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Radial Adding socket into radial circuit protected by 32amp RCB

Discuss Radial Adding socket into radial circuit protected by 32amp RCB in the Looking for Domestic Electricians? area at ElectriciansForums.net


New EF Member
Just after a bit of advice regarding the following. First up I am not electrically qualified and know limits of what I would and would not attempt but am electrically read.

The situation is that I have a radial power circuit fed from the consumer board and protected by a 32amp RCB feeding 5 sockets on the ground floor in living areas. The kitchen and appliances are served by a separate radial and a dedicated radial for the oven.

It is in good condition and the existing radial arrangement is wired in 6mm cable (mostly; see below).

I need to add a double socket into this arrangement close to a telephone point to enable use of a router and lamp at the end of a hallway which is around 4 metres away from the circuit.

Currently the first leg of the radial exits the consumer board and enters a junction box (JB1) from which a single socket has been fed as a spur (this is in 2.5mm cable I presume this was an later addition. I understand this is allowable under reg in 2.5mm from a 4mm or 6mm 32amp radial providing it is a SINGLE double spur due to overload protection at the socket end, maximum of 26amps being drawn).

The main radial then continues to a double wall socket in the rear room and onward to a further junction box (JB2) which then radiates out to serve three separate sockets in a parallel arrangement. See image for a better idea:

The planned additional twin socket is shown. What are my options for adding this in to the current arrangement?

1. Can this be an extension of the existing junction box spur (JB1). If so, I would plan to replace the 2.5mm cable here with a higher rated cable to avoid the issue of potential overloading with a 32amp RCB or derate the whole system to 20amp.

2. Alternatively could the planned double be taken directly from the existing junction box thus creating two separate spurs off this point in the circuit as below? Which is the better arrangement?

Having calculated the voltage drop I understand 4mm cable will be suitable but would this create any issues with the existing 6mm wired system? Am I better just harmonising to 6mm throughout any wiring extension or spur? Having done this sort of this before I know this can create issues with cable crowding at the points of connection.

Any help from an experienced sparky is much appreciated!

alban moffitt

Regular EF Member
if you can easily replace the cable to the single socket fed by JB1 to 4 or 6mm then that would be better as you could then feed your new double from the single socket. if you were to feed it from the 6mm then i would probably put in a new jb for that supply because it will get quite crowded at JB1 with two 6mm's and two 2.5mm's.
idealy get a qualified sparks in to do it as the work is Part P notifiable


Regular EF Member
When did adding a new socket require building control

Partpee only covers new circuits and judging by most jobs I see nobody even bothers with it anymore


Electrician's Arms
I've always liked that you only need to notify in special locations and a lot of people think this means the work doesn't require testing with a calibrated tester and the knowledge how to use it.

Because obviously its ok to get electrocuted/start a fire in say a bedroom or lounge!


Scouser and Proud of It
Respected Member
from that diagram, you can tap in any socket using 2.5mm cable for new socket.


New EF Member
Thanks all. Makes sense. I think I will take the new 4mm from the JB1 and then onward to single socket replacing the existing 2.5mm and continue to the new double as suggested by alban

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