Radial questions.

[EddieJT]

Does it matter if a single radial serves 2, or even 3 different rooms of the house, or should a single radial serve just a single room?

I have thick brick walls. Is it ok, on a radial circuit to put sockets "back to back" as it were either side of an internal wall, providing of course the wall is thick enough (which it is). Can the wiring just pass directly through the wall?

 

[ColJack]

no it makes no odds how many rooms a radial serves, the only limit is it's length for volts drop..

putting sockets back to back is not a good idea ( not easy to fix them in for a start ) but as far as I know, it's not against a reg.. ( please corract me if I'm wrong, it's always good to learn.. )

 

[JohnD]

A domestic radial circuit is installed with a max load of 20A; so it will struggle if you have two large loads such as a washing machine, tumble drier, dishwasher, oven, combi microwave. If you have any two of these running, and add a third, or turn on a kettle or toaster, it is likely to trip.

A ring can be rated to 32A, which is enough for most homes (though it is a good idea nowadays to provide a dedicated circuit for the kitchen and/or utilty room as it has so many appliances.

 

[ColJack]

you're making a false statement there John...

a radial power circuit wired in 2.5mm² cable should be protected by no more than a 20A OCPD..

a radial power circuit wired in 4.0mm² cable should be protected by no more than a 32A OCPD..

 

[JohnD]

I wondered if I should try and go into that

decided to be simple

thinking also about cost of doing it in 4mm, you might as well just make it a ring if you want 32A

I like rings

 

[ColJack]

rings were a shortcut introduced to save copper..

they are technically more dangerous than a radial as well..

break a wire in a ring and all the sockets still work, but now you've got a 2.5 radial protected by a 32A OCPD..
at least with a 4mm radial you would notice some sockets not working and the remaining ones would still have their supply cables protected properly..

 

[EddieJT]

Thanks guys.

Next question, my kitchen will obviously be on it's own circuit (radial). It would be easy for me to continue this radial from the kitcen through an internal wall to the next room, for just one socket.

Is this allowed i.e. have a living room, with one socket that is actually on the kitchen circuit, while the rest are on the living rooms own radial?

 

[plugwash]

It's not forbidden but many frown on it because of the possibility of someone making an incorrect assumption while working on the circuit.

If you do go down this road at least make sure you produce good documentation for the installation (you should really be doing this anyway but it's doubly important if you do anything unusual)

 

[ericmark]

The main problem with radials is the volt drop or before use of RCD's on all sockets the earth loop impedance.
With a ring main with 2.5mm cable we have a maximum of 106 meters of cable. With 4mm cable as 25A radial this is reduced to 26 meters and with 6mm cable rated at 32A it is 20 meters.

Cable is rated Reference Method 100# (above a plasterboard ceiling covered by thermal insulation not exceeding 100 mm in thickness) for:-
2.5mm 21A
4mm 27A
6mm 34A
Since 2.5mm is doubled up in a ring main it is 42A total so can be used on a 32A fuse. The Corrigendum to 17th Edition does show 4mm used on a 32A MCB or fuse but it does not state how the cable is to be installed and with clipped direct 4mm is rated at 37A so it would be permitted where cable is exposed it states:- "Cables should, wherever practicable, be fixed in a position such that they will not be covered by thermal insulation. Where a cable is partially or completely covered by thermal insulation, reference should be made to Regulation 523.7."
The 523.7 gives all de-rating values for where cables are not clipped direct.

I think MK sockets will take 2 x 6mm cables although I have not tried to fit them. Many sockets will not take 2 x 6mm cables and even with 2 x 4mm cables are unlikely to be able to take a further 2.5mm cable to allow a spur to be taken.

Although in theory radial circuits are safer and if I was re-wiring my own house it is likely I would use a radial system once one leaves the 2.5mm cable one does have problems. Using 90degC cable like Ali-tube does increase the current one can draw and 2.5mm Ali-tube could have a 25A MCB and 4mm a 32A MCB but one has to consider if the higher temperature would cause problems where it is run. 70degC is hot and 90degC may melt or char things it is clipped on and I would be very wary of using the extra power rating. The same goes for mineral insulated cable and I know one can do a 32A ring in 1.5mm Pyro but don't think I would want to do that in a house!