new kitchen oven & hob circuit

[WhydidIstart]

Hi, this involves a new oven and hob supply in a kitchen. I'm aware of Part P and notification. Is the following acceptable please?

Am I correct in thinking T&E burried direct in plaster in a concrete wall has the same CCC as "clipped direct" = 47A for 6mm T&E?

If so, is it acceptable to feed 6mm T&E from a 45A RCBO in the CU to a 45A cooker switch (with drop to outlet for a 7.8kw hob) AND then continue from the first switch to another 45A cooker switch which has a drop to outlet for an oven rated at 3kw please?

The 45A RCBO should protect the installation and although the total load could be ~47A, diversity would mean that would be unlikey to happen and 47A would take a very long time to trip a 45A RCBO - is that correct please?

 

[EFLImpudence]

Am I correct in thinking T&E burried direct in plaster in a concrete wall has the same CCC as "clipped direct" = 47A for 6mm T&E?

Correct.

If so, is it acceptable to feed 6mm T&E from a 45A RCBO in the CU to a 45A cooker switch (with drop to outlet for a 7.8kw hob) AND then continue from the first switch to another 45A cooker switch which has a drop to outlet for an oven rated at 3kw please?

Yes. Assuming the distance is no issue.

The 45A RCBO should protect the installation and although the total load could be ~47A, diversity would mean that would be unlikey to happen and 47A would take a very long time to trip a 45A RCBO - is that correct please?

10.8kw / 240 = 45A - 10 x 0.3 + 10 = 20.5A

Use a 32A RCBO

 

[WhydidIstart]

Great, thanks for the very quick reply and diversity calculation!

To help me understand a bit better, can you explain why you would use a 32A RCBO rather than a 45A one please?

I can understand if fitting a smaller protection device means cheaper / easier to work with cables can be used, but if I am using 6mm T&E that can take 45A, wouldn't using the larger RCBO mean less risk of a nuisance trip, say on Christmas Day when everything was on at the same time?

I know that's unlikely, but I remember many years ago a friend blew the 100A service fuse to their house! No faults found and replacing it cured the problem so I guess they must have just happened to overload it, unless it just failed due to age? I think this was back when they would fit 100A and not the 80A normal maximum now fitted?

 

[EFLImpudence]

To help me understand a bit better, can you explain why you would use a 32A RCBO rather than a 45A one please?

Because it is adequate.
The vast majority of cooker circuits are 6mm² cable and 32Amcb/30A fuse.
It will cope with 15kW cookers.

I can understand if fitting a smaller protection device means cheaper / easier to work with cables can be used, but if I am using 6mm T&E that can take 45A, wouldn't using the larger RCBO mean less risk of a nuisance trip,

Nuisance trips on RCDs/RCBOs are not due to overload.
You could. Not all manufacturers make a 45A, so 40A.

Actually 4mm² cable would do for 32A.
It is (sort of tradition) because older 30A BS3036 fuses had, because of their fusing characteristics, to be installed on 6mm² cable (not 4 which is rated at 37A).

say on Christmas Day when everything was on at the same time?

Folk lore.

I know that's unlikely, but I remember many years ago a friend blew the 100A service fuse to their house! No faults found and replacing it cured the problem so I guess they must have just happened to overload it, unless it just failed due to age?

You won't overload a 100A fuse.

 

[EFLImpudence]

double post

 

[WhydidIstart]

Great, thank you very much.

I'll go with 32A and keep with 6mmsq cable.