How to Size a Junction Box

From household appliances to industrial machinery, they need the proper arrangement of the electricity and the right circuit. But how? Fortunately, the equipment plays an important role in this arrangement job, which includes conduits, raceways, and the devices that this post covers: junction boxes. Before deeply understanding the modern equipment, we also need to know what the junction electrical does and other basics of the junction box.

What Does A Junction Box Do?

The junction box is an electrical box that houses and protects the electrical conductors at the points where they are spliced, tapped, and pulled. This good electrical junction box offers easy access for electricians to perform work, meanwhile protecting internal conductors from damage and unauthorized access.

The video is present the electrical junction box----customized aluminum box
Someone may misunderstand the “junction box” is used to indicate an electrical fixture box. As a matter of fact, from the perspective of the technical, the wires in the electrical junction box should connect only to other wires and raceways. As for the actual application, this term is also could be used to refer to many other electrical enclosures in which wires connect to a fixture such as a ceiling fan, light switch or wall socket.

Various Types of Junction Box

Because of the actual application of the junction box, the electrical junction box has been developed into many shapes to adapt to the actual environment. Today, I would introduce some common shapes of the junction enclosure.

Round Pan Electrical Enclosure

The round pan electrical enclosure is a relatively small case ---- usually ½-inch to ¾-inch deep. The round pan electrical junction box is commonly found in the ceiling or the wall for light fixtures or smaller objects. The round pan junction box is ideally be used with items that are less than 50 pounds.

Octagon & Round Junction Box

These two shapes of the junction offer more flexibility in the actual application, according to the sizes of the box. They usually are seen in the ceilings, walls or light fixtures and ceiling fans up to 50 pounds. Furthermore, they provide you with more room for wiring. Therefore, You can use larger units as junction boxes where needed.

Square Electrical Junction Box

Standard square boxes are usually 1-¼ inches to 2-⅛ inches deep. The square electrical enclosure is the most commonly used electrical box. It frequently is applied as the light switch boxes or electrical outlet boxes.

Why Size a Junction Box?

Okay, it is time to make the topic back to the trail. Sizing a junction box is the most critical factor for ensuring your junction box makes sense. If the electrical box is too small, the electrician has to bend conductors excessively to fit them fully inside the box, which may cause damage to the insulation of the wires.

Furthermore, most nations have released a list of national electrical codes which contains the junction box sizes in detail, depending on the number and size of conductors and fittings enclosed within the box.

How to Size the Junction Box?

Sizing the junction box also can be divided into two methods, depending on the size of the electrical enclosure.

Sizing A Junction Box (Under 4 AWG)

If the conductors in our junction box are of size 18 AWG through 6 AWG, you could use the tables in NEC 314.16 to determine the minimum volume of your junction box. The following charts would help you to understand the size of the junction box.
      Minimum Volume Maximum Number
of Conductors
MM Box Trade Size(in) Junction Box Shape CM3 IN.3 18 16 14 12 10 8 6
100x32 (4x1¼) Round/Octagonal 205 12.5 8 7 6 5 5 5 2
100x38 (4x1½) Round/Octagonal 254 15.5 10 8 7 6 6 5 3
100x54 (4x2⅛) Round/Octagonal 353 21.5 14 12 10 9 8 7 4
100x32 (4x1¼) Square 295 18 12 10 9 8 7 6 3
100x38 (4x1½) Square 344 21 14 12 10 9 8 7 4
100x54 (4x2⅛) Square 497 30.3 20 17 15 13 12 10 6
120x32 (4Lx1¼) Square 418 25.5 17 14 12 11 10 8 5
120x38 (4Lx1½) Square 484 29.5 19 16 14 13 11 9 5
120x54 (4Lx2⅛) Square 689 42 28 24 21 18 16 14 8

Sizing A Junction Box(4 AWG and Above)

If the size of your electric box is larger than the 4 AWG, you need to follow a different set of standards for sizing your junction box. When the larger conductors are forced into a small electrical box, their insulation would easily get damaged. Therefore, it is necessary to calculate the minimum size based on the arrangement of connectors and their size.

The three following parts are the basic procedures for how to size a junction box with 4 AWG conductors or above, depending on what kind of connector arrangement you’re using inside the box:

●Straight Pulls: The distance from conductor entry to the opposite wall of the electronics enclosure box should be at least eight times the trade size of the largest raceway entering the enclosure.

Example: If you want to install a 4-inch straight pull through the junction box, the components enclosure would need to be at least 32 inches on one side to accommodate the pull. (8 x 4 = 32)
Angle Pulls: The distance from the conductor entry to the opposite wall of the enclosure must be at least six times the trade size of the largest raceway entering the enclosure

●Angle Pulls: The distance from the conductor entry to the opposite wall of the enclosure must be at least six times the trade size of the largest raceway entering the enclosure. Then, we need to plus the sum of the trade size of any other raceways on the same row and wall.
Example: If we make the angle pull of a 4-inch raceway with another 3-inch raceway entering on the same wall, the box would need to be at least 27 inches on the other side.  ((6 x 4) + 3 = 27)
●U-Pulls: The above-mentioned distance must be at least six times the trade size of the largest raceway entering the enclosure, plus the sum of the trade size of other raceways on the same wall.
Example: If we want to make a U-pull of a 3-inch raceway that exits the box through another 3-inch raceway on the same wall, the enclosure for electronic would need to be at least 21 inches on one side.

Last Words

All in all, the electrical junction box is a type of special industrial enclosure. And, when you are finding the custom electronic enclosures, especially sizing the junction box, you should pay attention to the arrangement of the wires and the size of the electrical box.