Elec.. Guide.. Build.. Regs Table 4.1.2a - Is this a cooker mistake?

[Ian1981]

I'm glad that even seasoned professionals are struggling to find agreement here.

I accept the argument that for the purposes of future proofing a new cooker circuit you would wire it in 6mm.

Is the radial in 4mm at 32 Amp something to do with greater potential diversity. However if you were doing a new installation then you would make design decisions based on anticipated current demand?

Just my novice interpretations.

If you had a 15 kw cooker.
15000/230 = 65 amps
Diversity first 10 amps plus 30% reminder.
So that's 65-10 is 55
30% of 55 is 16.5
16.5 + first 10 amps = 26.5 plus 5 amp with socket if any =31.5 amps applying diversity.
So choose a cable that has the ccc of at least 31.5 amps.
Subject to reference method,volt drop and any other derating factors.
If 4mm is acceptable then it's acceptable.

 

[SparkyChick]

I get the calculation side of it, the maths is not the problem. I just have this issue in my head that I struggle to get over that someone could quite easily make the cooker pull it's full 65A on a cable we've rated for 31.5, so for the sake of argument lets say 4mm clipped direct (37A) and obviously the breaker would be rated 32A.

From the breaker perspective, using the trip time charts in the BYB, a 32A with 65A on it is going to trip somewhere in the region of 100-200 seconds.

In that time, what would 65A do to our 4mm cable? Would it be damaged? And from the breakers perspective, surely it's not going to do it any good if this is a regular occurrence?

I know this is unlikely be a regular occurence, but in the software world where I come from, you have to take note and deal with such things. I think that's why I struggle with diversity and wrapping my head around the fact that we knowingly install cables and MCBs that may not be able to cope under the full load of the devices connected to it. And well, I'm just concerned about how this might affect the installation and I'll be honest, that's down to a lack of experience of it.

 

[Wilko]

FWIW - I think the cable will be ok for a few minutes at 65A. I don't think this would happen in reality, but for the discussion ... At 65A the 4mm is dissipating less than 1W per metre and that doesn't seem enough heating effect to cook it in that time. Just my thought, but I avoid this worry by using 6mm .

 

[telectrix]

install a gas hob.problem sol ved.

 

[Paul228]

I'm with sparky chic on this. Take Christmas day at my mum's, both ovens on for six hours and all four hobs on for 3days cooking the sprouts.
Also it's unrealistic to assume the feed to an average kitchen would be clipped. They are nearly always burried in a wall.
For the cost and time, always go with 6mm.
Having said all this, my new twin fan oven came with a pre moulded 13A plug on it..

 

[Richard Burns]

I tend to feel that cooker circuits as they are recommended to be installed (6mm² on B32A) are one of the least safe circuits, because of the designed in overload, even if only for a short time.
However practical experience over many years shows that this is a generally a safe installation, terminal overheats aside.
This is because the operation of a cooker is such that it is practically very unlikely in domestic use to be able to cause any long term problem. I think HHD did describe s student let where the hob was used continuously for heating many pans of water for a bath as the immersion was not working, but this must be the exception.
I did some rather rough calculations on current draw in high use (14kW all switched on simultaneously) and found that the limits of the cable were exceed only for very short periods of time (5 mins) in the initial instance and that as time went on the cable was never overloaded and the circuit breaker was never closer than 15mins away from tripping.
The thermal cycling of the elements means that so long as the cooker is not being misused or the thermostats have not broken (which would lead to more rapid problems much faster than melting the cable) that the circuit will work without deterioration (even on Christmas day) since the current ratings are for 24 hour continuous loading 5 mins will reduce the life of the cable slightly over time but probably no more than a hot summer may do to the rest of the cables installed near the limits of their CCC.

 

[Midwest]

I've not really read any of the previous posts, my mind still in Rhodes. I've seen loads of cooker circuits in loads of houses, loads of electric ovens connected to said circuits, and loads of hobs connected to said circuits. All been on 32amp 6mm supplies, never seen any thermal damaged cables or mcb's overloaded. Only ever had to connect a aga monster, and it's connected to a 32amp supply. There that's technical analysis of the issue.

 

[Baker1988]

I personally use 6mm on 32A MCB but with people saying if the cooker and hobs are all turned on then this will damage the cable as it will draw too much current, remember that the hobs don't stay on full for the complete time you are useing it (well mine doesn't any way) it will go to the max heat so it's bright red but then the element turns its self off for a bit then when the heat drops it turns its self back on all so it will not be constantly pulling the full load it will only pull it for a few seconds then one or two will turn off and by the time that one turns back on another one has turned off so it is constantly fluctuate in power it's pulling.

This is just my opinion and I could be totally wrong but it just won't be pulling the full load constantly even if every thing is turned on at once.

 

[Paul228]

Another hull sparky..welcome mate

 

[Baker1988]

Another hull sparky..welcome mate

Thank you. It's a pleasure to be here.