The old electric hob and oven were supplied together from a single supply which has a 32A MCB at the CU, but goes to a kitchen mounted switch box containing a 32A MCB (serving old hob) and a 16A MCB (serving oven).
I want to install an induction hob which (I assume -- haven't got the hob yet so I can't confirm the installation specs) will require 32A alone. I'm concerned about retaining the old setup because the 32A MCB at the consumer unit implies that the supply is only good for 32A. If this is the case then I will consider running the oven from the kitchen ring main (the oven requires only 13A), and dedicating the cooker supply to the hob alone. But if the cooker supply is actually
48A (as implied by the 16A + 32A MCBs in the kitchen mounted switch), then I'll be OK to run the new hob and the old oven using the previous setup.
What should I do?
(I'm aware of the need to notify building control if I do make changes)
The 16A MCB is probably there to provide protection to the oven. Whilst the *total* load cannot exceed the 32A of the main MCB you can (probably) use a 32A induction hob on the same circuit on the assumption of diversity ie the oven and hob won't be fully on at the same time (oven will cycle on its thermostat, very unlikely you will have all rings on max power on the hob simultsaneously).
On 27 Mar 2007 04:24:28 -0700 someone who may be "Duncan" wrote this:-
We are even less able to guess than you what the rating of the particular induction hob you propose to buy will be. It should be easy enough to come up with a candidate that fulfills your requirements and then for you to find out its ratings.
One hopes so. However, it would be worth checking the size of the cable.
Not something to even consider until the ratings of the hob are known.
This statement confirms that you don't know enough about electricity to do this electrical work yourself.
Get someone who knows what they are doing to do the work.
Of course. The one I ordered is a Neff t4243, but I'm unable to find the ratings. I expect it will come with a handbook. However, isn't it reasonable to assume (until I can open the box and actually check) it will require a 32A supply, because similar induction hobs with similar power outputs require a supply rated at this current?
The cable between the consumer unit and the box in the kitchen is
6mm^2 twin and earth. It runs through the joists in the loft and down behind the plasterboard wall in the kitchen. The total length is 6 m, give or take a metre.
Could you explain why? Obviously the reason I'm posting here is that I want to learn. But of course, you might be trying to protect me from myself, which is fine and quite understandable.
Anyway, my understanding is that the MCBs are meant to trip if an appliance is faulty such that it draws more current than usual, or if the current exceeds the safe limit for the cable. If there is a dedicated cable for a built in applicance, then the MCB used to protect that cable should be rated at or a little above the appliance's maximum current requirement, and below (of course) the rating for the cable. The oven requires a 13A supply (it says so) and its cable is protected with the 16A MCB. If the total cooker supply is rated at 32A, this only leaves 19A for the old hob, and if this is all that it needed then why would the installer have put a 32 MCB on it? This is why it implied to me that the power supply might be more than
32A. But there are other explanations, which I was expecting to read from your replies, for example perhaps is no such thing as a 22A MCB, so a 32A MCB is the appropriate protection for a hob that draws only
19A.
Well, I have an electrician coming in to give a quote the afternoon the hob arrives. But even so, I'd like to know what's wrong with my question. Thanks,
On 27 Mar 2007 07:28:56 -0700 someone who may be "Duncan" wrote this:-
Have you asked the manufacturer and / or supplier?
With the ratings of each hob one can do a diversity calculation to see what the likely maximum load is. Without them speculation will not get one very far.
The 32A protective device in the consumer unit will limit the current that can be drawn at the kitchen end of the circuit. Even if there were two 32A MCBs at the kitchen end the circuit would not be able to supply any more current than the 32A MCB in the consumer unit will permit. Thus the existing oven and hob are limited by the
32A protective device in the consumer unit.
As an aside a 32A MCB will probably allow 50A to flow for many minutes, perhaps an hour (I haven't bothered to look at the curves to check this time).
