What Is a Junction Box?
A junction box is a box that protects the cable ties when connecting cables.
It is used for electrical work. The technical standard for electrical equipment stipulates that the facility should be designed so that the charging part of the cable to be energized is not exposed. Therefore, connections of cables used at low voltage are generally protected with heat-shrinkable tubing or vinyl tape.
Junction boxes are used to prevent people from accidentally touching these connections and to improve their appearance.
Uses of Junction Boxes
The following are examples of locations where junction boxes are used:
- Relay points for conduit pipe-laying work
- Branching points of electric lines
- Relay point for street light poles
Junction boxes are rarely used in homes because electrical wiring is generally housed inside walls. You may see them in commercial and public facilities.
Some junction boxes provide wiring protection with a simple cover, while others include terminals and lids.
Principle of Junction Boxes
A junction box consists of a main body, a lid, and an entry port.
1. Main Body
The main body can be made of either resin, often using vinyl chloride, or metal, mainly using iron. Milky white or rat color is used, and a junction box of a similar color to the exterior is basically selected to make it inconspicuous. In the case of metal, products plated with hot-dip zinc are also available for corrosion resistance.
2. Lid
The lid is screwed to the main body. Screws made of steel are used even for plastic junction boxes.
3. Entry Port
Openings are provided on one or all of the four sides of the main body for cable entry. Wiring is not directly put into the cable entry opening but rather it is passed inside the box by connecting a capacitor or conduit.
Types of Junction Boxes
There are various types of junction boxes, such as plastic, metal, explosion-proof, and relay boxes with terminals.
1. Junction Box for VVF
This is a junction box exclusively for VVF. These are VVF-specific junction boxes that pierce and connect VVF wires and are generally made of resin.
2. Outlet Box/Pull Box
These are junction boxes used in various places, such as factories and outdoors. They are used by connecting thin steel pipes, VE pipes, or other conduit pipes to the entry ports. It serves as a relay point for cables passing through conduit pipes, where the cables are connected, branched inside the box, and then rerouted through conduit pipes to different locations.
Products that are drip-proof and can be used outdoors are available, as well as products with multiple terminals for easy change of wiring and branching.
3. Junction Boxes With Explosion-Proof Construction
They are used in combination with thick steel conduit pipes, etc. Explosion-proof construction is used in flammable gas atmospheres, etc., and is a sealed construction used to prevent ignition by electricity. It is mainly used in chemical plants and hazardous material storage facilities where ignition is a concern.
It has a sturdy structure, and wire entry is done by a packing-type cable gland with explosion-proof packing. The lid and other open/close parts are also sealed with packing.
4. Round Exposed Box
A round exposed box is a round junction box. It is also called a round box. It is widely used for remodeling, factory wiring, etc. The lid and body are generally screwed together at two points, top and bottom.
Round exposed boxes are standardized by some standards and are smaller in volume than pull boxes and other types of boxes. Therefore, they are used for relatively small-scale wiring connections. The lower part of the box may be opened on all four sides without wire entry and used as a drainage hole.
How to Select a Junction Box
Junction boxes are selected in terms of type, size, and color. There are many types available as described above, and they should be selected according to the intended use. In general, there is no set rule, so select a type that is easy to use. However, an explosion-proof Junction Box is always required in explosion-proof areas.
If a pull box or other type of box is selected, it can be of various sizes. In general, the larger the number of cores, the more wiring connections to be stored, so a large Junction Box should be selected. The color should be selected to blend in with the surrounding color. If the required color is not available, painting is required separately.