4.2Kw Oven install

[JohnW2]

And when the unlikely event happens and the cable overheats and fails, does one say "Oh dear me I wonder why that happened" or even "I wonder why there are horrible smelly fumes in my kitchen?" ... Alternatively does one think ahead and say "For a few pounds more, that can't happen."

Few things are impossible, but one sensible thing about the regs is that they recognise that it is not necessarily sensible to anticipate happenings are 'very unlikely'. Maybe you should marry bernard

Furthermore, there are clearly some large safety margins built into the figures we work with, so I very much doubt that (even if the 'very unlikley' were to occur) 2.5mm² cable protected by a 32A MCB would ever come to any harm, let alone result in any "horrible smelly fumes". .... If the 'tabulated CCC" of Method C 2.5mm² cable were just 19% higher (i.e. about 32.13A), you presumably would be perfectly happy for it to be protected by a 32A OPD - do you really believe that (given the 'safety margins') that 19% make the difference between 'fine' and 'catastrophic'?

Kind Regards, John

 

[SUNRAY]

Few things are impossible, but one sensible thing about the regs is that they recognise that it is not necessarily sensible to anticipate happenings are 'very unlikely'. Maybe you should marry bernard

Furthermore, there are clearly some large safety margins built into the figures we work with, so I very much doubt that (even if the 'very unlikley' were to occur) 2.5mm² cable protected by a 32A MCB would ever come to any harm, let alone result in any "horrible smelly fumes". .... If the 'tabulated CCC" of Method C 2.5mm² cable were just 19% higher (i.e. about 32.13A), you presumably would be perfectly happy for it to be protected by a 32A OPD - do you really believe that (given the 'safety margins') that 19% make the difference between 'fine' and 'catastrophic'?

Kind Regards, John

Youd be surprised how much damage occurs to a 2.5mm² T&E running at 40A.

 

[JohnW2]

Youd be surprised how much damage occurs to a 2.5mm² T&E running at 40A.

I would be very surprised if a current of about 46A (**) flowing for 1 hour would do appreciably more damage to a cable with a tabulated 'CCC' of 27A as compared with one with a tabulated 'CCC' of 32A.

[ ** 46.4A being what a 32A MCB might allow to flow for 1 hour ]

Kind Regards, John

 

[mrbiscuit]

Taking a look at what you're discussing I think I see the problem, but as we've already ascertained here, I'm no electrician.
The wire from the CU to the current cooker connection is 6mm and on a 32A MCB.
This connection I now plan to swap to a double cooker connection.
The hob is rated max 20A
The cooker is 17.5A
So perhaps on a very busy day it could exceed the rated current for the 6mm cable, but the MCB would trip at the max rated current for 6mm cable that's trunked and chased into the wall.
...So I guess my question is, with the new setup, i.e using the double cooker connection and running a cable round to the oven via a double pole switch, is everything OK?

 

[EFLImpudence]

Youd be surprised how much damage occurs to a 2.5mm² T&E running at 40A.

Not to mention the much smaller - 1mm²? - conductors inside the oven.

The oven is 4.2kW @ 240V - 17.5A. It likely has two elements, so say 2.1kW, 8.75A each.

That means each has a resistance of 27.4Ω.

For the oven to draw 40A, one of the elements will have to decrease its resistance to 7.68Ω and draw 31.25A instead of its designed 8.75A - around 4 times as much.
How would a damaged element achieve a satisfactory connection to allow this to happen?

I know you have said you have come across such a situation but it seems almost impossible.

 

[SUNRAY]

Let's not get bogged down with arguements etc.

Simply put there is an existing 32A circuit, part of which needs to be extended. If it was any other power circuit (the royal) we would be up in arms at the suggestion of using an undersized cable... and quite rightly so.
I know how these things can change, we only haveto see how many questions there are on here about cooker circuits, and I've personally corrected several cooker cables, following EICR, in the last few years where just such a scenario exists and 'someone' has installed a full cooker on the existing 2.5mm²T&E.

For the sake of the minor additional cost of 4mm² (or even 6mm² to match the existing circuit) this silly point simply fades away into oblibion forever.

 

[SUNRAY]

Let's not get bogged down with arguements etc.

Simply put there is an existing 32A circuit, part of which needs to be extended. If it was any other power circuit (the royal) we would be up in arms at the suggestion of using an undersized cable... and quite rightly so.
I know how these things can change, we only haveto see how many questions there are on here about cooker circuits, and I've personally corrected several cooker cables, following EICR, in the last few years where just such a scenario exists and 'someone' has installed a full cooker on the existing 2.5mm²T&E.

For the sake of the minor additional cost of 4mm² (or even 6mm² to match the existing circuit) this silly point simply fades away into oblibion forever.

