3-inch raceways that contain the same conductors?


(c) 24 inches

(d) 28 inches

The angle pull is the only applicable calculation here: 6×3 in.=18 in.

Therefore, the answer is (a), 18 inches.

Example No. 3: A pull box contains two 4-inch raceways on the left side and two 2-inch raceways on the top.

What is the horizontal dimension of the box?


(c) 24 inches

(d) 30 inches

The left to right angle pull calculation is the only applicable step here: (6×4 in.) +4 in.=28 in.

Therefore, the correct answer is (a), 28 inches.

What is the vertical dimension of the box?


(c) 24 inches

(d) 14 inches

Again, the top-to-bottom angle pull calculation is all that applies: (6×2 in.) +2 in.=14 in.

Therefore, the answer is (d), 14 inches.

What is the minimum distance between the two 4-inch raceways that contain the same conductors?


(c) 24 inches

(d) 28 inches

The angle pull is the only applicable calculation here: 6×4 inches=24 in.

Therefore, the correct answer is (c), 24 inches.

To strengthen your ability in this area, rework the examples above, but change some numbers. For example, size a pull box that contains two 3-inch raceways on the left side and three 2-inch raceways on the top. Don't forget to draw out the problem. Then you can see just how good you're getting as you practice.

Putting it all together. By now, you should feel fairly confident in sizing boxes, enclosures, and fittings. Art. 314 can appear intimidating at first. However, once you understand some basics and work through some examples, you can correctly size these items every time.

sizing junction boxes

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