Electrical Wiring - How to Wire a Junction Box to Extend Circuits for Sockets and Lights

Summary: How to wire an electrical junction box. A junction box is used to add a spur or to extend circuits and direct power to lights and additional sockets. Advice on wiring electrical junction box with easy to follow junction box wiring diagrams, including information on 20 and 30 amp junction boxes. Be sure which type of junction box should be used for ring main, radial circuits and lighting circuits. Also includes safety tips and information on fixing junction boxes and ensuring they are accessible.

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Warning: To complete electrical works you must comply with Electrical Regulations - Click here for more information.

Working With Junction Boxes Safely

Part P of the new building regulations could involve a check on any additional circuitry by qualified electricians when you sell your home. This can affect your sale, you could be breaking the law and your house insurance may not be valid. Please be absolutely sure you know what you are doing and get all of your work checked by a qualified electrician. Make sure you turn all power off and isolate the circuits you are working on.

What are Junction Boxes Used for?

Junction boxes can be used to connect additional sockets to circuits, add lighting points, extend circuits, and in general they are a way of getting power from an existing source and taking it somewhere else. They can also be used for repairs.

If a wire has become damaged, the damaged section can be cut out and teh two sections joined back together again using a junction box.

In short they are used for joining wires or running a spur from an electrical circuit safely. Whenever you need to join two or more electrical wires, then you should use a junction box. As a result they are commonly used and very useful for anyone involved in wiring and electrics.

You might be thinking that block connectors do the same job and you would be right, but they do leave quite large sections of wiring exposed and if you have removed long lengths of the protective sleeve from wires, bare live wires can be easily accessible and cause an obvious danger.

Junction boxes have an advantage here in that all wiring and joins are housed within the junction box itself, leaving no exposed bare wires that can be touched or knocked.

Junction boxes can also be used to add lights and switches. For more information please see our lights and switches project.

Heavy duty 30A junction box

A typical junction box - These can be purchased from our online store here

Types of Electrical Junction Box Explained

Electrical junction boxes come in a range of different types. There are two key differences that you will need to look out for when purchasing your junction box:

  • Rating
  • Terminals

There are also different styles of junction boxes that you can use. Each will differ slightly depending manufacturer (terminal layout, slightly different mouldings etc....), but the essentials should all be the same:

  • Standard Junction Box
  • Maintenance Free or Wagobox Junction Box

We will explain the differences between these types of junction box and their various benefits shortly, however it is important that you have the right rating whichever type that you choose.

Junction Box Amp Rating

Junction boxes are rated in amps to protect them, you and the circuit. Make sure you have the right box for the right job. A ring main (more about ring mains can be found in our project here) and also a radial circuit (more about radial circuits in our project here) will need a 30 amp junction box, whereas a lighting circuit (although you can buy 5amp junction boxes) will need a 20amp junction box.

The rating that you use must reflect the current of the circuit that it will be used on. It is possible to use a higher amp rated junction box on a lower rated circuit, but never the other way around - To summarise:

  • Ring mains and radial circuits = 30 amp
  • Lighting circuits = 20 amp

Junction Box Terminals and Wiring

Junction boxes come with either 3, 4, 5 or 6 terminals so work our which you will need to complete your project. The terminals are where the connections will be made by inserting the wire core from either side or the wiring run and then screwing the terminal close to create a safe electrical junction and join.

Joining wires using a junction box

A 3 terminal junction box being used to join two sections of wiring together

Essentially there are three types of terminal that are used in a junction box - these are:

  1. Single screw terminal – The terminal has one screw which is screwed closed on to the cable cores being joined together to secure them all in one single connection point. These are typically found in the Standard junction box, and allow the connection of a larger number or sizes of cable cores that the other terminal types
  2. Single Screw type terminals inside a junction box

    Single Screw type terminals inside a junction box

  3. Bussbar screw terminals – The cable cores are inserted into their own holes and then secured by screwing down on them to trap them securely. It is important to ensure that the terminal hole is sufficiently large for the cable core to fit. These terminals are relatively quick and easy to work with
  4. Bussbar type terminals in a junction box

    Bussbar type terminals in a junction box

  5. Easy connect/maintenance free terminals - the cable cores are each inserted into their own hole in the terminal. The cables are either secured by levers, or are push fit. The number of cores that can be connected at each terminal is limited by the number of holes in each terminal – Generally 2 to 8. They are extremely quick to connect and easy to use.
  6. Easy connect maintenance free type terminal connector

    Easy connect maintenance free type terminal connector

Round or Square Junction Boxes

There is no requirement for a junction box to be either round or square, and they are commonly both. As a rule of thumb the more common round junction boxes are typically used for lower current cables.

