5 core induction hob cable (1.5mm?) to 10mm supplied cooker outlet

[AndyPRK]

And where do we stand on the fact that the 1.5mm flex is only 1 meter long, in free air. Does that still mean it needs a 16A MCB John?

Also consider that the conductors are doubled up in this case. (only at one end though). 1 driving 2 'hot plates' and 1 driving the other pair.
Yes, if you get a short you need protection, but the benefit of having pairs of live conductors is lost if you stick ONE 16A MCB for both live conductors. (2 16A MCB's would be fine - Then it would be like europe)

In this rare instance the manufactures are happy with it, and we are not, they must have satisfied themselves its ok!
(unlike fan manufactures requesting a 3A fuse etc.)

Also note, these electronic induction hobs, automatically limit their current to 32A, (in fact in the 'settings menu', you can probably limit them to 16A and 10A - NEFF) so it's only under failure conditions.
e.g a short on one cable, or one of 4 triacs? shorted, turning on one 'plate'

 

[BobT007]

OP here, lots of interesting stuff, some a bit over my head! but couple of things to chip in...

So, is the general consensus that the 32A MCB is the most appropriate here, rather than dropping to a 20A or something?

And, what significance may the length of the 5G 1.5mm2 flex be? Would being 2.15m long, rather than 1.1m long make any difference at all to anything?
(Standard supplied length is 1.1m, but if longer required [which I might need] Bosch have a 2.15m cable available [at £31 plus postage!])

 

[EFLImpudence]

So, is the general consensus that the 32A MCB is the most appropriate here, rather than dropping to a 20A or something?

Yes. No need to alter it.

And, what significance may the length of the 5G 1.5mm2 flex be? Would being 2.15m long, rather than 1.1m long make any difference at all to anything?

Not electrically.

You might not be able to connect a built in oven if the cable is too short.

(Standard supplied length is 1.1m, but if longer required [which I might need] Bosch have a 2.15m cable available [at £31 plus postage!])

 

[AndyPRK]

Just goto B&Q to get your heat resistant flex (HO5BB-F or HO3V2V2-F)
must be cheaper! £19 for loads (not for collection though)

https://www.diy.com/departments/time-white-heat-resistant-3-core-flexible-cable-2-5mm-x-5m/188974_BQ.prd

Hob Wiring

My induction hob glass cracked so I bought a replacement, same manufacturer almost identical but not exact same model. The hob is on its own circuit, 6mm T&E RCD protected, 32A MCB. Cooker isolation switch is in cupboard adjacent. Anyway I wired it in at it was wired before - 6mm T&E into the...

www.diynot.com

 

[JohnW2]

And where do we stand on the fact that the 1.5mm flex is only 1 meter long, in free air. Does that still mean it needs a 16A MCB John?

"We" (and BS7671) do not consider the length of a cable. If so much as one inch of a cable run is 1.5mm² (the rest being bigger) we seemingly feel that the OPD should be appropriate for 1.5mm². I suppose that makes some sense, given that many fuses afre only about an inch long, yet reliably 'melt' at the intended current.

As for "in free air", it certainly does not make much sense to me that BS7671 says that 1.5mm² flex (almost inevitably used 'in free air') has a CCC of only 16A, whereas 1.5mm² T+E has a CCC of 22A in free air or 20A when 'clipped direct'. I therefore take that with a bit of salt, and personally would be very comfortable with 1.5mm² flex protected by a 20A OPD.

Also consider that the conductors are doubled up in this case. (only at one end though). 1 driving 2 'hot plates' and 1 driving the other pair.
Yes, if you get a short you need protection ...

It's that possible short circuit that we need to protect the cable against.

In this rare instance the manufactures are happy with it, and we are not, they must have satisfied themselves its ok!

Count me out of that "we". I am personally perfectly 'happy with it'. Maybe you were thinking of Sunray?!

Also note, these electronic induction hobs, automatically limit their current to 32A, (in fact in the 'settings menu', you can probably limit them to 16A and 10A - NEFF) so it's only under failure conditions.

Sure (and I think that some induction hobs can be set to li it current to 13A, so that they can be 'plugged in') - but it's those (rare) 'failure conditions' that we have to protect against.

 

[ebee]

So,
if both ends of the flex (parallel lengths of conductors) were connected together rather than just at the cooker outlet plate but also links in the cooker terminals end making it , effectively 2 x 3.0mm csa on the Lives with a 1.5 cpc would anybody be more concerned or less concerned with a 32A MCB for any fault to be protected against ?

 

[AndyPRK]

That would be an improvement and ok

 

[JohnW2]

So,
if both ends of the flex (parallel lengths of conductors) were connected together rather than just at the cooker outlet plate but also links in the cooker terminals end making it , effectively 2 x 3.0mm csa on the Lives with a 1.5 cpc would anybody be more concerned or less concerned with a 32A MCB for any fault to be protected against ?

'Not concerned at all', one would imagine, since, even using BS7671 CCCs for flex, 2 x 1.5mm in cable would have a CCC of 32A.

There is, I suppose a theoretical risk that a 'short' close to the upstream end of the flex (incredibly unlikely, I would think, might result in a small bit of that cable being 'overloaded' - but that feels like barrel-scraping' to me!

As I've said and implied,I personally would not think, let alone worry, about ti issue and would simply use whatever the manufacturer had advised!

 

[plugwash]

It's that possible short circuit that we need to protect the cable against.

