What Is A Junction Box – The Ultimate Guide

If you are wondering which type of junction boxes are there, their parts, working principle, installation criteria or recommended ratings – read this guide.

It has all information you are looking for about junction box.

What Is A Junction Box?


Junction Boxes can be described as plastic or metallic enclosures used for housing electrical wiring connections. Its main purpose is to protect the wires that are prone to harsh environmental conditions or physical contact.

Junction Box
Junction Box

It also allows for easy access in the event of maintenance or when alterations are carried out.

How A Junction Box Works

A Junction Box basically works by taking a single input power source and splitting it. In simple terms, we can have one power source supplying several electrical components.

When you are doing installation, you need to identify the wires you require and strip them. Observe color coding and connect all similar colored wires.

You will require wire nuts for the connections since they strengthen the joints I their position. Your electrical circuit is now complete and you can now close the junction box and power it.

Junction Box Flow
Junction Box Flow

Pros and Cons Of Junction Boxes

Some of the advantages of that clearly stand out include;


One of its major uses is enclose the electrical wirings. This ensures that there is reduced or no contact with human beings thus prevent the risk of electric shock.

Cost Effective

We have the plastic box whose cost is very cheap compared to other box types. Contractors also prefer using this type since it can easily be hang anywhere without using clamps.


Plastic boxes cannot conduct electricity for the one obvious reason, plastic is non-conductive. This means it is not really necessary to ground them.

The Disadvantages Include;


This mostly affects the plastic boxes. The nail brackets together with the box are literally made of plastic.

You easily break them when carrying out maintenance or when trying to install a replacement. This eliminates your mounting totally once the bracket is broken.

Metal Conduits

It is very obvious that you cannot use metal conduits with plastic boxes. This is very important in specific installations where the wires are not concealed in walls.

You can still apply metal boxes and conduits in surface mounts. This is because they guarantee the best safety and protection in such areas.

Types Of Junction Boxes


You can find Junction Boxes in a variety of shapes and sizes depending on your intended purpose. A critical look at the common ones can shed more light;

Round Pan

Normally, they have a depth of approximately 0.5”-0.75”, making them really shallow. Generally, you can use them ceiling wirings or very light wall fixtures.

Round Pan Junction Box
Round Pan Junction Box

Only three wire connections can be made due the size of the box. Caution should be taken not to overstuff them because you stand the risk of electrical fire.

Standard Regular

Generally, they are shallow with depth size of approximately 2”-3”. Their main purpose is the safeguarding of single wire connections and regular power outlet receptacles.

Multifaceted connections allow for the ‘ganging’ of this boxes. This means that you can use detachable features on their sides to link many boxes.

Standard Regular Junction Boxes
Standard Regular Junction Boxes

Standard Square

We can also call them 4-inch square junction boxes having a depth range of about 1.25”-2.125”. Its square corners increase its volume allowing it to have conductors, connectors and very many other applications.

Square Junction Box
Square Junction Box

Standard Round And Octagon

Having a range of depth of around 1.5”-3”, they can accommodate more connectors unlike the Round Pan. When you’re dealing with heavy lighting connections on ceilings, then this is your best bet.

Round and Octagon Junction Boxes
Round and Octagon Junction Boxes

Old work applications most make use of this box too. They have fasteners on them that you can conveniently use to attach them to ceiling walls or surfaces.

Ceiling Fan Junction Box

Just like their name, their main purpose is mounting fans on ceilings. Their regular shape is usually round but they still come in various types and sizes.

Ceiling Junction Box
Ceiling Junction Box

Outdoor Junction Box

You can place them outside on walls and decks. They are manufactured to withstand extreme weather conditions.

When you have a complex outdoor electrical set up, then this is the most suitable to apply. The material used to make the plastic is usually PVC but of very high impact.

Outdoor Junction Box
Outdoor Junction Box

Durable Aluminum can sustain the metal type through any weather extremities.

Installing Junction Boxes

When you are about to install your Junction Box, ensure the power supply is turned off. You can test if the supply is still there by turning it on after opening the circuit breaker.

