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Definition – Junction box is such an enclosure, in which two or more point’s joints together.
Instrumentation point of view – Junction box is such an enclosure in which two or more wires joints together with T.B. (Terminal Blocks). Junction boxes protect electrical connections from the weather, as well as protect people from accidental electric shock.
What is a junction box Used For ?
When a junction box is used for every electrical connection in a building, it is much easier to locate and repair any electrical problems that may arise. When some connections are made without a junction box, such as inside wall cavities or in concealed ceiling or attic spaces, the potential for future problems increases.
In instrumentation, junction boxes are used for branch cables and joints of multicore cables. Multicore cable comes from control room to field and multicore cable comes from instrument to junction box. The joint of multicore to branch cable is through terminal board.
Junction Box Wiring Basics
Essentially, a junction box contains wire connections to split power from a single source to multiple outlets. For example, a junction box may contain a single power source that is connected to multiple wires to power several different lights.
Junction boxes are usually two & half to three & half inches tall and made of metal or hard plastic. Differences include installation, with plastic junction boxes generally being quicker and easier to install than metal ones. However, a standard junction box designed for simple covered wire splices can be either metal or plastic.
Metallic vs. Plastic Junction Boxes(pvc junction box)
Junction box enclosures can be made of metal (typically aluminum or stainless steel) or plastic (typically polycarbonate or ABS). Choosing between plastic and metal junction boxes is the first major decision point when specifying an enclosure type for a commercial application.
The choice depends on what cable and conduit material you are using. Metal-sheathed cable and metal conduit usually require a ss junction box to provide grounding. Plastic junction boxes are the standard choice for non-metallic cables.
Essential Features for Commercial Junction Boxes
Material and size aren’t the only two factors to think about when choosing a commercial junction box. The enclosure features we’ll discuss below are also relevant in many different business applications.
NEMA Rating or IP Rating
These two rating systems measure the protective properties of electrical enclosures, including junction boxes. (Read our NEMA vs. IP guide to understand the difference!) Basically, these enclosure ratings allow you to choose an enclosure that provides the level of protection your wiring connections require.
Check local commercial electrical codes to find out if you need to use a junction box with a specific NEMA or IP rating. In most cases, the commercial electricians and engineers working on your electrical system will be able to tell you the NEMA or IP rating required by local codes.
By definition, junction boxes almost always require cutouts and/or knockouts to accomplish their function. Most junction boxes will require cutouts for cables and conduit to enter and leave the enclosure. However, it also means making sure the enclosure maintains its NEMA or IP rating where cables enter or exit.
Polycase’s custom CNC machining options make it easy to obtain the precise cutouts you need on your commercial junction box. What’s more, we offer several different models of cable glands for protecting cable entries in enclosures. For truly instant customization, we offer the SK Series with built-in knockouts designed to fit standard cable sizes.
Junction boxes in commercial systems are often required to be marked with identifiers or instructions. Technicians may need to know which specific connection junction boxes house or which boxes require additional safety precautions. For this reason, businesses may hire specialized printing companies to customize their junction boxes, or they may print the labels and attach them in-house.
When to Use a Metal Electrical Box
You should use a metal electrical box if:
- You are using metal-sheathed BX cable. Metal junction boxes are used in connection with metal-sheathed cable to ground the wires.
- It is extremely important that the electrical box is securely attached to the studs. Metal electrical boxes are strong and resistant to warping, making them compatible with the heavy-duty screws you’ll need for an extra-secure stud attachment.
- Your project is in an exposed interior location. Some interior locations, such as unfinished basements, will have unrecessed electrical boxes attached directly to a surface (usually a masonry wall). Since these applications expose the wires more, you should use a metal junction box to protect them from damage.
- You require an electrical box that is more resilient. If put under strain, some plastic electrical enclosures may get destroyed.Metal electrical enclosures are generally more flexible.
When Not to Use a Metal Electrical Box
Conversely, there are also some reasons why you might choose a plastic junction box for your project instead. A plastic electrical box may be a better choice for your application if:
- You are using non-metallic sheathed wire (often called Romex wire). Romex wire and plastic junction boxes are often used together, but metal enclosures may also be acceptable (depending on the project). If you are using metal clad wire, you may be able to use plastic enclosures, but you will always need to take the proper steps to ground the wire first.
- It is safer to use metal or plastic enclosures if you want to save money. Plastic electrical boxes are generally less expensive and require fewer additional tools, such as clamps, than metal electrical boxes. (Remember, however, that no amount of savings is worth trading away safety if the application requires metal enclosures.)
- You want a junction box that is easy to handle and work with. Some types of sheet metal enclosures have sharp edges capable of causing injury, although these risks can usually be eliminated by wearing gloves or choosing a finished sheet metal enclosure.
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