As a general rule of thumb at least twice the current that the protective device is rated at needs to flow before the protective device will operate with any sort of rapidity.
The safe limit for a cable depends on how and where the cable is installed and for how long the current is present, there is not one current for a particular sort of cable. For five minutes a cable can usually easily carry twice its continuously rated current without bad effects. This is very useful, provided the designer knows what they are doing.
To some extent this depends. Some bits of equipment have even higher starting currents than is typical and need special consideration.
Some way below in the case of some protective devices.
What is the rating of the oven? I would take a guess it is equivalent to around 10A. That is before diversity calculations.
Assuming no diversity.
Is it normal for the oven to be turned on full at the same time as every ring on the hob is turned on full? I suspect not and thus one can allow for the fact that the oven and rings will be cycling on and off by doing a diversity calculation.
Pass. A 45A double pole switch would have been fine by the sound of it.
BTW they failed to provide discrimination between the two (32A) MCBs.
My Neff induction hob is on a 32A MCB'ed 6mm2 radial all to itself. I think installation instructions call for a separate radial.
My Neff cooker and microwave are also on a 32A MCB'ed 6mm2 radial as well.
It is better to connect fixed appliances on their own circuits as they are high power this will affect the loading of any house ring mains and if on their own MCB can easily be isolated via the MCB's on the CU.
Anyway the induction hob is the "doggies danglers" of cooking methods, I see why some of these cooks on TV rave on about their induction hobs (Heston Blumenthal), its just so controllable and clean and quick compared messy gas and slow electric.
Thanks for your response. I don't think I misunderstand anything fundamental, I just wasn't very articulate in my original question. I understood that the 32A MCB at the consumer unit limits the amount of current from the CU, and having 32A+16A MCBs in the kitchen doesn't increase this. What I wanted to know was why the rating of the two MCBs in the kitchen exceeds the maximum current that can be drawn from the supply through the single 32A MCB at the CU. It seemed to me that they can't all be the correct choice of MCB.
2350W, so it needs 10.2A -- good guess. I know this is the maximum it will draw, not the average, because the heating element isn't on all the time, even when the oven is set to Broil. But I was being conservative by taking the worst case (also why I used 230v not 240v to give the current above). So I was being simplistic, but not dangerous - it's not dangerous to underutilise a circuit.
As I said, I don't know the rating of the hob and I'm not inclined to sit on hold listening to Vivaldi for half an hour to find out when I'll be able to look in the box myself in a couple of days, but let's say, just for the sake of my education, that it's rated at 7kW, 29.1A. Again, I understand that this is the maximum possible, not the average.
So how does one use a diversity calculation give a figure for the actual current requirement of these actual and virtual appliances? And why isn't this dangerous? I mean it's unlikely that the cycling of the four hobs and the oven heater will all coincide, but eventually they will, and then their combined current draw would exceed a 32A supply for a short time. Why is it called a *diversity* calculation? Is it because diverse appliances (eg air con and heater) are unlikely to be needed at the same time?
Well, that's what I didn't understand. It seemed to me that either it was wrong to use 32A+16A MCBs in the kitchen, or it was wrong to use a
32A MCB in the CU. I thought they couldn't both be the correct choice.
I have had a 4 "ring" Induction Hob ( Neff) since approx year 2000 and can fully concur that its the dogs danglies as far as well everything is concerned - not least the speed and ease of keeping clean
I had it wired with its own 32A breaker and 6mm cable and the same for the cooker ( which was definitely overkill as it also takes only some 10A) with a breaker in the CU for each and an Isolating switch (45A) under/behind each with heatproof cable for the last metre or so..
.....and for nearly 7 years it has worked flawlessly. This was obviously well before regs were tightened up, but I tend to err well on the side of safety and overspecify cables required for the job...
Unless its quite a long run 6mm or even 4mm may be adequate - TLC has cable heating and voltage drop tables to help assess requirements..
