Electric cooker installation

[kissinger]

Hello. I'm looking into how to install my new electric cooker, but I'm a novice so could really use some help. I'm hoping that it's just a matter of wiring the cooker to the terminal outlet already installed in my kitchen (with all of the electrics turned off at the mains, of course), but I know it's not always as simple as that because there's the consumer unit, wire thickness, etc to think about as well.

So I was hoping if I post up all the details I've got so that I could get some advice on how to proceed. The cooker has a total load rating of 12.6kW (although I suppose this is only if every hob and both ovens are on at max power at the same time). It's this cooker:

http://www.smeguk.com/product/suk62cmx8/

I will have to buy the cooker wire to connect it to the terminal outlet, but the wire leading to the consumer unit is 6mm. Here's some pictures of the CU for the other necessary details:

So am I good to go, or am I looking at a major PITA installation involving ripping cable out my walls

 

[ban-all-sheds]

You're good to go with no wall-bashing or cable ripping required.

The cooker has a total load rating of 12.6kW (although I suppose this is only if every hob and both ovens are on at max power at the same time)

You suppose right, but even when (if) they are, once they are up to temperature they all start cycling on and off, so your 12.6kW cooker is actually regarded as a 5.2kW one:

http://www.tlc-direct.co.uk/Book/6.2.1.htm

 

[StudWall]

the best thing to do is to fit 6mm2 cable from the outlet to your cooker, that way the cable is protected correctly by the MCB, you could use 4mm2 on a 32a but for the couple of quid difference you'll be better off with 6mm.

worst case scenario there is you turn everything on at once and it trips, you'll soon learn not to do that

 

[skenk]

Some cooker instructions specify heat resistant flex which is better than t&e in any case imo http://www.tlc-direct.co.uk/Products/CA4TQ3slash50.html

 

[JohnW2]

the best thing to do is to fit 6mm2 cable from the outlet to your cooker, that way the cable is protected correctly by the MCB, you could use 4mm2 on a 32a but for the couple of quid difference you'll be better off with 6mm.

Yes, but the OP's cooker circuit is protected by a B32 MCB - so 4mm² would (unless de-rated by installation method) be 'protected correctly by the MCB'. In practice, many would probably sue 2.5mm². As has been said, heat-resistant flex is probably better than T+E.

Kind Regards, John

 

[skenk]

I would give a 12kw cooker connected via 2.5mm cable and protected by 32A breaker a code 2. Not suitable imo.

 

[JohnW2]

I would give a 12kw cooker connected via 2.5mm cable and protected by 32A breaker a code 2. Not suitable imo.

Interesting. Is it the application of diversity, in general, which worries you, or specifically the protection of 2.5mm² cable with a 32A Breaker?

Another way of asking that question would be to ask how you would feel about (and code!) a 12kW cooker connected via 2.5mm² cable (Method C) and protected by a 25A MCB?

Kind Regards, John

 

[ban-all-sheds]

I would say that a 12kw cooker connected via 2.5mm cable and protected by 32A breaker was potentially dangerous and that remedial action was urgently required.

Why?

 

[skenk]

John - It'd be a code 3 because even after diversity 25A is too low for a 12kw (52.17A) cooker (if only just). Also there's a good chance it would trip when the cooker was in heavy use. No problem with a 25A breaker protecting 2.5mm flex (method C..) in principle though.

BAS - a possible max load of over 50A for 17 minutes is too much for 2.5mm and potentially dangerous imo.

 

[JohnW2]

John - It'd be a code 3 because even after diversity 25A is too low for a 12kw (52.17A) cooker (if only just).

I'm not sure that I follow your maths. Even if, as you have done, one assumes that the 12kW relates to 230V (it is almost certainly quoted at 240V, which would equate to about 47.9A at 230V), giving you that 52.17A figure, I then make the after-diversity current about 22.65A (i.e. less than 25A) - so why Code 3 if the breaker is 25A (as you've agreed, a 25A OPD is not, in itself, a problem with Method C 2.5mm²)?

