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Wiring a Junction Box

Making the right connections

Almost every electrical installation has a junction box, which in simple terms is a means of getting power from one point to another. Junctions boxes are used to provide a connection between fixed wiring and lights and can be used to extend circuits and for repairs. They also provide mechanical protection.

Here we answer the most common questions about installing an electrical junction box:

1. How do I know which junction box to use?
2. What guidelines are there for location of junctions?
3. How do I avoid mechanical strain?
4. What is the most common cause of loose connections?
5. What regulations apply?

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1. How do I know which junction box to use?

   First and foremost, make sure it has the right amp rating — this is crucial to protect the junction, the circuit, and most importantly, you. A 30 amp junction box is needed for a ring main or radial circuit and a 20amp junction box for a lighting circuit. Secondly work out how many terminals the job needs — junction boxes come with 3, 4, 5, or 6.

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2. What guidelines are there for location of junctions?

   According to Electrical Safety guidelines connections should be easily accessible and located where there is room to work around them.

   It is possible to use maintenance–free terminals, however electrical junction boxes with screw terminals must be positioned where they can be easily reached for inspection, testing and maintenance. For example, if the box is in a the loft or under floorboards then an access point must be made readily available and easily accessible, for example via a loft or trap door.

   Placing a box under a fitted carpet or behind a covered hole in a ceiling will make it inconvenient and difficult to get reach, and if the junction box needs pulling out of its position to reach it there is a danger that the terminal connections could be damaged.

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3. How do I avoid mechanical strain?

   Terminations of cables and conductors used as fixed wiring can suffer mechanical strain if they are not sufficiently supported.

   An effective means of support is to clamp cables coming in and out of the junction box. This will also prevent conductors from being exposed.

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4. What is the most common cause of loose connections?

   Since the loose or disconnected cables can’t be seen within electrical junction boxes, every more stringent safety standards, including Part P, have placed junction boxes under the spotlight for close inspection.

   A common problem with connections occurs because the same terminals are used for both the twin and CPC cables, as well as the light fitting’s flexible cable. This means that the thin strands of a flexible cable’s conductors can become easily damaged, possibly causing a loose connection.

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5. What regulations apply?

   Junction boxes are covered under the 17th Edition of the Wiring Regulations, but work involving them often fails to meet the requirements due to the lack of availability of suitable products.

   However, in order to comply with regulations and pass inspection tests, electrical connections must be accessible, providing “durable electrical continuity, and be suitable for the conductor CSA, shape and the number of conductor strands.”

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