A problem I believe exists is the fashion for individual units for hob and oven seems to be moving to 900mm 'range style' cookers and the installer (probably kitchen fitter) sees the cooker outlet plate and in goes the new 6mm...

 

[mrbiscuit]

OK, so I get a roll of 6mm cable to run from thenew double cooker connection and round the kitchen.
Will that arrangement of extending the original circuit keep me in step with the local authority and safety regulation?

 

[JohnW2]

Simply put there is an existing 32A circuit, part of which needs to be extended. If it was any other power circuit (the royal) we would be up in arms at the suggestion of using an undersized cable... and quite rightly so.

I obviously don't know (but can guess!) what it takes to make you "be up in arms" but if it were part of "any other power circuit" which was providing a dedicated supply to a load considered "unlikley to result in an an overload", then many (maybE most) of (the royal) us would not be "up in arms".

Kind Regards, John

 

[JohnW2]

OK, so I get a roll of 6mm cable to run from thenew double cooker connection and round the kitchen. ... Will that arrangement of extending the original circuit keep me in step with the local authority and safety regulation?

It would - but, as has been said, the same would be true if it were 4mm or 2.5mm cable. Even Sunray presumably has to accept that such would be 'in step' (complaint) with the ('safety') regulations, even if he personally does not like those regulations.

Kind Regards, John

 

[mrbiscuit]

Simply put there is an existing 32A circuit, part of which needs to be extended. If it was any other power circuit (the royal) we would be up in arms at the suggestion of using an undersized cable... and quite rightly so.

To be fair, the only cable that will ever run near it's rated load is the existing 6mm trunked supply for the radial, that is protected by a 32A MCB. My proposed extension will only be able to pull a fraction of the rating for 2.5mm.
Am I missing something?

 

[JohnW2]

To be fair, the only cable that will ever run near it's rated load is the existing 6mm trunked supply for the radial, that is protected by a 32A MCB. My proposed extension will only be able to pull a fraction of the rating for 2.5mm. Am I missing something?

What you are 'missing' is that there is an (incredibly small) possibility that a fault in the oven could theoretically cause it to draw a current in excess of the theoretical current-carrying=capacity of 2.5mm² cable - but, as I've said, even the regulations do not require that possibility to be addressed if it is considered "unlikely" (and, as EFLI and myself have said, I would regard it as 'incredibly unlikely').

When people raise issues about the 'incredibly unlikley', I have to wonder whether they travel in cars,. cross roads, let alone (heaven forbid!) smoke, drink alcohol eat unhealthily or don't do as much exercise as they should

Kind Regards, John

 

[SUNRAY]

What you are 'missing' is that there is an (incredibly small) possibility that a fault in the oven could theoretically cause it to draw a current in excess of the theoretical current-carrying=capacity of 2.5mm² cable - but, as I've said, even the regulations do not require that possibility to be addressed if it is considered "unlikely" (and, as EFLI and myself have said, I would regard it as 'incredibly unlikely').

When people raise issues about the 'incredibly unlikley', I have to wonder whether they travel in cars,. cross roads, let alone (heaven forbid!) smoke, drink alcohol eat unhealthily or don't do as much exercise as they should

Kind Regards, John

I accept the regulation which has given me a decent enough income with the amount of repairs I've done, so yes my interpretation of "unlikely" is somewhat steered by those repairs/alterations.
The following piece of 1.5mm² flex was indeed installed according to that exact regulation. It was the original flex on a lab grade ~2.5KW device, 3 of them were running on a 30A fuse & 32A RCBO, all very correctly installed and very correctly passed previous inspections, that is just one of the ocassions where I've encountered such a failing and why I don't like having cable unprotected.

 

[JohnW2]

.... The following piece of 1.5mm² flex was indeed installed according to that exact regulation. It was the original flex on a lab grade ~2.5KW device ....

What was this 'device'? Only a relatively small proportion of loads (primarily those which consist only of resistive heating elements) qualify as "being unlikely to result in an overload current".

Kind Regards, John

 

[SUNRAY]

What was this 'device'? Only a relatively small proportion of loads (primarily those which consist only of resistive heating elements) qualify as "being unlikely to result in an overload current".

Kind Regards, John

The best way I can describe it is a miniature oven with internal dimensions of something like 8" cube. Very very simple just basic thermostat and heating element and quite old. There were 3 of them and for a while this had been the preferred unit as it heated quicker than the other 2. Without looking for notes I recall this was pulling ~40A instead of 10A it was absolutely the correct equipment to qualify for oversized OPC.

Everytime I have come across something like this the thing that goes through my mind is: "If only this circuit had been designed correctly, this damage wouln't have happened". Luckily the load of all 3 machines did trip the RCBO on overload but not before the cable had started melting.