Using a Junction Box for Electrical Work

How to Make Sure You Have the Right Junction Box

Junction boxes are one of the most commonly used devices for both professionals and DIY-ers when working with electrics and cabling in the home. A junction box is a type of ‘connector’ that is used any time one cable needs to be joined to another, or many others. It provides a safe and secure connection.

When you are choosing a Junction box, you need to know two things above all:

• The amp rating required; this will be determined by what the circuit in which you are going to be using the junction box. As a rule of thumb, use 20 amp box should be used on lighting circuits and 30 amp on anything else. Although you can always higher amperage junction box than required, you cannot go the other way and, say, use a 20 amp junction box on a ring main.   Please remember that as a DIY-er you should not work on circuits that require 40 amp junction boxes, such as those for showers and cookers which is why we have not mentioned them, except in passing
• Number of Terminals; these are the number of connection points where you can connect the cores of the cables that need to be joined. The more there are the more cables that can be joined. They typically come in 3,4 or 6 terminals

For more details on the amp rating of junction boxes and the numbers of terminals you require please see our in depth project on fitting a junction box.

Types of Junction Box

There are several types of junction boxes design to choose from, although they all achieve the same thing – joining two or more electrical cables. There are advantages and disadvantages to all the types of junction box, but ultimately the choice comes down to your personal preference.

Here we will outline the three main types of Junction Box design:

1. Single screw terminal -This type of terminal design allows you to ‘trap’ all the cores being joined under a single screw. This has the advantage that many cores can be jointed under a single screw, although this can be tricky and time consuming to achieve in practic

   

2. Bussbar screw terminals – This design of terminal in a junction box is becoming very common as it is a little less fiddly and can be a little quicker to connect up, however they are limited to one cable core to each terminal hole. Each corresponding cable core being connected is inserted into either end of the terminal hole and the screw is tightened to trap the cable cores in the junction box terminal

   

3. Wago style terminals – These are push fit connectors, which use terminal holes for each cable core. They are incredibly quick and easy to wire up which will save a lot of time, particularly if you are doing a lot of wiring. The downside it that they are more expensive, so the time saving needs to offset the extra cost. Some people are prepared to pay the extra cost due to the simplicity and maintenance benefits. They are colour coded depending on the number of terminals, and will fit into a specially designed junction box

   

   More information on the types of junction box and which one is correct for the job you are doing can be found in our junction boxes project on the link above.

Electrical Safety

No blog about electrical DIY work is complete without a quick word about electrical safety, and rightly so. It is very important to stay safe, and at any point that you feel you do not understand what you are doing you should stop and seek professional advice. We have more information about electrical safety here, which we recommend you read even if you are confident.

Choosing your Junction Box

As we mention, it is ultimately down to what you prefer so you can choose any style of junction box you like. You will have to select the right amp junction box and number of terminals for the circuit that you are going connect the cables on.