Junction Boxes for joining cable
Please note that all electrical wiring and installation details given on diydata.com is for information purposes only. From 1st January 2005, the Building Regulations Part P requires, in England and Wales, that only certified persons can carryout electrical installation work, or the work must be certified upon completion - see this page for more details.
Always isolate any electrical circuit before working on the circuit.
Junction boxes are used to connect cables to add socket, lighting points, extend circuits etc. They can also be used to make repairs where a section of cable needs to be replaced. Junction boxes can be found as either round or square boxes, round ones are more common, square ones tend to be used for higher current cables.
Rating of Junction boxes
Junction boxes are rated in amps (typically 5, 20 or 30 amp). The size used should equate to the current rating of the circuit being connected into - you can always use a higher rated box than the circuit but never a lower rated box.
For a lighting circuit use a 20amp junction box and for a ring main or radial circuit use a 30 amp junction box.
Junction boxes have 3, 4, 5 or 6 terminals and the number required depends upon the application.
One thing to watch out for with the number of terminals, is the type of terminal used. Two basic types are available:
- Single screw point terminal - as the name implies, each terminal has just one large screw which is used to secure all the cable cores being joined together.
- Bus Bar screw terminals - with these types of terminals, each cable core has its own location hole and screw for securing to a terminal. The number of cores which can be joined is limited by the number of holes/screws in each terminal - these can vary from 2 to 6.
A problem when purchasing Junction Boxes is to know which type of terminal is fitted and, where Bus Bars are fitted, how many cable cores can be connected to each terminal.
Using Junction Boxes
When using junction boxes to extend ring mains or radial circuits, remember that the regulations limit the length of cable for both ring and radial circuits, and also how spurs are connected to ring mains. These regulations must be adhered to to avoid possible overloading the electrical wiring.
Junction boxes should always be fixed to a rafter, joist etc using screws through the holes in the back of them. They should always be accessible for electricians etc - they should never be buried in plaster or concrete.
The outer cover of the cables being joined should be stripped back only as far as necessary and not beyond the outside of the box.
Stripping the inner cores of the cable should be such that minimal conductor is exposed once the connection has been made.
Each of the earth cores of the cables should be covered with green/yellow earth sleeving within the box.
The cover should always be fixed in place using the screw (or screws) provided - the position of the internal wiring may need to be adjusted to allow the cover to sit on the body of the junction box before securing - never use the cover screws to force the cover down onto the cables, this can cause damage.
Examples of Junction Box use
Joining one length of cable to another.
Connecting a spur - see right.
UK Cable colours:
Pre March 2004
Post March 2004
Green and Yellow (see note)
Note: the earth feed is normally bare copper in mains installation cable, the Green and Yellow applies to the coloured sleeving which needs to be fitted where the earth is connected to a fitting or appliance.