We hapilly protect 4mm² T&E which has a 1.5mm² CPC with 32A breakers. So I'd be rather surprised if a 32A breaker did not provide adequate short-circuit protection for 1.5mm² flex (to be 100% sure you would need "let through" data for the breakers which seems surprisingly difficult to find)

Assuming the manufacturer has equally split the 7.4KW of load across the two line/neutral pairts, that leaves the risk of some fault in the appliance that causes it to draw more current than rated. Presumably the manufacturer considers that unlikely.

 

[JohnW2]

We hapilly protect 4mm² T&E which has a 1.5mm² CPC with 32A breakers. So I'd be rather surprised if a 32A breaker did not provide adequate short-circuit protection for 1.5mm² flex (to be 100% sure you would need "let through" data for the breakers which seems surprisingly difficult to find)

True. I confused myself into talking about fault protection ('short circuits' of negligible/zero impedance) when I shouldn't - since in virtually all cases the sizes of CPCs we have are more than adiabatically adequate for providing such protection (which will usually be cleared 'in milliseconds', anyway). Apologies for that!

What I should have been talking about a fault of non-negligible impedance (i.e. 'overload') that was close to the upstream ends of one of two parallel 1.5mm² conductors - but that would be so incredibly improbable that it's probably not worth even considering it!

Assuming the manufacturer has equally split the 7.4KW of load across the two line/neutral pairts, that leaves the risk of some fault in the appliance that causes it to draw more current than rated. Presumably the manufacturer considers that unlikely.

Quite so - and I would be inclined to agree with them!

 

[ebee]

So does anybody think that a link together at the cooker end might be beneficial therefore worth a consideration or not at all or alternatively not enough to be worthy of a consideration anyway?

I am trying to see to what extent a 1.5 HR flex is considered as OK or close to the call on for a 32A MCB, in other words Are we worried ? and by How much if we are?

A bit in line with John's observation of a completely understandable yet a bit irrational fear of flying even though he knows full well that his other risks deserves more consideration (and YES I do such things meeself too, I think we probably all do).
The obvious, to me, thing about that flight is the actual flight itself (including take off and landing whisch is proably the more risky part of that flight) is probably the safest part of the whole journey yet getting from home to airport and from airport to destination is far more times risky yet it seems to bother us a lot less.
Supposedly somebody once worked out that if you lived to a ripe old age and flew everyday of your life you could expect to crash once and you would probably survive it. OK it is probaby one of those universal myths but could we ever view a car jouney as being anywhere near as safe?

 

[robinbanks]

what would be the appropriate thing to do?

You change the breaker so it protects the current carrying capacity of the smallest cable in the run

Making extremes an example, you can put a 6a breaker on 10mm cable and run a light no problem - that cable is never going to set on fire before the breaker goes. You definitely don't put a 40A breaker on 1mm cable and try to run an oven, even if you live next door to a fire station

 

[AndyPRK]

ebee, I think the hobs already come with the copper links fitted anyway in this case. And you remove them when 'on the continent'. Well some do.
I best look at the instructions at the start of this thread to check!

EDIT: The neutrals have a link. Lives don't

 

[SUNRAY]

ebee, I think the hobs already come with the copper links fitted anyway in this case. And you remove them when 'on the continent'. Well some do.
I best look at the instructions at the start of this thread to check!

EDIT: The neutrals have a link. Lives don't

Oh...
That changes things, I thought I'd seen them linked in the initial image.

On that basis the answer from me is no, not unless the 2 'L' terminals are linked to make it parallel fed.




However many time some have mentioned the regs 'apparently' allow it for applances 'unlikely to cause an overload'

 

[JohnW2]

So does anybody think that a link together at the cooker end might be beneficial therefore worth a consideration or not at all or alternatively not enough to be worthy of a consideration anyway?

Since that would result in the 1.5mm² conductors being in parallel, hence effectively 3mm², I presume that it would be adequately protected by a 32A OPD, hence seemingly 'beneficial' - particularly since I can't think l of any reason for not doing it.

I am trying to see to what extent a 1.5 HR flex is considered as OK or close to the call on for a 32A MCB, in other words Are we worried ? and by How much if we are?

Regs-wise it clearly is not 'OK', but I doubt that many of us would be particularly worried, not the least because we probably think it extremely unlikley that (in the situation we are discussing) the cable would be subjected to a dangerous overload.

Having said that, as I've said, there's really no particularly good reason why one should allay any residual 'worries' by invoking diversity and reducing the OPD to one which was appropriate for the after-diversity current AND adequate to give 'no worries' protection to the 1.5mm² cable. As I wrote, a 20A (or even 16A) OPD ought to be enough for a 7.4 kW domestic cooking appliance, and that would 'fully protect' a 1.5mm² cable. However, I imagine that almost no-one would even think of doing that - which seems to suggest that they probably are not worried when it is a 32A OPD.

A bit in line with John's observation of a completely understandable yet a bit irrational fear of flying even though he knows full well that his other risks deserves more consideration (and YES I do such things meeself too, I think we probably all do).

As I probably mentioned, it's not totally irrational', particularly for younger people. Even though it is undoubtedly a much greater risk, I think that many/most people would ('understandably') be much less worried/frightened about "the risk of them dying in the next 20 (or 50, or whatever) years" as a result of a road accident or smoking than about "the risk of dying in the next 90 minutes" as a result of a plane crash.