Electric Hob

[dizz]

If what you are saying is right, doesn't that mean the flex of all kettles, toasters etc. plugged into a 32A ring must also be in 4mm, because in fault conditions the current will be way over the CCC of the flex?

No, because there's a fuse in the plug which protects the flex.

Oh my god what a freak I am - forgot about those little fuses!

BUT is there any merit in any of the other discussions regarding this because the sticker on the HOB says 2.5mm cable, as per manual and the manual for the oven says "A cable of the correct size rating must be used. Cable type H05VV-F, 3 x 1.5mm".

Certainly looking at the Oven, the appliance connection would not take the thickness of 3 cores of 2.5mm let alone 4mm - so that's a no-no. I would have to spur off to a 13A FCU. But the question remains regarding the 2.5mm onm a 32A - here is the techinical info.

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[EFLImpudence]

"When an oven shares the same circuit as a separate hob they should be regarded as one cooking appliance just as though it were an orthodox stand-alone cooker. Neither are capable of being overloaded, being as they are, fixed loads. The main consideration here is for short-circuit protection, usually due to the failure of just one of the heating elements. So in that regard, the 1.5mm flex to the oven will be adequately protected by a standard 30/32 Amp MCB.

Anyone unable to understand this principle, should take a look at the internal wiring of an ordinary cooker. It will (typically) have 1/1.5mm HR wiring to its individual elements - oven, grill and four hotplates. It has no need for 6mm internal wiring to match the supply cable and MCB.

Fixed load cookers have always been a special case in the Regulations. With six elements all likely to have an individual short circuit fault at some time, they are notoriously difficult to 'protect'. Added to this are such smaller items as fan motors and lamps which have no internal fuse protection. This is why the main consideration has to be short circuit protection - the load current will look after itself.


Lucia."

That is true.

However, there are several other regulations (433.3) with which you must comply if you wish, for some strange reason, to omit overload protection for the cable and these are not practicably achievable in a domestic situation.

In any case what would be the point?



Someone else used to sign their posts 'Lucia'.

 

[ban-all-sheds]

But the question remains regarding the 2.5mm onm a 32A - here is the techinical info.

No - that question has been answered - use 4mm², I'm sure it will fit.

Post a photo of the hob connections - it could well be that even if it won't fit directly you could use 4mm² by crimping ring terminals to it.

 

[EFLImpudence]

The instructions are clearly wrong in stating a minimum fuse of 32A.
Clearly, anything higher would never be installed or do you think it would be alright connected directly to the incoming cable and relying on the suppliers 60, 80 or 100A fuse.

Applying diversity to the hob, 16A would suffice.
So, obviously 20 or 25 would also be satisfactory depending on the cable.

 

[dizz]

The instructions are clearly wrong in stating a minimum fuse of 32A.
Clearly, anything higher would never be installed or do you think it would be alright connected directly to the incoming cable and relying on the suppliers 60, 80 or 100A fuse.

Applying diversity to the hob, 16A would suffice.
So, obviously 20 or 25 would also be satisfactory depending on the cable.

Yes got that, they don't mean minimum, agreed.

1. Does the maximum length referred to in the installation booklet of 2m of the 1.5mm / 2.5mm cable have any beraing on the ability of the cable to take the overcurrent.

I understand all the things you say - I just don't like going against the manufacturer instructions (" ... that's cos you tried to force 4mm cable into a 1.5mm ...."), but the very reason for my post was because of this dilemma.

 

[EFLImpudence]

Yes got that, they don't mean minimum, agreed.

So, are you treating everything else as gospel?
Perhaps they have wrongly spelt 6500.

1. Does the maximum length referred to in the installation booklet of 2m of the 1.5mm / 2.5mm cable have any beraing on the ability of the cable to take the overcurrent.

No (not unless it is literally miles long).

I understand all the things you say - I just don't like going against the manufacturer instructions

That's correct.
One of the regulations states we shall not.