You can now follow the following steps in installing your box;

i.Ensure the conductors are no longer connected by using a tester or voltmeter. After confirming there is no flow of current, you can move to step ii

ii. For interior box installation, the box should be mounted to a stud using nails. You must ensure the box face is level with the surface.

iii. For exterior boxes, you can fasten them using screws. You can as well remove the knockouts using the screwdriver.

A cable clamp will also be required for metal boxes. They reinforce the box by edging through the box from the exterior.

iv. You can then insert the cables into the box and make ½ inch marks on them just after the clamp. Remove the cables and splice them 6 inches after the initial mark.

You can now return the cables into the box and reinforce them with clamps for metal boxes. For plastic boxes will not require the same since their clamps are self-tightening.

v. You should ensure that you have the right wire stripper for your wire size. You can always place the conductors end to the stripper to ascertain this.

Now strip off about ¾ inches from each conductor end to be used. Ensure you don’t damage any wire while cutting.

vi.  After stripping all the wires, use color coding to join them. You may start with the insulated ground wires for metal boxes.

You can use a pliers to fasten them together and reinforce them using a wire nut. You will need a pigtail as an additional grounding on the metal boxes.

vii. Start by connecting the neutral then hot wires. To ensure the wire nuts are not loose, give them a slight tug. You neatly fold the wires and place them inside the box.

viii. Tighten the cover back with screws and turn the power source back on. Your connections should be perfectly working.

Safety Ratings For Junction Boxes


We have three safety ratings that govern the use of Junction Boxes. They are the NEMA ratings, UL Ratings and IP Ratings.

Let’s get a deeper understanding of each;

NEMA Ratings

NEMA stands for National Electrical Manufacturers Association. They have guidelines that have set the standard for production of quality equipment.

They normally have a non-progressive rating from 1 to 13 for each junction box. Rating a higher number does not necessarily translate to better quality.

Each number rating signifies a unique resistance to environmental extremities. In addition to this, they have included letters like R, S and X that put more emphasis on the additional resistance.

Underwriter Laboratory (UL) Ratings

This is an organization that functions autonomously in enforcing safety certifications. It has outlined its safety standards when it comes to application of junction boxes.

They focus majorly on boxes controlling high voltage equipment found in industries. They also offer quality standards on the manufacture of control panels.

International Protection (IP) Rating

Also called Ingress Protection, it contains defined rules that shape the level of protection for boxes. The set code is made a prefix, two numbers with a non-compulsory letter.

They majorly focus on how safe the box should be from extremities such water and their effect on electrical components. Furthermore, they protect you from dangers that may occur from inside the box.

Main Parts Of Junction Box

Simply put, a junction is a simple structure made up of an enclosure and wire connectors. The enclosure material is normally either plastic or Aluminum metal depending on its location.

The wires enter through an entry and exit point. For plastic boxes it can be a knock out hole and for metal boxes you can have glands.

The boxes have covers that are closed after all connections are done. Screws can be used to fasten them to the enclosures.

Junction Box Parts
Junction Box Parts

Commercial Applications Of Junction Boxes

In the commercial settings, we have electrical installations containing numerous junction boxes. Others may even contain hundreds of them.

The applications in use are what determine how the boxes are set up. We can have a critical look of such applications:

· Warehouses

The environmental surroundings of most warehouses are usually not that harsh. However, they consume a lot for their daily operations.

This requires them to have impact resistant boxes with sufficient space to for the many connections available. It would also be appropriate to have outdoor boxes for areas that have partially exposed connections.

· Medical Facilities

The electrical connections are normally very complex requiring rigid junction boxes. Because they handle lives and critical medical procedures, it is very crucial for the boxes to be easily accessible.

The common option for such facilities are indoor boxes although larger facilities may opt for outdoor rated boxes. This may be due to the several buildings available on its electrical system.

· Offices And Shops

These are some of the locations that you will least require the use of a Junction Box. However, you may also want to prevent unwanted access and hazardous contact to the electrical systems.

The boxes applied are normally larger than the regular ones in residential areas. You can easily use indoor boxes but outdoor ones are also an option in places with extreme environment conditions.

A rule of thumb is that you can acquire a Junction Box with slightly higher ratings than your requirement. This will assist in having an electrical system full of resilience.

