Second radial line to 16a MCB

Ira Jag · Mar 17, 2021

Hello
I am new on this website and first want to say hello to everyone. We moved to a house with old night storage heaters. We had to replace one in our bedroom this winter. The new electric radiator is 1,1kw with a plug. Night storage were hard wired 2,5mm cable with 16a mcb on night economy 7 busbar. The cable was connected to a day busbar. Hard wire was replaced with 13a socket. Now we think to replace night storage in second bedroom. But there is no 13a sockets near and no room for another mcb on day busbar. In Europe is not recommended to have two radial lines from one mcb, but not forbidden if both same size. Is it legal here to have one mcb for two 3 core 2,5m cables with 13a sockets at the end?

pc1966 · Mar 17, 2021

The UK regulations on wiring are a bit different to the EU and elsewhere due to the use of fused plugs. So we do not have any specific limit on how many sockets you connect to a cable providing the cable is adequately protected from overload by the MCB.

But the fact you can have many sockets does not mean it is necessarily a good idea!

Typically here we have a 32A MCB feeding either a 4mm radial, or a 2.5mm "ring final circuit" for the various sockets. The arguments for ring versus radial go on and on so I won't even start that discussion here, but just to say either of those would normally cover multiple rooms to around 75-100m^2 total area.

The safest option would be to get an electrician in to add the extra socket. If the cable is 2.5mm and not too much thermal insulation on its route they might decide to change the MCB to a 20A one which better serves several outlets.

A final note is socket outlets today should have RCD protection. Your existing distribution board may well have that, but it is something to check before deciding what to do.

Ira Jag · Mar 17, 2021

pc1966 said:
The UK regulations on wiring are a bit different to the EU and elsewhere due to the use of fused plugs. So we do not have any specific limit on how many sockets you connect to a cable providing the cable is adequately protected from overload by the MCB.

But the fact you can have many sockets does not mean it is necessarily a good idea!

Typically here we have a 32A MCB feeding either a 4mm radial, or a 2.5mm "ring final circuit" for the various sockets. The arguments for ring versus radial go on and on so I won't even start that discussion here, but just to say either of those would normally cover multiple rooms to around 75-100m^2 total area.

The safest option would be to get an electrician in to add the extra socket. If the cable is 2.5mm and not too much thermal insulation on its route they might decide to change the MCB to a 20A one which better serves several outlets.

A final note is socket outlets today should have RCD protection. Your existing distribution board may well have that, but it is something to check before deciding what to do.

Thank you for reply, we are RCD protected, but no room left in consumer unit. Changing the unit is another option, but we have limited space and it will be a separate question. Yes, extra socket can be added to the ring circuit, but it has to be done through the loft and it is much more complicated. My question were about two 2.5mm radials protected by one mcb, is it legal here?

pc1966 · Mar 17, 2021

Yes, it is acceptable to have two radials fed from the one MCB when it provides protection for the cable as this does. You might find it easier to take a spur off the existing heater socket rather than run another back to the CU.

However, you should note that adding a new circuit (add or change MCB value) to a CU, or changing the CU, is notifiable work under Part P of the building regulations covering England and Wales. For that you really should get a qualified electrician (a member of one of the competent person schemes like NICEIC) in to do it as it would work out cheaper than paying the building control folk to inspect/test and sign- off the work.

CPS members are able to self-certify their work due to their recognised qualifications & experience, so it is not a big extra charge.

Ira Jag · Mar 17, 2021

pc1966 said:
Yes, it is acceptable to have two radials fed from the one MCB when it provides protection for the cable as this does. You might find it easier to take a spur off the existing heater socket rather than run another back to the CU.

However, you should not that adding a new circuit to a CU, or changing the CU, is notifiable work under Part P of the building regulations covering England and Wales. For that you really should get a qualified electrician (a member of one of the competent person schemes like NICEIC) in to do it as it would work out cheaper than paying the building control folk to inspect/test and sign- off the work.

CPS members are able to self-certify their work due to their recognised qualifications & experience, so it is not a big extra charge.

Thank you. It really helps our planing. Unfortunately we had a problem with electrician here when we had kitchen insulated. Job was done and all walls plastered and painted and when we wanted to buy a new kitchen, we were told that some part of the job was not done correctly according local regs. I had extra cost fixing it, plastering and painting. So now we plan ahead. Currently we have two 10 module units, night and day, we would like to have 22 module day one, but there is no room for it, we will still try to get as big as possible, but at least we need one module less.

pc1966 · Mar 17, 2021

That is a disappointing experience. For future work you might find someone on this forum near you who will do a much better job.

22 circuits is a lot by UK standards!

My flat has 5 circuits, and even then one is a former water immersion heater that now provides a single socket in the kitchen where it used to have an isolator switch!

As I hinted at before, the typical UK system is one RFC per floor for sockets, one radial (really a "tree") per floor for lights, and then a couple of dedicated radials for high demand loads such as a cooker, electric shower, etc. Perhaps one or two for external stuff like outdoor lights or a shed/garage feed.

So even a 3 floor house might only have 12 used!