correct cable and fuse for cooker install

[bernardgreen]

Sorry but common sense needs to be applied. Regulations set a minimum standard, while they almost always lead to a safe installation they are a MINIMUM and if common sense and the laws of physics say the installation can be better then use common sense.

If it is possible for 37 amps to flow under normal circumstances then that is the value that should be used.

 

[JohnD]

but it would be quite sensible to fit a 32A breaker and 4mm cable. The diversity rules for cookers are very lax, and would let you go even lower, but I think 32A is very reasonable.

 

[EFLImpudence]

The MIs do state a minimum of 25A opd.

 

[EFLImpudence]

Sorry but common sense needs to be applied. Regulations set a minimum standard, while they almost always lead to a safe installation they are a MINIMUM and if common sense and the laws of physics say the installation can be better then use common sense.

Common sense would indicate that the figures we are told to use will be fit for purpose.
I would prefer to say the regulations state a minimum safe standard. What is 'safer'?
The laws of physics have been taken into account when stating that standard.

Your way would involve using a 40A opd which will allow more overheating of the cable before disconnection.
Or should we install 10mm² cable as another 'just in case'?

Do you slow down to 20 mph when approaching a 30 mph speed camera 'just in case' the camera has been wrongly calibrated?

If it is possible for 37 amps to flow under normal circumstances then that is the value that should be used.

So, why is the nominal voltage not 253V?

 

[EFLImpudence]

Further -

It is not really possible for the appliance to draw 37A, anyway.

However, even if we take this value, diversity would result in a current of 19A.

Perhaps the manufacturer has already thought about 'just in case' and erred on the side of caution when stating 25A.

 

[JohnW2]

Yes, strangely it seems that "schedule 4" allows protective devices to be changed without the need for any notification to building controls. So legally you can change the fuse/MCB/RCBO without notification, providing you do not create an unsafe situation.

Indeed, as I'm sure you know,it's actually even 'stranger' than that. 1(a) of Schedule 4 appears to allow any of the components of a CU to be replaced without notification. One might argue that includes the CU enclosure but, even if it didn't, 1(c) specifically allows the enclosure to be replaced (perhaps on a slightly different occasion) without notification. Although one is not allowed to "affect the circuit protection measures" (which I assume would include changing the rating or type of protective devices) when replacing the enclosure, there is no such restriction as regards replacing the contents of the CU under 1(a) (and I think that we are generally agreed that 'replacement' doesn't necessarily have to be 'like for like). so ratings/type of devices could presumably be changed as part of that 'replacement'. The only thing one isn't allowed to do without notificaton [1(a)(ii)] is to "provide a new consumer unit" - but if one is allowed to replace all of the contents (possibly be somewhat different items), and the enclosure, then that is pretty academic! Given what one assumes to be the spirit of the law, this does, indeed, all seem rather strange!

Kind Regards, John