Sometimes referred to as an “Electrical Box” or “Terminal Box” a Junction Box is essentially a protective box that shelters interconnected wires. There are several variations and material components that might be used in a junction box, depending on where it’s being used.

This includes

  • A Metal Junction Box
  • A PVC or Plastic Junction Box
  • ABS Plastic Junction Box
  • Fiberglass Junction Box (fiber-reinforced plastic)

The type of junction box an electrician recommends or chooses might vary depending on whether the box will be protected in the interior of a structure or exterior. Commercial and residential junction boxes may also have their own requirements.

Beyond protecting wires and components from the elements it also prevents problems with wire chewing animals, and possible complications caused by ambient dust. At the same time, a junction box also helps organize wiring per the building’s floor plan. Most include some form of labeling which further helps keep things organized for future contractors and service personnel.

A properly covered and secured junction box will also help prevent a short-circuit failure fire from spreading to the surrounding environment.

How Is A Junction Box Chosen?

There are several factors that can go into determining the type of junction box needed for your site or structure. This includes things like

  • Will it be located outdoors or indoors?
  • Does the junction box need to be waterproof?
  • Will it be a 4 terminal or 8 terminal junction box?

Other factors that might influence the type of box necessary include:

  • The method used to connect the wires within the junction box
  • The number and type of terminals being used
  • The general shape and size based on the available space

What Is A Junction Box Protection Rating?

The National Electrical Manufacturers Association sets clear standards and regulations throughout the United States for determining the necessary grades of electrical enclosure for specific conditions. Also known as NEMA their rating system serves as an outline for the type of junction box needed to withstand things like weather, as well as protecting against problems with dust, light, wind, and other environmental conditions.

The stated NEMA rating for specific site conditions essentially dictates whether a junction box can be used in hazardous or non-hazardous locations. Their rating system also defines how well a specific junction box will be protected against things like oil, coolants, and corrosives.

If an incorrect type of junction box is used for the given conditions, inspectors might fail the inspection, leading to significant delays. If a safety incident is involved in an improperly selected junction box, it could also be a potential case of liability.

What Is The Ingress Protection Rating System?

NEMA is not the only rating system that might have a bearing on a site’s junction box requirements. The Ingress Protection or International Protection rating system, also known as IP may be a factor with international electrical projects or job sites where extreme conditions are present.

An IP rating essentially describes the degree of protection a junction box provides against access from a foreign object such as dust, and moisture.

With these scenarios, there are varying levels of protection including things like water-resistant junction boxes, dust-tight junction boxes, and water-tight junction boxes. There are even submersible junction boxes, which need to operate underwater in cases where pressure might also be a factor.

What Are Junction Box Terminals?

By definition, a terminal is a point within a junction box where any two wires are interconnected. It’s not uncommon for there to be multiple terminals within a single junction box.

In these cases, there needs to be a good insulation resistance tester involved. This will essentially measure the characteristics of each terminal and ensure that one cannot affect another.

Most common junction boxes have 4 to 6 terminals. However, the exact number of terminals may depend on how just how many connections need to be made in that particular area. Before selecting a junction box there needs to be enough terminals to accommodate each pair of wires.

The wiring itself is then run through each terminal to produce the specific electrical junction. The voltage can then be easily measured across the junction by a standard multimeter.

It’s also worth noting that there are different ways to connect wires at a terminal. The one that is correct for that application will depend on how the wires are connected. Improper connection methods could also become an issue of liability or lead to a code failure.

Busbar Screw Terminals

This connection method is more common in quick installations where terminals are secured by screwing each of the cable cores into a specific slot or hole.

A Single Screw Terminal

These are designed to bring all the pertinent wires into one connection point. As the name implies, they are then joined together by a single screw. This is often the most accessible type of junction terminal when it comes to supporting a large number of wirings.

A Maintenance-Free Terminal

This is a relatively new type of junction terminal which is inserted, clamped or carefully pushed into place to secure wires and components without the use of screws. This type of junction terminal is intended as an easy to use in areas that have been defined as “inaccessible.”

What Are Some Common Junction Box Shapes?

In commercial and residential applications a specific utility area may have limited space to secure a junction box, the wires, and any other necessary external components. To answer these all-to-common concerns, modern-day junction boxes are available in various shapes and sizes.

This includes:

  • Square
  • Rectangular
  • Round
  • Octagonal
  • Ceiling

The one that is right for your site will be based on the available space, site conditions, and any other pertinent regulations.

Does A Junction Box Require Professional Installation?

Even the most talented and experienced do-it-yourselfer might be hard-pressed to install their own junction box. Not only is it dangerous to work with so many exposed wires and potential high voltage, using the wrong box or installing the wrong terminal connection could be a major code violation. This could lead to serious safety and liability issues throughout the life of the junction box.

Even a single incident could prove far more costly, than the simple service fees associated with professional installation.