Beast of a cooker

[brizospark]

OK I know this has already been covered but I have been asked to install a supply and isolation switch to a Rangemaster cooker which has a rating of 14kw.

This cooker has a couple of ovens and grills and induction hobs - it's a beast

My question is regarding the cable / isolation switch rating, I know there is some debate over the issue but I reckon being realistic there is no chance that the cooker will ever run at full capacity and that a 6mm cable covered by a 40a MCB (only 5m run from board no insulation under floor) and a 45a DB switch would suffice.

The other option would be a 63a MCB with 10mm cable and a 63a rotary isolator.

The house is occupied by a couple and soon to be baby.

What would you guys do in this situation?

 

[Rockingit]

Yep, as others have said, 6mm on a 32A will suffice, change it for a 40A if you want some double assurance.

I had this argument with AGA on behalf of a customer just before Christmas - they were buying a (rebadged Rangemaster) AGA range and the Aga installers were stipulating that it HAD to be installed on a 10mm cable and a 50A CPD. Story short, I battered the head of Aga Technical into submission down the phone until he'd done the maths himself....and the result, after using every individual item at the exact rating, was something like 25A (can't remember exactly). So AGA ended up looking like a right spanner, I saved the customer probably £500 for not having to install a new circuit and I got to charge for a couple of hours of my time, spent solely on the phone.

 

[Badged01]

To play the devils advocate, what was the rating of the cooker? Taking 6mm as good for 47A (11kW) and a 32A C/B good for 45A on slow overload, you're pushing the extreme of the cable and the end tolerance of the C/B?. Good design practise?

Changing for a 40A C/B could give you upto 60A on slow overload -- more than the cable can take?

 

[davesparks]

I know that diversity is applied for a cooker circuit, but what happens at Christmas for example, when she is running full tilt?

The same thing as will happen on any other day when it's on at full chat!
The current drawn will be that calculated according to diversity (apart from possible small/short term overloads)

 

[Badged01]

The same thing as will happen on any other day when it's on at full chat!
The current drawn will be that calculated according to diversity (apart from possible small/short term overloads)

Not to sure I agree there! Diversity takes into account the likelyhood you wont be at full power ...... if you are cooking Sunday lunch for the street you may well have everything on at 100%!

 

[davesparks]

Diversity takes in to account the operating characteristics of cookers.
Temperature is controlled by switching elements on and off via a thermostatic control of some type. This leads to only a certain quantity of the total power being connected at any one time.
Over many years the diversity calculations have been adjusted and improved to reflect this accurately.

 

[Badged01]

Totally agree and the manuf inst takes account for diversity and that is the loading characteristic it gives. Or have I been reading them wrong?.

 

[GLENNSPARK]

i dont know what it is with some in here...

but theres this continuing lack of understanding goin round about diversity....

as has been quite rightly said before its unlikely that all loads will be on at once...and even if a large portion of them were...once upto set/operating temprature the stats kick in...and just keep everything ticking over nicely.

now, taking into consideration that an MCB with characteristics to BSEN60898 will hold for upto 1.45 X its operating current for upto an hour before letting go...and the fact that the stats will be constantly making & breaking...then its unlikely that current draw will get anyware above 40A..is it

and in fact i dare say that a 32B would suffice...

there..i`v said it now...

 

[Badged01]

The key to that is whether the max demand given by the manuf is the max demand for each individual element added up or the maximum demand when everything is fully loaded and diversity is applied (i.e. the fact that the loads will be cycling on/off and not all on at once at full tilt!)

 

[davesparks]

I wonder if it's has its roots in the throwaway society? Apprentices don't get to do cooker repairs and so don't learn how a cooker works anymore?
It's just another magic box, just like the tellybox / microwave / fridge, I don't know the details but know the basic principal of their operation.

The same seems to be true with cars these days, so many people will happily drive them around but haven't got a clue about how they work.

 

[davesparks]

The ratings will normally state maximum connected load, ie. all the elements and any controls added together.

 

[Badged01]

The ratings will normally state maximum connected load, ie. all the elements and any controls added together.

I've seen both, where you can actually tot up the individual loads and come to the max load the manuf states, and others where the manuf states the max load includes diversity and it doesnt add up to the individual loads!

 

[jaypp]

Not quite.

14000/240 = 58.33

58.33 - 10 = 48.33

30% of 48.33 = 14.5 + 10 = 24.5A

+ 5A if socket.

should be 230 no?

 

[davesparks]

should be 230 no?

Only in the textbooks, in the real world we still have 240V.
And really it should be at the voltage stated on the ratings plate. These days you tend to find the rating at 240 and the rating at 230 on a lot of rating plates.

 

[jaypp]

Only in the textbooks, in the real world we still have 240V.
And really it should be at the voltage stated on the ratings plate. These days you tend to find the rating at 240 and the rating at 230 on a lot of rating plates.

of course I understand it's 240 in reality and why it's 230 to save manufacturers money, but I'm pretty sure calculations are made with nominal figure

 

[davesparks]

It's not 230 to save money! It's 230 because our European masters decided we should lower our voltage to harmonise with the rest of Europe (who ignored it, just like the colours change)

If an appliance is rated at 10KW at 240V you use 240 in your calculation, if it's rated at 230V then you use 230. You'll only introduce an error in to your calculation otherwise.