6mm for a 32a radial due to derating?

[mgx]

Hi, for a kitchen radial circuit supplying a total of 5.2kw of fixed load (dishwashers, washer/dryer, hob, etc) plus two 2g sockets, this would normally be fine for 4mm csa on a 32a rcbo, however the circuit passes through 2m of ducting with other circuits so derating brings it below 32a, and using a 20a rcbo could be a solution (allowed on radials not rings ) but is too close to the line (e.g. if both washer/dryer and dishwasher are on - very likely - they could draw just on 20a, add a kettle to that and it would trip), so to keep it at 32a rcbo, does it sound sensible to use 6mm? The circuit runs in stud will and then in surface boxes and mostly in conduit (since they are behind kitchen units). I am using radials for all my circuits (4mm in other cases), I guess the other option is to put this on a 2.5mm ring at 32a, but this would just double the physical cable sizes. One other issue is that the MK sockets say they can only take 2x 6mm STRANDED, will they actually fit solid core, but just make it harder for me . I'm not wiring the circuit yet, just planning. Thanks!

 

[LiamPope]

e.g. if both washer/dryer and dishwasher are on - very likely - they could draw just on 20a, add a kettle to that and it would trip

Possibly not before the kettle has finished boiling and switched itself off, though . Plus even if both washers are on, they are unlikely to both be drawing their maximum rated current simultaneously. The 20A RCBO would probably suffice in practice, but going 6mm and 32A would provide a more flexible circuit. T&E 4mm and above is stranded - i.e. live and neutral conductors are 7 (I think) cores each for flexibility. That is what MK are refering to (not wiring a socket up with flex!). So 6mm should fit fine in the sockets. Is there nothing you can do routing-wise to avoid the de-rating in the first place?

Liam

 

[Chri5]

Run 2 x radials at 4mm and split the potential load down?

Or run a 4mm ring.

Don't use 6mm, it's a real cow to terminate in and out feeds of 6mm in to a socket.

When you say duct, are you talking tiny or 6" with 3 circuits in it ?

If your up to it, post up your calcs.

 

[mgx]

Great, those are useful replies, many thanks. The ducting (~10cm x 3cm) has 5 circuits through it. I could split into two circuits, but would prefer not to use up an extra slot on the CU & cost of RCBO. It sounds like 6mm will work, even if it is a pain to wire up - but I can live with that pain since it is short lived to get a good result. The circuit will be LABC approved so I want to make sure there is a good margin on all the figures - I am guessing they might be pickier.

I just realised another option is to run 6mm from the CU (through the ducting) to the DP isolation switch, then run 4mm from there to the sockets. Is that allowable. Apologies for being new on this, on my learning curve and just trying to make sure I do it properly!

I will need to figure out the calcs next, and my maths is so rusty.