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

[SUNRAY]

Yes, I don't know where I got 17A from, either for each lead or a cockup on the arithmetic front.

I said it, thats what it works out to with diversity

I haven't done any actual calculations, my 'in head' guesstimate is based on 4½A/KW or about 32A total or 16A each (assuming the 2 terminals carry equal loads) , then diversity which I guestimated as under 20A would be 10 + 22/2 or 17.33 so I was a tad high there.

However the vast majority of my work has been commercial where for a start diversity is not normally applied, a 7.4KW load would automatically be 6mm² minimum (in fact 4mm² would only be considered for a circuit near the top end of 2.5mm² CCC) - calculated as 7400/230=32.17A (or bothering to calculate for the stated power at 240V would be 29.55A) for a cooker that would be 32A OCPD, if there was a 3KW oven to go with it that would most likely be an additional 16A/2.5mm² circuit. I'm not referring to a commercial kitchen here, for example a caretakers appartment in an office block or the office tea room.

 

[pete01]

I haven't done any actual calculations, my 'in head' guesstimate is based on 4½A/KW or about 32A total or 16A each (assuming the 2 terminals carry equal loads) , then diversity which I guestimated as under 20A would be 10 + 22/2 or 17.33 so I was a tad high there.

However the vast majority of my work has been commercial where for a start diversity is not normally applied, a 7.4KW load would automatically be 6mm² minimum (in fact 4mm² would only be considered for a circuit near the top end of 2.5mm² CCC) - calculated as 7400/230=32.17A (or bothering to calculate for the stated power at 240V would be 29.55A) for a cooker that would be 32A OCPD, if there was a 3KW oven to go with it that would most likely be an additional 16A/2.5mm² circuit. I'm not referring to a commercial kitchen here, for example a caretakers appartment in an office block or the office tea room.

I always wondered how an oven knew it was going into a commercial kitchen?

 

[SUNRAY]

I always wondered how an oven knew it was going into a commercial kitchen?

It's on the delivery note.

 

[pete01]

It's on the delivery note.

 

[JohnW2]

However the vast majority of my work has been commercial where for a start diversity is not normally applied, a 7.4KW load would automatically be 6mm² minimum .....

Fair enough but, as I've just written, "Why?". If one uses 4mm² cable which is protected by a 32A OPD, what do you see as 'the problem'?

 

[SUNRAY]

That's not what we are talking about. We're talking about a situation in which the OPD is of an appropriate rating to protect the cable, but with the (after diversity) 'design current' (for both cable AND OPD) a lot lower than the theoretical 'maximum load' (absolutely everything 'on'simultaneously) that the appliance could represent.

For example, take a cooking appliance which represented a theoretical total load of 15 kW at 230V - just over 65A. Allowing for diversiity, that would be a current of about 26.6 A, hence fine with 4mm² cable or even (just) 2.5mm² Method C cable. However, the OPD would be a 32A one, hence not quite adequate to protect either 2.5mm² cable - so one would presumably use 4mm² cable (which would be adequately protected)

Probably none of those 'maybe's but, rather, the fact that you are talking about something different from the application of diversty. As above, there is nothing about the concept of diversity which allows (or makes it sensible for) an OPD to be used to 'protect' a cable whose CCC is less than the In of the OPD. As you say, the OPD is there primarily to protect the cable, and therefore must have an In appropriate for protecting the cable concerned.


Fair enough, but you could. Even if the 7.4 kW related to 230V (which t rarely would!) - that's only about 32A - which, after diversity, becomes about 16.6A - so fine with Method C 1.5mm² cable - and that cable,in turn,would be (just) adequately ;protected by a 20A OPD.

What was the CCC of the cable and what was the In of the OPD protecting it?

Since you say it was "according to regs", do I take it that the latter was no greater than the former? If that really was the case (and assuming no /'poor connections' etc.), yet the cable came to harm, you would appear to be questioning the correctness of the tabulated cable CCCs which we all use, all of the time. Are you sure that there were not any 'poor connections' in the equation?

This thread has meandered around a bit but it started but it started with a 32A MCB and what size cable, 1.5mm² was suggested as suitable and in other threads is frequently suggested as suitable for use on a 32 or even 45A MCB.

 

[SUNRAY]

Fair enough but, as I've just written, "Why?". If one uses 4mm² cable which is protected by a 32A OPD, what do you see as 'the problem'?

I don't see that as a problem, as long as derating factors don't kick in.

In a commercial environment it would mostly be 6mm² - IME. I don't know if that is a hangover from the days of 7/0.044" (when the CCCs were lower compared to metric cables) as seems to be the case in domestics.

 

[JohnW2]

This thread has meandered around a bit but it started but it started with a 32A MCB and what size cable, 1.5mm² was suggested as suitable ....

That would only be possible (per regs) with Method C 1.5mm² in the context of a BS7671 ring final. That is the only situation in which BS 7671 allows a cable to be protected by an OPD rated higher than the CCC of gthe cable.

Your recent comments have been expressing concern about application of diversity but, as I've said, applying divefrsity does not allow one to have a cable protected by an OPD with a higher rating of the cable - and so long s the cable is adequately protected (in the eyes of BS7671), I don't really understand why you should have concerns about diversity - the 'very worst' that can happen is that the OPD would operate.

... and in other threads is frequently suggested as suitable for use on a 32 or even 45A MCB.

