Electricity is dangerous, we all know this and are taught from a young age to treat it with care and respect. Despite the obvious dangers, if you are competent and understand how to work with electricity safely, it is more than possible for you to add your own circuits, including radial circuits.
In order to work with electricity safely you must understand how to correctly isolate circuits to prevent injury while you’re working, how to test your work to ensure it’s safe and there are no faults, ensure that you have used the correct components of the correct size or rating and that you are aware of any relevant rules and regulations.
For more information on working safely with electricity, please see these projects:
If you are not sure on how to isolate circuits correctly or test your work then please do not attempt any electrical work yourself, get the professionals in!
If after reading the above you are confident that you tick all boxes as far as safety and competency go, then please read on and find out all about radial circuits and how they work.
What is a radial circuit?
A radial circuit is a linear mains power circuit found in all homes to feed sockets, lighting points and other specific items like showers, cookers, boilers or immersion heaters etc. It is simply a length of appropriately rated cable feeding one power point then possibly going on to the next. The circuit terminates with the last point on it. It does not return to the consumer unit or fuse box as does the more popular circuit, the ring main.
To explain a little further, using sockets as an example, a radial circuit runs from the consumer unit or fuse box out to one or more sockets that are connected in a line, with each subsequent socket being supplied with power by the one before it.
With this in mind, the cable running from the power source (consumer unit etc), runs to the first socket, where each wire (live, neutral, earth) is connected to the corresponding terminals on the back if socket face-plate e.g. live to live (L), neutral to neutral (N), earth to earth (E).
The second socket in the run is then connected from the first. To do this, a second cable connects to the terminals on the first socket, so this first socket then has two wires for each terminal e.g. two brown wires in the live terminal, two blue wires in the neutral terminal and two earth wires in the earth terminal.
This pattern is then repeated for any further sockets until the end of the run is reached where it then terminates, so the last item in the run will only have one set of wires.
Aside from the traditional socket-based circuit, there are other types. A cooker circuit is also a radial as is an electric shower circuit. Although similar in principle, these more specialist circuits have very specific requirement and so are subject to much more stringent regulations, and should only be attempted by the pro’s.
Lighting circuits are also radial circuits as they start at the consumer unit or power source and terminate at the final lighting point on the run.
One final point to make concerns fault finding. With basic circuits of this type, tracing faults is comparatively easy due to the fact that it’s linear
Using sockets as the example, as each socket in the chain is powered by the one before, in most cases, all you have to do is find the point in the chain where the sockets stop working and the fault should reside with the unit just before that.
If none of the sockets are working, any faults should are then likely to reside with the consumer unit or power source.
How many sockets can I have on a radial circuit?
In principle, you can add as many sockets to a radial circuit as you like, but in reality there are some caveats to this.
If you adhere to the principles above e.g. a radial circuit running from a consumer unit with its own MCB of the correct rating can have as many socket as you like, as long as it’s in an area not exceeding 50 square metres (or 540sq ft).
That said, you will also need to consider what is likely to be plugged in to the circuit.
If it’s a pretty standard setup; 16amp MCB, 2.5mm cable, 15 sockets (more about this below) and you want to plug in and run 10 tumble driers all at the same time, then the draw on the circuit as a whole will be far too much for the MCB and it will trip straight away.
However, if you are only wanting to run a TV, charge some phones, power a router and a few other items then chances are you will have no issues.
One other point to consider is if you are creating a new sub-circuit from an existing radial circuit, even though this will also be a linear circuit it is technically not a radial circuit, but a spur.
In essence, a spur is similar in concept to a radial which can cause some confusion between the two if you are new to the world of electrics as they can be wired up in the same way (more about spurs can be found in our project here) but they are essentially quite different.
A radial circuit in it’s own right emanates from the consumer unit or similar point of origin, where as a spur is a branch off of an existing circuit.
With this in mind, a spur should only feature one additional new socket or appliance, unless it is preceded by a fused unit, in which case it can again feature as many sockets etc as you like providing the draw on the circuit does not exceed that of the fuse in the fused unit. Also the secondary fuse should never be more than the orignal rating of the MCB. I.e. a spur on a 16A socket ciruit for which multiple additional sockets should be protected by a fused unit with a fuse of 13A or less, and again should not exceed 50sqm rule stated above.
What Size Cable Should I Use?
If you have ever done any basic electrics, you will be aware of the various different types of wires. If not, then there are quite a few, these include twin and earth (flat cable) or flex (or flexible cable) which are the most common and all come in various sizes which should be selected approprately for their intended purpose. More information on the different types and sizes of cable can be found in our cable sizes project here.
As in this instance we are talking about sockets, the most appropriate cable to use is 2.5mm twin and earth, or 2.5mm 2 core and earth as it’s also known.
Although in almost all situations, certainly domestically, 2.5mm twin and earth will be the wiring choice to go for for sockets circuits, the size of cable will be in some ways be dictated by the size of MCB in the consumer unit that the circuit will run from.
