cooker point into radial circuit?

[WhydidIstart]

Hi,

Firstly this will be notifyable under part P - if possible at all.

I have a cooker point in the kitchen fed via standard 6mmsq cable with a socket at the cooker switch point and then more 6mmsq cable to the cooker connection point.

I want to remove the cooker and use this 6mmsq cable from the consumer unit to serve 3 double socket outlets. The cooker will be fed with a new 6mmsq feed from the consumer unit on the other side of the kitchen. The layout of the doors, a concrete floor, etc mean it is hard to run a new ring main to the sockets I want, or take them as spurs.

So am I correct in thinking I can take the 6mmsq cable to the first outlet, then 4mmsq to the next one and on to the next one again as a radial circuit? I would fit a 32A MCB as the loads will be high (dishwasher, kettle, toaster, etc) and the 4mmsq cable is rated for it.

If this is ok, my next problem will be the physical connections as I think it will be fun trying to get a 6mmsq cable into an outlet, let alone 6mmsq and 4mmsq in the same one!

Thanks in advance for any thoughts you may have.

 

[electronicsuk]

Hi,Firstly this will be notifyable under part P - if possible at all.

I'm not sure if that's a question or a statement, but yes, the work will be notifiable to your local building control before it commences, and will require inspection on first and second fix.

So am I correct in thinking I can take the 6mmsq cable to the first outlet, then 4mmsq to the next one and on to the next one again as a radial circuit? I would fit a 32A MCB as the loads will be high (dishwasher, kettle, toaster, etc) and the 4mmsq cable is rated for it.

You can't just assume that 4mmsq will be up to the job. It is rated for 32A clipped direct, but as your installation method may vary, you'll need to confirm this.

If this is ok, my next problem will be the physical connections as I think it will be fun trying to get a 6mmsq cable into an outlet, let alone 6mmsq and 4mmsq in the same one!

With good quality accessories you should just about be able to get away with anything up to 2x 6mmsq conductors, but you'd want to use 47mm deep boxes.

 

[Taylortwocities]

To expand, if a circuit leaves the consumer unit as 6mm, it should continue as 6mm all the way, unless it stops at a fused point.

An example of this would be 2.5mm ring with a 32A fuse terminating on a fused spur unit with a 3A fuse and then feeding an extractor fan with 1.0mm cable.

An alternate solution could be to terminate your 6mm into a mini fuseboard in the kitchen with (eg) 3 x 20A contact breakers. You could then feed the three sockets as separate radials.

What will be on those 3 new sockets?

Yes to repeat, all notifiable work!!

 

[WhydidIstart]

Hi, thanks for the good replies - yes the Part P was a statement.

There isn't really space for another distribution board and thanks very much for the comment re 4mmsq cable rating reminder. From the table I'm looking at in 7671 the rating falls from 37A clipped direct to only 27A embedded directly in an insulated wall, so I was wrong.

Loads are dishwasher = 2.5kW, kettle = 3kW and toaster = 1kW so 28.3A max. However diversity will probably enable a 25A MCB to be used and not give nuisance trips, but still protect the 4mmsq cable. Do you agree?

The 47mm boxes are a very good idea, thanks.

Re: using 6mm cable throughout, is that a 'should' be used or a 'must' be used? Your example with a 3A spur is understandable, but what about the following:

Ring circuit with only 2 sockets in 2.5mm. A spur is taken which should be in 2.5mm to a FCU with say 5A fuse and then 1.5mm taken to a light. Say for some reason the property was being converted and the sockets weren’t required any more, electrically it would seem safe to replace the sockets with blanking plates and junction boxes, FCU with an unufused unit and replace the MCB for a 6A one, with a note explaining not all the circuit was 2.5mm. Although not very nice at all, would this be against 7671?

Thanks again in advance.

 

[WhydidIstart]

Hi, I was wondering if anyone had any ideas about if the circuit should continue in 6mm or must continue in 6mm, assuming the MCB is rating for the lowest rated part of the circuit?

 

[plugwash]

Afaict it is not actually a regs requirement anymore but it is certainly considered best practice.

 

[BS3036]

433.3.1 says you can reduce csa without an fcu provided the whole circuit is adequately protected. The only danger then, is if insufficient documentation exists for the installation to make a future installer aware of the cable size reduction.

 

[sparkyspike]

In your case, although less than ideal, you can step down to 4mm but mark the circuit clearly at the DB!

Otherwise wire the whole lot in 6mm on a 32A breaker like a standard radial. Good luck with that! TIC

 

[WhydidIstart]

Will do. Thanks very much for the advice.

 

[Wooshzz99]

The last time I posted here I got shot down by everyone. However a 2.5mm ring main is protected by a 32a mcb generally or maybe a mcbo. Why not convert the 6mm to a ring main at the end in the kitchen or have I missed somethng? Thanks All.