Builder says we need expensive kitchen rewire - do we really?

[JohnW2]

Was that a 20A breaker or a 16A one? If 20A, I really wouldn't expect that set up to cause it to trip. At the very most, they would represent a 26A load (and the kettle would only be on for short periods of time) - a 20A MCB should allow 22.6A to flow indefinitely, and would take up to an hour to trip with 29A, so probably a very long time to trip with 26A (even if the kettle was on continuously, for 'hours').

It was a 15A mcb and it would trip as the kettle neared boiling point. Got fed up after a few months and got the gas kettle. Much easier than trying to upgrade the cable or circuit, Lol.

Fair enough 26A would trip a 15/16A MCB fairly quickly. As I implied, an alternative (to getting a gas kettle) would have been to change the MCB to a 20A one (very probably possible with the 2.5mm² cable) - since, as I said, that would almost certainly not have tripped with a dryer plus (electric) kettle.

Kind Regards, John

 

[ban-all-sheds]

What's the Ib for a dryer + kettle?

 

[JohnW2]

What's the Ib for a dryer + kettle?

As I said, it is potentially around 26A. Although that would take a very long time (well over an hour) to trip a B20 MCB, it ought to trip a B16 in a maximum of 4-5 mins (and a B15, if that's what he had, would presumably trip even quicker at 26A).

Kind Regards, John

 

[ban-all-sheds]

As I said, it is potentially around 26A

Then you may not use a 20A breaker, as you know full well.

 

[JohnW2]

As I said, it is potentially around 26A

Then you may not use a 20A breaker, as you know full well.

The problem here is about the identity of that 'you' and what meaning one can ascribe to Ib in a sockets circuit. The designer of a sockets circuit does not have a crystal ball. S(he) will frequently design a 32A sockets circuit which has, say, 8-10 double sockets. Even if no-one ever plugged in more than 20A into each of those double sockets, that is a potential load of 160A-200A .. so what is the Ib of such a sockets circuit?

You will probably argue that in this case we 'know' that the load will sometimes be 26A for at least a few minutes, and therefore that we should take that as the 'design current'. However, 20A radials, with multiple sockets, seem to be quite popular with some designers, and those designers must 'know' that it is inevitable that some (maybe most) such multiple-socket circuits will at least occasionally be asked to carry currents in excess of 20A, yet that does not stop them from designing 20A, multiple sockets, radials, with 20A OPDs.

Because designers have no crystal balls, a sockets circuit is a design nightmare (really a design impossibility). However, the OPD is, of course, there to protect the cable in situations in which the actual load ever appreciably exceeds to notional 'design current' (as perceived by the designer) of the circuit - a situation which is very possible in any sockets circuit.

Kind Regards, John

 

[deadshort]

I fail to seen the argument, if the load requires 26a then that's what it requires. I never understand these people who skimp on the electrics when they spend ££££££ on a new kitchen, odd

Regards,

DS

 

[JohnW2]

I fail to seen the argument, if the load requires 26a then that's what it requires. I never understand these people who skimp on the electrics when they spend ££££££ on a new kitchen, odd

As I said, the problem with any sockets circuit is that it cannot (by definition) be designed properly, since the load is an unknown quantity. The concept of a multi-socket 20A (or even 16A) radial is fully accepted, yet the person who designs it cannot possibly know what the load will be in the future - the next occupier (or maybe even the present one) may use the sockets for a TV and a phone charger, rather than a kettle and a dryer. More to the point, if it had been the other way around and the circuit had been 'designed' on the assumption that the loads would be a TV and phone charger, someone could have come along the next day and plugged in a dryer and kettle. As I wrote, this is one of the main reasons we have OPDs.

In any event, in the case in point, we're not really talking about anything other than 'diversity' (over time). The fact that 13A worth of kettle may occasionally take the real-time demand beyond the rating of the OPD for 2 or 4 minutes is no different from a cooker taking the real-time demand well over the rating of the circuit's OPD for short periods of time if multiple elements are switched on from cold simultaneously.

Do you not believe in 20A multi-socket radials, given how easy it would be for a user to plug in more than 20A worth of load?

Kind Regards, John

 

[ban-all-sheds]

The problem here is about the identity of that 'you'

Which one?

The second was definitely you. The first also, but could be replaced by "one", if you wish.

As for the rest, I agree with everything you say about the vagaries of Ib for socket circuits, and the problems which a designer faces when trying to determine what it is, or what value he should use.

Which is why I asked you what value you were using. And you said you were using 26A. If you are, then you may not specify a 20A device, as well you know. There is no special exemption in 433.1.1 for socket circuits, and what you were designing for was a known case of simultaneous loads which you as designer judged to be an Ib of 26A. What if the dryer and the kettle were not plugged in but were each hardwired into FCUs with flex outlets? From an electrical and a regulatory POV nothing changes.

 

[JohnW2]

As for the rest, I agree with everything you say about the vagaries of Ib for socket circuits, and the problems which a designer faces when trying to determine what it is, or what value he should use. ... Which is why I asked you what value you were using. And you said you were using 26A.

Well, what I meant was that the instantaneous load was potentially 26A, but ....

There is no special exemption in 433.1.1 for socket circuits, and what you were designing for was a known case of simultaneous loads which you as designer judged to be an Ib of 26A. What if the dryer and the kettle were not plugged in but were each hardwired into FCUs with flex outlets? From an electrical and a regulatory POV nothing changes.

As I've just written to DS, I personally see the specific example we're talking about as a situation of 'diversity' - and, as you know, the regs themselves say absolutely nothing about when and how diversity can/should applied, so I feel free to make my own judgement about that. As I wrote, an occasional 2-4 minutes of an extra 13A has almost no impact on the average demand on the circuit as measured over any reasonable period of time. I suppose, therefore, that I asked for your response by answering your earlier question incorrectly - whilst I would have to regard the instantaneous load as potentially being 26A, I would not regard that as the 'design current' for such a circuit, since I would 'apply diversity' when attempting to estimate the design current (Ib). If, in estimating that diversity, one assumes that the dryer (13A) is on continuously and that the kettle (13A) is on for 3 minutes every half-hour, that would represent an average load (which I would take as the 'design current' for that particular setup) of about 13.65A. Indeed, even with the ludicrous assumption that the kettle was going to be on for 15 minutes of every half-hour, even that would still only be an average current of about 19.5A.

So, in that specific case, I would have no problem (using my judgement about diversity) to protect a circuit which I 'knew' would only ever be used to supply a kettle+dryer circuit with a 20A OPD. In the real world, if there were also other sockets, goodness knows what I would regard as a realistic estimate of 'design current'. As I've said, proper design of sockets circuits is impossible, which is why we rely on OPDs to protect cables against 'the unexpected'!

Kind Regards, John