For a radial socket circuit, the size of MCB used should be no more than 16 amps. This will ensure that if the circuit is overloaded, the breaker will trip before the wiring itself gets so hot that it melts and possibly catches fire.
Using a 16 amp MCB and 2.5mm T+E wiring will be ample for any general domestic appliances such as TV’s computers, washing machines etc.
For any more heavy duty appliances, such as a cooker, a more substantial cable is required. The size of wire and MCB breaker that’s used will very much depend on the rating of the appliance that’s going to be connected to it, so this will need checking first.
One final point to mention when it comes to wiring is the colours of the internal cores. Before March 2004 and the introduction of amendment 2 to British Standards BS 7671, standard wire colours were red for live, black for neutral and green and yellow for earth (or bare copper with a green and yellow sleeve).
After March 2004, wire colours became “harmonised” e.g. changed to the same colours as the rest or Europe, as this is as follows:
- Live = Brown
- Neutral = Blue
- Earth = Green and Yellow
With the above in mind, be aware that wires of different colours may be present so take note of these colours and what they represent. For more information on this, please also see our project on the New Wiring and Cable Colours.
The Importance of the Correct Size Cable and Breaker
We have already touched on this point, but it’s an important one, so it’s well worth going over once more in it’s own right.
When it comes to maintaining the safe operation of an electrical circuit, having the correct size of cable protected by the correct breaker/MCB or fuse is essential.
To prevent injury and the possibility of fire due to overheating, the correct size of MCB should be used so that it trips the second a fault is present on the circuit or the overall draw of power across the circuit is greater than it is built to allow and in turn, too great for the size of cable or wire used.
In the sections above, we used the example of plugging in 10 tumble driers on a radial circuit formed from 2.5mm twin and earth. As a tumble drier requires a large amount of current to run, with 10 of them all trying to draw the amount of power they need from such a small gauge of cable, causes the cable itself to heat up.
If this load or draw of current is not stopped (by the MCB tripping or protective fuse blowing), the cable will continue to heat up to the point that it’s plastic outer sheath melts and then catches fire to any flammable items around it.
So, as you can see, using the correct size of cable and MCB is of the utmost importance.
Can I add sockets to a radial circuit?
As with the point on cable size above, we have also already mentioned this, but as it’s such a common query, we thought we would again give this topic it’s own heading.
The simple answer to this question is yes you can and this is known as a spur, or spurred socket, but there are a few things that you nee to be aware of.
You can add one further accessory or socket off of a single socket present in a radial circuit and no more.
If, however you put in a fused unit between the existing socket and the new socket you can add as many subsequent sockets to that spur as you want as long as you do not exceed an area of more than 50 square metres.
In terms of where the spur can actually be taken from, this can be directly from the socket itself or from the wiring running to a socket by using a junction box.
How to Wire a Radial Circuit
In this scenario, we will be running a length of 2.5mm twin and earth from an MCB in the consumer unit out via a radial circuit containing 5 x 2-gang (double) sockets to small garage conversion which will be used as an office.
Firstly, an MCB of the correct size is installed in the consumer unit. In this case a 16amp MCB is used as the items to be used in the garage-come-office are going to be relatively low powered e.g. a TV, some computer monitors, laptop etc.
The MCB is wired up correctly with the outgoing live connected to the live out of the MCB and the neutral and earth is connected to the relevant neutral and earth terminals within the consumer unit (in accordance with the manufacturers guidelines).
The cable is then run out to the location of the first socket and the wire looped through a hole in the wall.
In this case, the garage is being lined out with OSB board, so a hole saw is used to cut a hole and the wires pulled through.
The cable is then run to the next socket point and the same process was followed in terms of creating a loop to run to the next socket.
When the final socket in the circuit is reached, the remaining wire is pulled through.
The next job is to then wire up each socket face-plate.
Starting at the first socket, the wire loop is cut in half so that there is now two individual wires, one from the consumer unit and one going to the next socket.
The cable sheath is stripped back as were all the wires and then each core (live, neural, earth) from both individual cables is connected to the relevant terminals on rear of the socket face plate, resulting in each terminal having two wires in it.
The first socket had now been successfully wired up. The next job is to repeat this previous process for the rest of the sockets until the final socket point is reached.
The connection of the final socket in a radial circuit is exactly the same as the others, only this time there’s only one set of wires to be connected up.
Once all sockets have been wired up correctly, a full and complex series of tests must be carried out to ensure all is working as it should, and there are no faults. This must be done by a competant and registered electrician and an appropraite certificate issued. This certificate and all the data contained in it, is your assurance as a DIYer that your work is safe and complies with BS7671 IET Wiring Regulations and Part P of the Building Regulations. This is something that your are obliged to ensure if you carry out elecrical work on your own home.
Once the test has been done and all is well, you’re now free to use your new sockets.
Wiring in a new radial circuit is a fairly straightforward job if you are competent and have a good understanding of electrics, but if not, you should feel no shame in getting the professionals in.
All project content written and produced by Mike Edwards