Kind Regards, John

 

[ban-all-sheds]

a possible max load of over 50A for 17 minutes is too much for 2.5mm and potentially dangerous imo.

Where did 17 minutes come from?

 

[JohnW2]

a possible max load of over 50A for 17 minutes is too much for 2.5mm and potentially dangerous imo.

Where did 17 minutes come from?

I left you to ask that, and just addressed my half of skenk's post

I fear that there are going to be some intangibles here. We know that a Method C 2.5mm² cable would be deemed to be adequately protected by a hypothetical B27 MCB, regardless of what current the load attempted to draw - implying that, for example, the cable could safely tolerate about 39.15A for an hour. However, a B32 could obviously allow a higher current than that to flow for an hour, which could fuel skenk's concerns. The question to which we're probably not going to find an answer is whether a 12kW cooker could conceivably draw more than 39.15A for in excess of an hour (particularly given the the absolute max instantaneous current would be only about 47.9A at 230V), or 'equivalent' even higher currents for shorter periods of significant length. I certainly suspect not, but I'm not sure that we could find any chapter and verse to quote.

Pragmatically, I am personally very comfortable about the (hypothetical) situation being discussed.

Kind Regards, John

 

[skenk]

John - yeah sorry i made a mistake with the cooker diversity maths, so it would be under 25A, but still risking tripping when it was in use.

BAS - 17 minutes is the approx time it would take a 32A MCB to trip at 52A, according to the red book anyway.

It seems the instructions specify 6mm heat resistant cable for single phase installation in any case.

There's no way 2.5mm can be used on the 32A breaker and compliance with the regs achieved (even after the amendment comes in which says MI's are only a guide if indeed that happens).

You certainly don't design or install to take advantage of 'overheads' built into cable CCC's

32A MCB > 2.5mm cable > 12kw load is rough as f*** whatever way you look at it.

 

[deadshort]

I was giving up hope Nice to meet someone from the real world !


Kind regards,

DS

 

[JohnW2]

John - yeah sorry i made a mistake with the cooker diversity maths, so it would be under 25A, but still risking tripping when it was in use.

I would think that risk is extremely small. As you go on to say ...

BAS - 17 minutes is the approx time it would take a 32A MCB to trip at 52A, according to the red book anyway.

I would have thought that the probability of every element drawing power continuoulsy for 17 minutes would be incredibly small (even if absolutely everything was switched on simultaneously from cold) - and, in any event, as I said, the peak current is probably a bit lower than you are using for calculations (perhaps nearer 47.9A).

There's no way 2.5mm can be used on the 32A breaker and compliance with the regs achieved (even after the amendment comes in which says MI's are only a guide if indeed that happens). ... You certainly don't design or install to take advantage of 'overheads' built into cable CCC's

I certainly agree with that final statement. As you will be aware, the regs allow complete omission of overload protection (let alone 'merely' protecting a cable with a CCC of 27A with a 32A breaker) if it is considered that the cable is "not likely to carry overload current". The question is therefore whether that is talking about true instantaneous currents (which may, for at least brief periods, exceed the cable's CCC) or whether they are thinking of the current averages over a reasonable period of time (i.e.'after diversity') currents. I can'tread their minds.

32A MCB > 2.5mm cable > 12kw load is rough as f*** whatever way you look at it.

As you have acknowledged, provided one accepts the concept of diversity, there's nothing wrong with 2.5mm² cable feeding a 12kW cooking appliance load. The only issue is the 32A MCB protecting the 2.5mm² cable - and, as I said, that issue could (assuming Method C installation) be overcome by using a 25A MCB (if one were available for the CU). However, I never actually advocated 2.5mm² cable. I said that 4mm² would be OK, but added that "many people would probably use 2.5mm² (which I still suspect is true) ...

Yes, but the OP's cooker circuit is protected by a B32 MCB - so 4mm² would (unless de-rated by installation method) be 'protected correctly by the MCB'. In practice, many would probably use 2.5mm².

I would certainly personally say that, whatever the MIs might say, 6mm² is unnecessary on a 32A MCB.

Kind Regards, John