So what would you do if you had a freestanding electric cooker where your method of cable sizing (no diversity) results in a larger size than the terminals can accommodate because the maker has used diversity?
Induction hob cable
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So going by the above statement, I assume that you are going to recable your socket circuits in your property as they will no doubt be cabled in 2.5mm with a max capacity of 27a yet are protected by a 32a mcb!
That's not quite the same because there are two of them with a maximum of 54A.
As such a ring is considered compliant with a 30A BS3036 fuse, then a 40A MCB should be acceptable.
As such a ring is considered compliant with a 30A BS3036 fuse, then a 40A MCB should be acceptable.
There is a 13A fuse in there somewhere.
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Then you are stuffed, as a cable that does not exceed the terminal size has to be used.
Well you're not stuffed, you just apply diversity and fit that cable.
What do you think people install for big range cookers? A 25mm² SWA? No
Only if the ring has the 2.5/1.5cpc and not the older 2.5/1.0cpc!
we all know how a ring final works and why, that wasnt my point, the point was the op stated that he will never have a cable rated less than the appliance rating, and by virtue has a 27a cable on a 32a mcb or 30a fuse
ok, you crack on thenThere is a 13A fuse in there somewhere.
Isn't that what they originally had with the 30A fuse?
...but you were talking about the sockets, hence the ring, therefore he will have two conductors connected to the MCB or Fuse.
Bad analogy. It has been brought to your attention by others that a ring has two 27A cables, giving 54A with a 32A mcb. Also, no appliance can be connected to the ring with a fuse larger than 13A. You can plug in a 3kW heater to every socket but the 32A mcb cuts out. Diversity is used on a ring. A proper installation has high power appliances on their own circuits & mcb, etc. So the likelihood of plugging a 3kW heater in every outlet on a ring is very, very low, as any electric heating would have its own circuit.
Thanks, I will crack on.
You are stuffed it you want to fit a cable larger than what the appliance terminals can take for sure. So you fit a cable that the terminals can accommodate.
Going by diversity, most domestic homes have oversized cables. Many have 6mm cable direct to an ordinary hob.
But you need fuses and mcb's about an installation to ensure cables are protected and do not get too hot being a fire risk.
I would use the diversity factor with caution in high-rise flats. Someone on this thread said the diversity factor dictates that a 2.5mm cables can be the final flex cable to a 7.4kW induction hob. This when the diversity formula has to be treated with caution.
But you need fuses and mcb's about an installation to ensure cables are protected and do not get too hot being a fire risk.
I would use the diversity factor with caution in high-rise flats. Someone on this thread said the diversity factor dictates that a 2.5mm cables can be the final flex cable to a 7.4kW induction hob. This when the diversity formula has to be treated with caution.
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Whilst I agree with you and others that this was not a very good analogy, in the interests of accuracy ....
With a ring final circuit using cable with a 27A CCC, it is only true that this is equivalent to a 54A CCC if the load were applied in a totally symmetrical fashion to the ring - the simplest example being a single load exactly at the midpoint of the ring.
The moment one moves away from symmetrical loading of the ring, the current becomes unequally shared between the two legs of the ring and, in the most extreme case of loads all applied near one end of the ring, almost all of the current can flow in one leg, hence the effective CCC falls to almost 27A (with a 32A MCB) ... and, of course, the regs allow the CCC of the cable to be as low as 20A, in which case one could have a cable with an effective CCC not much above 20A being protected by a 32A MCB.
Designers are, of course, expected to take steps to avoid such a situation but, unless they avoid having sockets close to an end of the ring, they cannot guarantee what will be plugged into the sockets, and where.
Kind Regards, John
Indeed - and that's the whole point....
What you seem to be missing is the fact that the 2.5mm² cable would have to be adequately protected by an appropriate MCB. If the cable were 'clipped direct', that would mean a CCC of 27A, hence a maximum MCB rate of 25A (if available for one's CU, otherwise 20A) (remember that the after diversity current of a 7.4kW cooking appliance is around 17A).
Returning to your first comment, the whole point of the CCC tables we use is that they indicate what MCB rating will ensure that the cable "does not get too hot and become a fire risk", regardless of what load is applied to the cable. In other words, one can be confident that a 2.5mm² cable will not overheat if it is protected by a 25A or 20A MCB, even if a load is applied that tries to take much more current than that. For example, the way in which CCCs and MCB ratings are presented require that a cable can carry 1.45 times its 'rated' CCC for 1 hour without suffering any damage, and certainly without representing a fire risk. In other words, a cable with a tabulated CCC of 27A has to be able to safely carry an average of about 39A for an hour, whilst, at the same time a 25A MCB would trip if an average current of about 36A flows for an hour. In other words, the MCB is 'sized' to ensure that the cable never gets unacceptably hot.
As a final comment, whilst I fully understand why it's happening, I think we have to be careful about making comments about fire safety issues specifically in relation to "high-rise flats" (or high-rise buildings in general) - since more people die as a result of fires in 'low-rise' buildings than high-rise ones.
Kind Regards, John
Where?There is a 13A fuse in there somewhere.
And how does it act to prevent the circuit being overloaded?
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