4mm Ring Rather Than 2.5mm Ring??

[FourPoleRelay]

Hi - I specified a 2.5mm2 T&E ring circuit feeding general 13A socket outlets in two small bedroom and an adjoining hall, 32A RCBO at distribution board. I calculated a 5 amp connected load with a 20mm cable run.

The main contractor is making a fuss about the ring being a 4mm2 one - why is this? I can't see any issues with volt drop of the current carrying capacity of the cable.

 

[Spark123]

Is thermal insulation involved?

 

[FourPoleRelay]

No the cables would be run in the floor void as far as I know (I'm fairly sure this is what will happen).

Or perhaps run in the ceiling void and then dropped to the sockets under capping.

 

[Spark123]

20mm cable run?
Just ask the electrician why he feels the need to use 4mm - he has seen the installation and we haven't.

 

[FourPoleRelay]

20m cable run rather.

Even if the cable was to be in insulation (for lets say 5m) I would have thought 2.5mm would be man enough?

 

[JohnD]

Is he doing it as a 32A radial?

 

[FourPoleRelay]

Yes, 32A RCBO radial circuit. Given such a short run and only several socket outlets I can't understand why 4mm is necessary - unless it's something to do with the 17th Edition.

 

[Spark123]

A Ring is a circuit which starts at the CU, loops to each accessory on the ring and then returns to the consumer unit.
A radial leaves the consumer unit and feeds each accessory in turn, it doesn't return to the CU.

It may be viable to run a 2.5mm radial protected by a 20A protective device. 4mm is correct (depending on correction factors and lenght etc) for a 32A radial circuit.

 

[FourPoleRelay]

Spark, sorry mate I did mean ring I'm getting myself confused now lol!

However you raised an interesting point - why the larger cable size for a ring?

Thanks for everyone's correspondence by the way

 

[Spark123]

The only reasons for upping the size is for volt drop which is associated with length or for a correction factor such as it being in a high ambient temperature, laid in thermal insulation or grouped with other circuits.
Only the electrician can answer this for you unfortunately.

 

[FourPoleRelay]

Hi Sparks - no that's brilliant thanks - I think he's in the wrong but I shall investigate.

 

[M4tty]

You cant put 2.5mm T&E on 32A Breaker on a Radial Circuit only can do 20A Breaker with 2.5mm cable or 32a Breaker 4mm cable. or ring circuits normally 2.5mm 32A. Look at Back of BRB

 

[comms]

Last 2 extensions I've wired I put new ring finals wired in 4mm...thermal insulation in ceiling void & in stud walls.

 

[ban-all-sheds]

Worst case, even 4mm² is not rated high enough for a ring final....

 

[GaryMo]

You cant put 2.5mm T&E on 32A Breaker on a Radial Circuit only can do 20A Breaker with 2.5mm cable or 32a Breaker 4mm cable. or ring circuits normally 2.5mm 32A. Look at Back of BRB

Depends on what function the 32A MCB is providing (fault current protection, not overload).
Certain circumstances would allow a 32A MCB to supply a single or twin socket via 2.5mm² t&e.

I wouldn't do it but it's perfectly safe.