However, misprints and mistakes happen.
You should, perhaps, contact the manufacturer to clarify the matter.

(" ... that's cos you tried to force 4mm cable into a 1.5mm ....")

What do you mean?
4mm² is not that big.

It won't have a 1.5mm² hole for you to fit the wire.
It will likely have a screw fitting with a metal clamp.

but the very reason for my post was because of this dilemma.

Your first question was because you quite correctly thought the 2.5 too small.


I refer the honourable poster to the reply given earlier:

4mm flex it is then.

 

[dizz]

I understand that 4mm can carry 36A and hence the reason why this was quoted as an appropriate cable size for the MCB, but the maximum current for the 6.5kW hob is 28A without diversity.

I can see for other circuits that carry various loads (ring) the cable may need to carry those loads for extended periods up to the MCB rating and hence why it would be important to match the CCC of the cable to the MCB rating.

I am trying to see why the manufacturer and some other forum posts suggest 2.5mm is OK on a 32A MCB for a hob? Is it because the maximum load is already pre-determined and fixed at 28A, as we cannot load the cooker circuit with other appliances? So the conditions would be :

1. 0A up to 28A max, or :
2. L/E Fault

When we talk about a L/E fault, those currents are very high - at 0.2 ohms we're talking about 1kA - At these high fault currents the duration is very short before the breaker kicks in. Agreed the CCC of a 2.5mm cable is only 28A, but the CCC of a 4mm cable is only 36A also - so both cables have to carry 1kA for a short period of time under fault conditions. Doesn't the matching of the cable to the MCB become rather redundant when we are talking a binary scenario for a hob - that being:

1. The hob is either carrying the current of the appliance or less, or :
2. It's carrying a L/E fault?

 

[EFLImpudence]

As I said in reply to your other quote regarding the fact that the oven cannot overload the cable - That is true - but...

... one of the conditions for the omission of overload protection is that it is not where there is a fire risk (433.3).
If you do not consider an oven and hob in a kitchen to be a fire risk then that is up to you.
Another is that the requirement for protection against fault current in the smaller cable shall be met. Is it?

Will you do any testing?

In any case I will not advise the omission of overload protection remotely on a forum especially when it is completely unnecessary, pointless and required purely for pedantic reasons.

 

[dizz]

... one of the conditions for the omission of overload protection is that it is not where there is a fire risk (433.3).

You refer to 433.3 and it seems obvious that a kitchen should present a fire risk, so I have to agree with everything you say, as you have many times more understanding than me, so it got me reading the regs a bit more ...

I have also looked at Regulation 434, Protection against Fault Current - I have read 434.2 & 434.2.1 over and over.

434.2 says that where a cross-sectional area change takes place (causing a reduction in the CCC), a device providing protecting against fault current shall be installed at the point where the reduction takes place.

434.2.1 says that 434.2 shall not be applied to installations situated in locations presenting a fire risk ....

So if a kitchen is a fire risk, 434.2 should not be applied - and we know that many protective devices are located in kitchens where the csa is reduced (in fact we discussed installing a 13A FCU as an option).

434.2.1 says that a device for protection against fault current may be installed other than that specified in 434.2 provided that the point of reduction in cross-sectional area change and the protective device shall not exceed 3m.

Eureka - So the maufacturer is claiming that a kitchen does not represent a fire risk, this enables them to :

1. quote cable size appropriate to the power rating of the appliance with no regard for the breaker size.

2. tell you to use a cable of MAX 2m length obviating the need to put in ANY protective device between the reduction in csa and the appliance, since you have up to 3m before you need to install a protective device.

3. quote a minimum rating of 32A, because it is allowed to be higher (after all they are claiming overload isn't a consideration, it's only fault current that is a consideration).

I don't think any of their literature is wrong now - I think it's the stance they are taking regarding the appliance, and I presume after taking advice. I may not agree with it ...

