New oven & hob using old wiring

[willpom]

Hi

I'm upgrading my oven and hob and am looking for confirmation on my calculations. Currently it is 6mm2 wiring through the isolation switch with a 5A socket included. Outlet just under 14m from consumer unit, B40 MCB & RCD, cable running through ceiling void (some with glass fibre insulation), then into wall in PVC conduit.

New oven & hob are a higher rating. Using diversity I've calculated the following.

@ 230V
Oven: 4.9kW=21.3A
Induction hob: 7.2kW=31.3A

As 2 separate units have to split so: (10A + (11.3Ax0.3))+(10A+(21.3Ax0.3))=29.78A

This would be ok for 6mm2 cable.

However +5A socket = 34.78A which isn't. So my easiest solution without ripping out wiring etc is to change the current isolation switch so it doesn't have a socket.

Am I correct or am I missing something. Thanks for any input.

 

[EFLImpudence]

Not quite. The splitting is irrelevant.

The ratings will be quoted at 240V, so
4.9kW will be 20.42A, and
7.2kW will be 30A, so

50.42A with diversity is 40.42 x 0.3 = 12.13A + 10A + 5A = 27.13A.

This will be even less at 230V.



A 32A 6mm² circuit (with no derating factors for installation method) with socket is good for 16kW.

 

[EFLImpudence]

Also -

6mm² T&E (with no derating factors for installation method) has a maximum rating of 47A so that is not a problem.

 

[willpom]

Thanks @EFLImpudence. Just updated my post with run, MCB etc. 3m is insulated.

But it would appear it's all good

 

[EFLImpudence]

Unless the cable is completely surrounded by the insulation, it will be fine - as it has been for however long.

 

[willpom]

Clipped to joist. But if necessary I can access that from above as I'll be doing the bathroom floor next which is where it's insulated.

 

[EFLImpudence]

That's fine. Just make sure there are no signs of cable damage when you do it.


Some stickler will probably say you should reduce the MCB to 32A but that wouldn't alter the current drawn at all.