What is a Junction Box (Electrical Box)?

A junction box – also known as an ‘electrical box’, ‘jbox’, ‘or ‘terminal box’ – is a protective box where wires are interconnected.

Junction boxes are found both indoors and outdoors. Waterproof junction boxes are commonly used outdoors.

Junction boxes are often built into the plaster of a wall, in the ceiling, or within concrete. They are standard within most homes, buildings, and factories.

There 4 main electrical junction box types:

  • Metal junction box
  • PVC junction box (plastic)
  • ABS junction box (plastic)
  • Fiberglass junction box (fiber-reinforced plastic)

Most places you find electricity, you’ll find a junction box.

What Does A Junction Box Do?

A junction box serves a variety of purposes:

  • It protects people from coming into contact with live wires
  • It protects your wires from dust and dampness
  • It prevents small critters (rats) from chewing on your wires
  • It organizes the electrical floor plan into units
  • It prevents fires within the junction box from spreading (when properly covered)

In short: junction boxes serve as a protective casing for a section of wires, similar to a feeder pillar.

They stop unwanted objects – like water, rats, and your hands – from coming into contact with live wires.

When properly covered, they stop fires that start within the junction box from spreading.

They also help simplify all the electrical wiring within a building. Each junction box in a building typically indicates a different section of wiring within the overall floor plan.

This type of organized arrangement makes doing any kind of electrical work a whole lot easier (just like using the right set of electricians tools). An electrician’s life would be a lot harder if it weren’t for junction boxes.

Choosing The Right Junction Box

There are many different types of junction boxes. The right junction box for you will depend on what you need it for.

Do you need a waterproof junction box? Is it an outdoor or indoor junction box? Is it a 4 or 8 terminal junction box? The list of questions goes on.

So let’s boil the selection down. These are the four main factors you need to consider when choosing a junction box:

  1. The protection rating of the junction box
  2. How the wires are connected within the junction box (the type and number of terminals)
  3. The material the junction box is made from
  4. The size and shape of the junction box

Junction Box Protection Rating

The National Electrical Manufacturers Association (NEMA) defines standards used in North America for various grades of electrical enclosures (i.e. junction boxes).

The NEMA ratings outline the environmental conditions that a junction box can withstand, such as whether the junction box can protect against dust, light, wind, snow, and other weather conditions.

For a weatherproof junction box, you want at least NEMA Type 3 (Type 3X/3RX/3SX if you also need protection against corrosion).

The NEMA rating will tell you whether a junction box is suitable for use in hazardous or non-hazardous locations, and indoor or outdoor use. The rating also indicates how well the junction box is protected against oil, coolants, and corrosives.

Another popular rating system is the Ingress Protection (IP) or International Protection rating system. This is commonly known as an ‘IP rating’.

The IP rating describes the degree to which a junction box provides protection against the ingress of foreign objects, dust, and moisture.

Ingress of foreign objects’ is just a fancy way of saying ‘what can and can’t enter into’ the junction box.

There are varying levels of protection – for example, dust or water resistant junction boxes, dust tight junction boxes, watertight junction boxes, and submersible junction boxes. The exact level of protection will depend on the IP rating.

Common IP rated junction boxes include:

  • IP65 junction boxes – IP rated as “dust tight” and protected against water projected from a nozzle.
  • IP67 junction boxes – IP rated as “dust tight” and protected powerful jets of water or against heavy seas (up to 1m depth of submersion)
  • IP68 junction boxes – IP rated as “dust tight” and protected against complete, continuous submersion in water (suitable for continuous submersion at depths below 1m. Manufacturer will usually give details of maximum depth and/or pressure conditions)

Lastly, you have the electrical protection of a junction box. This protection rating is generally given in amps. This rating is used to determine if a junction box can handle the maximum fault current of your intended circuit.

It’s always a good idea to overrate your junction boxes rather than underrate them. This overrating acts as a safety buffer – preventing human error in rating calculations or unexpected fault conditions from causing harm.

Junction Box Terminals

A terminal is a point within the junction box where two wires are connected together. You often have multiple terminals within the same junction box, with each terminal serving as a connection point for two different wires. Make sure to use a good insulation resistance tester if you need to measure the insulation resistance. If you need to remove the insulation from the tips of the wires, you can use a set of wire strippers to strip the insulation off before connecting the wire to the terminal.

You can think of each terminal within a junction box as a dock within a shipping port. Just as each dock at the port is intended for a different line of shipment, each terminal within a junction box is intended for a different electrical connection.

Just as a shipping port can have multiple docks, so can a junction box. The most common junction boxes are 4 or 6 terminal junction boxes. That said, the exact number of terminals will always depend on how many connections you need to make. When choosing a junction box make sure that you have enough terminals for each pair of wires you wish to connect.

Wiring is run through each terminal to create an electrical junction. You can measure the voltage across the junction using a multimeter. There are a few different ways to connect wires at a terminal. The type of terminal depends on how exactly the wires are connected:

  • Busbar screw terminals: intended for quick installation, these terminals are secured by screwing each of the cable cores into their own holes.
  • Single screw terminals: these terminals bring all the cables into one connector point, joined together by a single screw (make sure you’re using a good screwdriver set). They are the most accessible junction terminal, supporting a large number of wirings.
  • Maintenance-free terminals: these are new types of junction terminals which are clamped, pushed or inserted to secure their various parts rather than screwed on. They are intended as an easy, stress-free solution for junction boxes located in ‘inaccessible’ areas.

Types of Junction Boxes

What a junction box is made of (its material composition) plays a big role in how durable and reliable it will be. Most junction boxes are either made of metals or polymers.

