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

Summary: How to wire a junction box. Using a junction box to extend circuits and direct power to lights and additional sockets. Advice on electrical junction box wiring with junction box wiring diagrams including information on 20 and 30 amp junction boxes and which 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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Please also see our project on the New Wiring and Cable Colours.

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.

Turn all power off and isolate the circuits you are working on - See our electrical safety project for more information

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.

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 or radial circuit will need a 30 amp junction box, a lighting circuit (although you can buy 5amp junction boxes) will need a 20amp junction box.

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

Junction boxes come with either 3, 4, 5 or 6 terminals so work our which you will need to complete your project.

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

Wiring a Junction Box

Double checking that the power is off, connect your junction box as shown below.

Junction Box Wiring diagram

Junction Box Wiring diagram

Junction Box with Spur wiring diagram

Junction Box with Spur wiring diagram

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.

This does not mean they have to be visible and 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!

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.

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.

Some useful books and guides when it comes to domestic wiring and electrical jobs:

Wiring and Lighting book from Amazon

Wiring and Lighting book from Amazon

Electrical Wiring Domestic book from Amazon

Electrical Wiring Domestic book from Amazon

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