Ovens and hobs - why no tech specs provided?!

[DNS1]

My parents are having a new kitchen installed soon and had someone out today to check over the room before they make their final decisions.

My mum wants a double electric oven and induction hob, but the kitchen fitter has said that the cable installed (which is completely hidden and inaccessible!) isn't sufficient and that they'll be needing a new circuit run, which will require a new consumer unit as there is no space available in the existing one.

I'm very suspicious of this conclusion as the current (un-used) cooker circuit is in 6mm and protected by 40A MCB. Besides, this forum has taught me not to trust the electrical abilities of kitchen fitters.

Decided to do the calculations (diversity etc) for their new cooking set-up myself, but ran into a problem... none of the manufacturers seem to mention the maximum current draw of their appliances...

Anyone know why this is? What figures should I be using?

 

[Guitarist]

What do you mean by "completely hidden and inaccessible"? 6mm cable shouldn't really be protected by a 40amp MCB in the first place so I would upgrade this to 10mm if possible then any oven/hob combo would be covered.

 

[DNS1]

Not sure I follow... 6mm is good for 47A at ambient temperature.

 

[DNS1]

Anyone?! Please, I'm stuck!

 

[Amp David]

Do you have the make and model of the oven and hob?

 

[Silly Sausage]

....none of the manufacturers seem to mention the maximum current draw of their appliances...

I can't believe that any of these appliances haven't got a Power (Watts) rating on them somewhere...a label or instructions

 

[oldtimer]

Ok I will have a stab first a bit of history when electric cookers came out it was just that an electric cooker with hob and oven combined so 6 mm and a 30 amp fuse at the time was suffice ok.

Now years later they split the hob and the oven now you could connect both to the 6mm circuit and if you have read a previous post on the subject you will see the thoughts on connecting them and that fine So whats the problem now well now there are double ovens and induction hobs the double oven can now take the load of an old combined cooker hence why the guy who has surveyed it has calculated that the supply cable either has to be uprated or he has to run another 6mm circuit and have the oven on one and the hob on the other.

One thing to be aware of Christmas day ?????? you say well some houses have a 60 amp main fuse but people want bigger and better so double ovens 10Kw showers , immersion and daughter blowdrying her hair and bingo you could blow the main fuse when all 3 are going at the same time plus this does not include any other equipment that is on at the same time.

 

[DNS1]

Thanks for the help.

My parents haven't picked an oven or hob yet, I'm just practicing a few calcs and checking the kitchen fitters work!

Just curious about the diversity calcs now. According to 17th I can say that the circuit will be rated at 10A + 30% of the remainder (+5A for a socket, but this doesn't matter in my case). So a 6mm T+E would be compliant in most cases, but just bad practice? Am I getting the right idea?

 

[Engineer54]

If a 6mm cable complies/is compliant with your calculated loading, ...How can you then call it bad practice?? ...So NO your not getting the ''right'' idea!!

Please look at the numerous other threads on this subject at the bottom of this page....

 

[DNS1]

Just wondered if it's "one of those things"...

Was having the discussion the other week about crimping cables. Perfectly ok with the regs, but many consider it bad practice.

 

[Guitarist]

Not sure I follow... 6mm is good for 47A at ambient temperature.

Ok, so 47 amps at ambient temperature clipped direct. No problem there. Now look at method 100, above a plasterboard ceiling covered by insulation not exceeding 100mm... 34 amps. Then add some voltdrop for good measure and any other factors and you can see why 6mm is normally protected by a 30/32 amp device.

If you are telling me that this cable is within a few feet of the CU, does not pass through any insulation at all and has no other derating factors, then I stand corrected. :smile5:

 

[DNS1]

I wondered if that was what you were getting at, fair point.

Although in this case, you stand corrected I'm afraid! :wink5:

Consumer unit is in the kitchen and the run to the cooker will be about 10 feet, embedded in or clipped to plaster.

Another quick question about the diversity calculations.

Will the total load be:

10A + 30% of remainder (hob + oven)

or

10A + remainder of hob + 10A + remainder of oven

Cheers!

 

[Guitarist]

10A + 30% fl of each appliance plus 5A for the outlet.

I do indeed stand corrected. Personally I always go 10mm on 40A breakers :wink5:

 

[DNS1]

Thanks for the advice. Having looked at the consumer unit myself, I've found an unused way (used to be for bell transformer) so if I can get the a new MCB then that will be used for one of the appliances.

Done the calculations anyway for practice. Does someone mind checking my maths? pleeeease!

Induction hob: 7.2kW = 31A

Oven: 4.5kW = 19.5A

Total without diversity = 50.5A


Diversity

Hob: 10A + 21 x 30% = 16.3A

Oven: 10A + 9.5 x 30% = 12.85A

Total = 29.15A


Cable installed: 6mm, 8m run, ref method C, ambient temperature

Voltage drop = 1.8v

Protective device - 40A Type 2 (max Zs for 5 sec disconnect = 0.66 ohm)

(R1+R2) per metre = 10.49mohms

Zs = 0.01049 x 8 + 0.35 = 0.43 ohms - compliant


Hope this makes sense, just basically typed up my handwritten notes from earlier.

Does it all look ok?

 

[plugsandsparks]

The only thing i would add is diversity is not an exact science and the use of the appliance can still be a major factor. Sure when ervything is up to temp and the heaters are coming in and out under thermostat control you can see diversity working for you. BUT if the user fires up all rings and oven elements from cold (unlikely i know) but quite possible then you are looking at full load against the rating plates.
No biggy but worth keeping in mind