Wago type terminal connector

High current junction box - Image courtesy of Bartec

Increasingly common are the “Maintenance Free” Junction boxes. These are designed to replace the standard round junction boxes commonly use in household wiring. The reason that they have become so popular is that they can save nearly 75% of the time required to wire a traditional junction box.

Maintenance free type junction box

Maintenance free type junction box

Wiring a Junction Box

This section explains how to wire a traditional or standard style junction box. It doesn't matter about the rating or number of terminals, the process is the same in each case.

Joining Two Sections of Wire Together

Double checking that the power is off and connect your junction box as shown below:

Junction box wiring diagram

Junction Box Wiring diagram – Connecting cables using a junction box

Creating a Spur using a Junction Box

If you want to spur off from a current circuit to provide power to a new socket of light, again, make sure the power is off and the circuit is isolated and wire your junction box using the method below.

Junction Box with Spur wiring diagram

Junction Box with spur wiring diagram – Adding a Spur using a Junction box

Fixing cables into the Junction Box

The cables should be stripped to reveal the cable cores, but ensure that you do not do this back beyond the junction box itself. The cable leaving the junction box should be complete and intact.

The cable cores will need to be stripped before being inserted into the terminals. Try to expose the minimum amount of the conductor as necessary. 10mm will be typically sufficient for this.

Use green and yellow earth sheathing to cover the earth which will most likely be uncovered.

Which Colour Wires are Which?

UK wiring colours went through a change in March 2004. The traditional red live and black neutral were changed to brown live and blue neutral. The earth wire remains as yellow and green if sheethed but it can also come bare. As stated above, if bare, you should cover with some sleeve just to indicate that it is the earth.

For more information, please also see our project on the New Wiring and Cable Colours for a full explanation about the cable colours in use.

  Before March 2004 After March 2004
Live Red Brown
Neutral Black Blue
Earth Green and Yellow or Bare Core Green and Yellow or Bare Core

Securing a Junction Box Once Installed

Junction boxes must be fixed solidly to a firm, suitable surface and must be accessible. These rules are stipulated in the 17th edition regulations and must be adhered to. Screw the base of the junction box to a joist or rafter through the securing holes indicated in the image below.

Parts of a junction box

Parts of a junction box

This does not necessarily mean they have to be visible, they can be fixed in floor or roof voids. Accessible means that a builder or electrician can easily remove part of the floor or ceiling etc.... and access the junction box when required. Junction boxes may not be buried in plaster or other similar materials.

Likewise, the cables running into and out of the junction box should also be secured to a suitable, stable surface. If not, then “mechanical strain” could take place and over time, wires may become loose causing loose connections. Cables should be clamped using cable clips or clamps.

Please also check the rules very carefully for ring mains and radial circuits. You are limited in the length of cable you are allowed to use in both circuits and long spurs could make you exceed the limit and over load the circuit!

If this is the case you are asking the circuit to use much more energy than the circuit is designed for. More energy = more heat and cables can catch fire!

For detailed information on the rules and for more specialist advice please download the latest Approved Document P available from the Planning Portal website.

You can also read a brief synopsis on the electrical guidelines and what they mean by visiting our Approved Document P project here.

Once the junction box has been fixed to a secure surface, and the wiring has been connected to the terminals and tested you can close the junction box, they must never be left open and exposed. The cover is screwed closed using the provided securing screw (or possibly screws) through the cover and fixing it to the base. It is often necessary to adjust the wiring inside the junction box to allow the cover to close. Avoid simply forcing the cover closed with the screw as this can risk causing damage to cables.

Messy wiring in junction box

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