Metal Junction Box

Also known as a Metal Electrical Box. Metal junction boxes have the advantage of being stronger, more UV stable, and more flame resistant compared to plastic junction boxes (PVC, ABS, and fiberglass). They are also able to maintain their strength at both high and low temperatures.

The downside is that metal junction boxes are higher cost, heavier, and conductive. Because metal is conductive, you’ll need to ground your metal junction box.

Metal junction boxes also have the downside of being susceptible to corrosion – although this depends on the type of metal used. There are three main types of metal junction boxes:

  1. Mild steel junction box
  2. Aluminum junction box
  3. Stainless steel junction box

While mild steel junction boxes are the cheapest of the three, they are also the heaviest and most vulnerable to corrosion. As such they should only be used indoors or in non-coastal and non-corrosive outdoor locations.

Aluminum junction boxes are more lightweight, with better protection against corrosion than mild steel junction boxes. However aluminum is not as strong as steel, and corrosion can still occur in harsh environments.

Stainless steel junction boxes are harder and structurally stronger than aluminum or mild steel. Stainless steel is also extremely resistant to corrosion. The biggest downsides are cost and weight – stainless steel is expensive and heavy.

So you need to evaluate your use case for the junction box to determine whether or not the benefits are worth the extra upfront cost.

What We Like
  • Extremely strong & durable
  • UV Stable
  • Strong flame resistance (fireproof)
  • Great temperature tolerance
What We Don’t Like
  • Higher cost
  • Heavier than plastic junction boxes
  • Conductive (must be grounded)
  • Vulnerable to corrosion

PVC Junction Box

Also known as a PVC Electrical Box. PVC (Polyvinyl Chloride) junction boxes are becoming very popular – and for good reason. PVC is lightweight, cheap, and strong.

PVC is a nonconductive material. So your PVC junction box won’t conduct electricity if it’s touched by a live wire, a fixture, or a switch (in the case of a fault). Because of this, PVC junction boxes don’t need to be grounded.

You can easily punch holes out from the back or sides of a PVC junction box. Combined with how light they are, it’s very easy to install and work with a PVC junction box.

What We Like
  • Low cost
  • Lightweight
  • Non-conductive
  • Corrosion-resistant
  • Decent chemical-resistance
What We Don’t Like
  • Not as strong as metal
  • Not recommended for ceiling fan installations
  • Less fire resistant than metal
  • Low UV resistance (coating can improve this)
  • Brittle at cold temperatures

ABS Junction Box

Also known as an ABS Electrical Box. ABS (Acrylonitrile-butadiene-styrene) is a hard, rigid, easily configurable, low-cost plastic. It is very similar to PLC and shares many of its advantages and disadvantages when compared to materials like metal and fiberglass.

The biggest differences between PVC and ABS junction boxes:

  • ABS can withstand colder temperatures and has a slightly higher impact resistance
  • PVC is slightly cheaper and has a slightly higher UV resistance

ABS has a similar chemical resistance to PVC. Like PVC, ABS is a non-conductive material. So ABS junction boxes do not need to be grounded.

What We Like
  • Low cost
  • Lightweight
  • Non-conductive
  • Corrosion-resistant
  • Decent chemical-resistance
What We Don’t Like
  • Not as strong as metal
  • Not recommended for ceiling fan installations
  • Very low UV Resistance (coating can improve this)
  • Less fire resistant than metal

Fiberglass Junction Box

Fiberglass is also known as Glass-Reinforced Plastic (GRP) or Glass-Fiber-Reinforced Plastic (GFRP). So you may see these labeled as GRP junction boxes.

Fiberglass (or GRP) is strong, durable, and stable over a wide temperature range. As fiberglass is non-conductive, fiberglass junction boxes do not need to be grounded (similar to PVC and ABS junction boxes).

Fiberglass junction boxes are a sort of ‘middle ground’ between metal junction boxes and ABC/PVC junction boxes.

For instance, fiberglass is lighter than metal – but heavier than PVC and ABS. It’s easier to modify than metal – but not as easy as PVC and ABS. It’s stronger than PVC and ABS – but not as strong as metal. Keep this in mind when choosing your junction box.

Fiberglass enclosures are usually chosen when you need a strong resistance to chemicals. Fiberglass is resistant to mineral acids, oxidants and reducing agents, acid solutions, salt & sea water, greases, oils, and some alcohols.

What We Like
  • Low cost
  • Lightweight (although heavier than PVC)
  • Non-conductive
  • Corrosion-resistant
  • Excellent chemical-resistance
What We Don’t Like
  • Not as strong as metal
  • Not recommended for ceiling fan installations
  • Less fire resistant than metal

Junction Box Size And Shape

Junction boxes come in many shapes and sizes. The choice comes down to what you’re using the junction box for.

For example, if you’re looking to get a junction box for a single terminal connection in your wall, then a 4-foot stainless steel junction box isn’t the ideal solution. Get a junction box that isn’t under or oversized, with a design that fits the system you’re encapsulating.

Here’s a list of the different shaped junction boxes:

  • Square junction box
  • Rectangular junction box
  • Round junction box
  • Octagonal junction box
  • Ceiling junction box

Picking The Right Junction Box

To summarise: figure out what you need from your junction box and work back from there.

As each type of junction box has different advantages and disadvantages, choosing the perfect junction box will depend on what you need it for.

About Blake Sutton

Blake has worked as an electrician for over 10 years, receiving his Journeyman Electrician license in 1998. Looking to take his professional electrical career further, in 2008 he received his Bachelor of Science in Electrical Engineering (BSEE) from the University of Texas in Austin. Blake now works full time as an electrical engineer, specializing in power systems.