Material Composition Of Junction Boxes

The main material used to manufacture Junction Boxes for indoor use is normally plastic. You may also have outdoor boxes made of high grade PVC to withstand the environmental conditions.

For outdoor boxes, you can have them in steel with fiber glass. But the most common metal used is Aluminum alloy because its durable and can withstand harsh weather.

How Junction Box Compares To Terminal Box

A Junction Box is simply an enclosure used to house electrical wire connections. Their material is usually aluminum, stainless steel or a plastic.

Terminal Box
Terminal Box

On the other hand, a Terminal Box contains insulated terminal blocks where two or more wires can be joined. Mostly used in factories to ensure termination of all wires and secure them.

Box Extenders In Junction Boxes

We can also call them extension rings. Their work is basically to increase the size of the box by installing it in front of the box.

It can also be installed behind the box to enhance the flush with any surface material it is mounted on. They are available in many shapes and sizes depending on the type of Junction Box.

Their shape is usually similar to that of the box minus the back panel. You should be careful to match either a metal or plastic extender with your box material.

 Wiring Connection Of Junction Boxes

They have a very basic wiring connection. It composed of a main power supplied to it with the connector ends spliced.

All the wires required from the components spliced at the ends. They are connected by matching colors to the main supply and fastened using wire nuts.

Features Of Junction Boxes

Junction Boxes Contain the following major features;

i. An enclosure with a material composition of either a plastic polymer or metal

ii. Various wire connectors

iii. The enclosed surface contains entry and exit points for the wires

iv. Gable glands may be used for the entry and exit points instead of just holes

v. They have various shapes and sizes based on area of use

vi. Their connections are specific with an allowance for future expansion

Precautions When Installing Junction Boxes

There are certain procedural and safety precautions you need to observe when installing a Junction Box. Let’s have a look at some significant ones;

i. You must mount the box on the walls using only the designated holes for such

ii. Every wire that you put inside the box should be stripped ¾ inch off its end

iii. Any exposed end of wires of the same color should be twisted together using pliers

iv. All the wires should be pushed inside the box to proceed with sealing it

v. You must turn off any power supply to the wires you are dealing with to prevent accidents

vi. Using a current meter to check the circuit will a 0A reading even when there is 120 or 240 volts in the circuit.

vii. You should attach a ground wire to all the metal boxes

viii. You must always install junction boxes in places you can easily access

Finding The Right Junction Box Size

First and foremost, you must consider the location of your Junction Box. Let’s take an example, having your box in a moist means you’ll have to choose one designed for that.

Now let’s focus on finding you the perfect size box in cubic inches. You first count total number of wires you require to enter into the box.

If you’re dealing with a 12-gauge wire, then multiply to total number by 2.25. If you have opted for the 14-gauge wire, then you can multiply the total by 2.

Now, pick the biggest ground wire you have and add 2.25 for a 12-gauge wire. Just add 2 for the 14-gauge wire.

The maximum number of conductors allowable in a Junction Box is called Conductor Fill Volume. The best option now will be settling for a box with a Fill Volume higher or equal to the total numbers of wires you have.

Keep in mind this the total number of wires entering the box after multiplying them by 2 or 2.5. All wires should have sufficient space since stuffing the boxes may lead to accidents.

Standard Size Of A Junction Box

The International Standard Junction Box begins from 4 by 4 inches. Let’s take an example of a cross junction box.

Let’s also assume that the pulls from both vertical and horizontal are both from conduits each ½ inch. Based on recent International Electrical Codes, the largest conduit must at least 1/8 of the total box length.

This gives a box length of 8 times for straight pulls since ½ inches by 8 gives us 4 inches. This should both the vertical and horizontal maximum length.


At KDM, we fabricate a range of junction enclosure boxes depending on your unique requirements and specifications.

We offer both standard and custom solution for your specific needs and requirements – contact us now for all your junction boxes.

More Resources:

Electrical Box – Source: Science Direct

Terminal Box Vs. Junction Electrical Box – Source: KDMFAB

Junction Electrical Box– Source: KDMSTEEL

Junction Electrical Box Vs Electrical Box – Source: KDMFAB

Metal Junction Box Enclosure – Source: KDMFAB

Scroll to Top