Electrical Wiring - How to Wire a Junction Box to Extend Circuits

Summary: How to wire a junction box. Using a junction box to extend circuits and direct power to lights. Advice on electrical junction box wiring with junction box wiring diagrams including information on 20 and 30 amp 30 amp junction boxes.

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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

Wiring and Lighting book from Amazon

Wiring and Lighting book from Amazon

Electrical Wiring Domestic book from Amazon

Electrical Wiring Domestic book from Amazon

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. 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.

Double checking that the power is off and 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. This does not mean they have to be visible and can be fixed in floor or roof voids. Accessible means to a builder or electrician who can easily remove part of the floor or ceiling etc. 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.

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