It may be that the 32A MCB is in there only because a MCB was required to make the connection in the enclosure selected (i.e. given a free choice, they may have preferred to have no additional protective device on the hob connection at all, and just have a 16A CPD on the oven).
The 16A MCB may be there because the oven is designed with the assumption that overload protection will be provided at no more than 16A.
(not unnecessarily relevant to this discussion, but see note [1] for difference between overload and fault protection)
Not dangerous, but if you design every circuit as if its momentary peak load is to be taken as the normal operating load, you are going to spend silly money on big cables and switch gear.
For a full description of the allowable calcs see Appendix 1 of the On-Site Guide [2].
For a cooker, you take 10A + 30% of the full load in excess of 10A (plus add on another 5A if there is a socket built into the cooker point). So for example, say your maximum load is 40A for oven and hob combined, and there is no socket on the cooker point, you get 10A + 30% of 30A or 19A
As you say the effect of the thermostats will modulate the power drawn over time, lowering the effective demand.
For a short time, some overload is acceptable (see notes on overload later) - it is the average ongoing effect that matters - since that is what will result in an overheated cable or not.
It is just allowing for the fact that in the real world there there is a good probability that you will be unable to use the full capacity of all the elements at once (and for that matter unlikely to want to).
Same logic applies to ring circuits used for general purpose power distribution. You may have tens of sockets per circuit even though in theory it would be possible to reach the full capacity of the circuit by fully loading just two double sockets.
Not really a problem in this case, but as a design aim one should try to arrange that when a fault occurs, it only causes a trip on the affected circuit and leaves the others working. In this case an overload on the
32A MCB may trip the one nearest to the hob, or it may also/instead trip the one in the main CU hence disabling not only the hob, but the oven as well. Hence it is not "discriminating" between circuits.
(a more serious example of the problem is a "whole house" RCD. A fault anywhere will take out all circuits including the lights. The repercussions of which may be more serious that the fault you were trying to protect against).
[1] There are two different things that you usually want to protect a circuit against: "overloads", and "fault conditions". Superficially these sound alike, but are somewhat different in practice.
An overload is a prolonged period of operation drawing more current that a circuit is designed for - say for example loading up a 32A power circuit such that the total load is 50A. Circuit protective devices like MCBs use a thermal mechanism (usually some form of bi-metal strip as used in a thermostat), to protect against overload. In this way it will permit the overload for a period of time, but should open before the cable reaches an excessive temperature. The amount of overload occurring will dictate the time it takes to open and this could be seconds, mins or even hours. Slight overloads are usually permitted indefinitely.
A fault condition is basically when something happens that results in a short circuit between phase and neutral or phase and earth (like nailing through the cable). The current that can flow in these cases (the "fault current") will typically be many hundreds (or even thousands) of amps. Here the protective device needs to open "instantly" to clear the fault. A MCB includes a magnetic trip mechanism for this type of fault activated by a solenoid. Another design criteria in the case of fault current is ensuring that the cable will survive long enough to be protected by the CPD.
[2] A must have book for anyone planing on doing electrical work:
On Tue, 27 Mar 2007 16:18:22 +0100 someone who may be "Ian_m" wrote this:-
They may well do, but that perhaps tells us more about those who assume all electrical installations are the same. In houses which have a traditional 45A cooker supply there is no need to run a separate radial circuit for a hob.
John, Thank you for the full and excellent reply, which I have read carefully and with interest. I bought a copy of the On-Site Guide this morning. A very useful reference, and a bargain at =A317.
On Wed, 28 Mar 2007 20:26:54 +0100 someone who may be Lurch wrote this:-
45A was the traditional rating of a cooker supply. The largest protective device in the house and only one device of that rating was to be fitted. Electric cookers used to gobble up electricity and there was also a need to allow for the 15A or 13A socket on the cooker control unit, which would probably be used for a kettle.
Since the 1970s or 80s it has been common to fit 30A protective devices to cooker circuits.
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