PS - EFLI - Thanks for your patience, your thoughts on these obseravtions much appreciated.

 

[dizz]

Another is that the requirement for protection against fault current in the smaller cable shall be met. Is it?

Do you mean overload current? 433.1.1 (ii) - If so, their stance regarding location not representing a fire risk dispenses with this requirement under 433.3

 

[ban-all-sheds]

So, are you treating everything else as gospel?
Perhaps they have wrongly spelt 6500.

Well they've got the cable spec wrong - they've written H0RR-F5, when they should have said H05-RR-F. ¹

Which raises another point....

Agreed the CCC of a 2.5mm cable is only 28A

The CCC of 2.5mm² H05-RR-F is 20A.

¹ Or should they? Maybe the 5 is not misplaced, maybe the 5 in 05 is simply missing. Could they have meant to write H05-RR-F5G2.5?

A photo of the terminals would be even more interesting now.

 

[dizz]

Well they've got the cable spec wrong - they've written H0RR-F5, when they should have said H05-RR-F.

Yes - they must mean H05RR-F because I have taken a look at an older instruction manual, and it reads H05RR-F with 2.5mm 3 Core. And I can see that, as you state, the CCC is 20A.

A photo of the terminals would be even more interesting now.

I'll see if I can take a photo later when I go back there, but there is a silver printed label on the underside that distinctly shows how to wire up the cable to L/N/E, etc, with a diagram showing "2.5mm" cable

I am awaiting response from supplier on this, and after agreeing with EFLI regarding the 4mm, which seems the right way to go, I tried to check around for heat resistant 4mm, but no-one has it - I know it's available, but not locally - which would suggest to me that these units are being wired up in either 2.5mm as the techincal manual suggests or 4mm standard cable.

Current plan is to wait for feedback regarding the hob, but stay with the 1.5mm on the oven by connecting to a 13 FCU at the CCU (A 4mm cable will definitely not fit - it's not the cores themselves, it's that the outer cable is too big for the plastic tie to prevent pulling), although according to the regs, if the kitchen is a potential fire hazard, I shouldn't put a FCU there when downrating the cable ...

 

[ban-all-sheds]

Yes - they must mean H05RR-F because I have taken a look at an older instruction manual, and it reads H05RR-F with 2.5mm 3 Core. ... there is a silver printed label on the underside that distinctly shows how to wire up the cable to L/N/E, etc, with a diagram showing "2.5mm" cable

OK - fair enough - I just wondered at the time if they were speccing 2.5mm² 5-core for a 3-phase supply.

although according to the regs, if the kitchen is a potential fire hazard, I shouldn't put a FCU there when downrating the cable ...

 

[dizz]

Yes - they must mean H05RR-F because I have taken a look at an older instruction manual, and it reads H05RR-F with 2.5mm 3 Core. ... there is a silver printed label on the underside that distinctly shows how to wire up the cable to L/N/E, etc, with a diagram showing "2.5mm" cable

OK - fair enough - I just wondered at the time if they were speccing 2.5mm² 5-core for a 3-phase supply...

Yes - the diagram on the back show 3-phase 5-core as well.

Feedback from supplier, who spoke to head office, who spoke to someone technical.

"2.5mm is adaquate, but they have had a lot of enquires reagrding this, and in the new set of manuals to be printed they will be stating 4mm."

When asked why adaquate now, they said "unlikely to use all hobs at same time for extended periods" - hmm - so they are using diversity to evaluate the choice of cable, but not backing that up with an (e.g.) 20A MCB?

OK, so while they say 2.5mm is perfectly fine, I need to put in 4mm, so the question now remains what 4mm should I put in?

HO5RR-F -. Max size is 2.5mm as far as I can ascertain

 

[dizz]

Any thoughts?

http://www.ebay.co.uk/itm/4mm-Heat-Resistant-3-Core-Cable-Butyl-Flex-/130711868249