If you're still talking about 1.5mm² cable, as above it's only acceptable with a 32A MCB if it is Method C and part of a ring final, and would never be acceptable with a 45A MCB.

 

[JohnW2]

I don't see that as a problem, as long as derating factors don't kick in.

Sure, I was obviously talking about Method C with no other de-rating factors.

However, provided only that that is the case, such that the cable is adequately protected, then it makes absolutely no difference (to the wellbeing of the cable) whether or not the concept of diversity has been invoked.

 

[SUNRAY]

That would only be possible (per regs) with Method C 1.5mm² in the context of a BS7671 ring final. That is the only situation in which BS 7671 allows a cable to be protected by an OPD rated higher than the CCC of gthe cable.

Your recent comments have been expressing concern about application of diversity but, as I've said, applying divefrsity does not allow one to have a cable protected by an OPD with a higher rating of the cable - and so long s the cable is adequately protected (in the eyes of BS7671), I don't really understand why you should have concerns about diversity - the 'very worst' that can happen is that the OPD would operate.

If you're still talking about 1.5mm² cable, as above it's only acceptable with a 32A MCB if it is Method C and part of a ring final, and would never be acceptable with a 45A MCB.

And yet we regularly recommend installing an oven with its 1.5mm² flex on the existing cooker circuit which is rapidly becoming 45A as standard. Despite the miniscule chance of cooking devices causing overload damage to such cables I seem to have copped enough of them to not like the situation.

Sure, I was obviously talking about Method C with no other de-rating factors.

However, provided only that that is the case, such that the cable is adequately protected, then it makes absolutely no difference (to the wellbeing of the cable) whether or not the concept of diversity has been invoked.

I have no issues with an adequately protected circuit, and to put things in perspective I frequently work with kit rated well above the OCPDs and cables, particularly in my temporary work such as stage lighting, one such show I had something around 20KW of lighting running on 3*13A sockets which I'm sure would have promptly taken out the 3 fuses if I tried a full-up.

 

[JohnW2]

And yet we regularly recommend installing an oven with its 1.5mm² flex on the existing cooker circuit which is rapidly becoming 45A as standard. Despite the miniscule chance of cooking devices causing overload damage to such cables I seem to have copped enough of them to not like the situation.

I have been talking about the fixed wiring. You may have a point in relation to the flex connected to the cooker, which usually (probably almost always) has a smaller CSA than the fixed wiring, although it is very rare to hear of any problems even with that (not the least because the fixed wiring itself is often/usually appreciably higher CSA than is actually necessary).

As for "... cooker circuit which is rapidly becoming 45A as standard", where does that come from? For those who believe in diversity, a 45A circuit would allow for a cooker of up to around 30 kW (about 126A at 230V), well in excess of the total capacity of any domestic single-phase installation!

 

[SUNRAY]

I have been talking about the fixed wiring. You may have a point in relation to the flex connected to the cooker, which usually (probably almost always) has a smaller CSA than the fixed wiring,

It's happened in this thread.

although it is very rare to hear of any problems even with that (not the least because the fixed wiring itself is often/usually appreciably higher CSA than is actually necessary).

As for "... cooker circuit which is rapidly becoming 45A as standard", where does that come from? For those who believe in diversity, a 45A circuit would allow for a cooker of up to around 30 kW (about 126A at 230V), well in excess of the total capacity of any domestic single-phase installation!

Yes we both know that but how many times do we see 10mm & 45A MCB/RCBO in questions relating to cookers?

 

[Mottie]

Are you sure yours has 2 lives and 2 neutrals? If so, maybe they’re using the same cable for the 3–phase and the single-phase versions. (Has anyone else ever seen this? What hob is it?)

Our Neff hob was the same. I think each live and neutral feeds different sides of the hob. I used the existing 6mm feed cable. Mine is also 7.4 kw.

 

[JohnW2]

It's happened in this thread.

Yes, but only because, after invocation of diversity, the wrong size of OPD has been used.

After application of diversity, a 7.4 kW domestic cooker represents a load of about 16.6A - so Method C 1.5mm² cable is adequate (** but see below). However, if any of the cable run has a CCC of only 20A, then it should be protected by a 20A MCB - and that remains true even if most of the circuit's wiring is ('unnecessarily') in 2.5, 4, 6 or 10 mm² cable. I suspect that's an aspect of invoking diversity that many people overlook.

[ ** I have to admit that, per BS7671, 1.5mm² flex has a CCC of only 16A (rather than the 20A for Method C 1.5mm² non-flexible cables), so I suppose one might argue that the OPD should be a 16A one (and I'd lose no sleep over that with an after-diversity current of 16.6A), but nor would I lose any sleep over there being a 20A OPD - not the least because I don't understand why the tabulated CCC is less for flex!) ... and,anyway, if (as is usually the case) the 7.4 kW relates to 240V, the after diversity current is a little under 16A at the design voltage of 230V, so OK with a 16A OPD if one was really concerned! ]

Yes we both know that but how many times do we see 10mm & 45A MCB/RCBO in questions relating to cookers?

I think times have changed, but in the opposite direction from what you seemed to be implying. There was a time when 10mm² cable protected by a 40A fuse/MCB was common (but I don't think I have personally seen a 45A OPD on a cooker circuit). However, in more recent times, cooker circuits very commonly have 32A OPDs and 6mm² (or 4mm²) cable.

 

[fixitflav]

I said it, thats what it works out to with diversity

Yes, I should have said. But I thought I'd checked in and agreed, got something wrong.