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)

Duncan

On 27 Mar 2007 04:24:28 -0700 someone who may be "Duncan"
wrote this:-

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.

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.

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.

One hopes so. However, it would be worth checking the size of the
cable.

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.

Not something to even consider until the ratings of the hob are
known.

But if the cooker supply is actually
48A (as implied by the 16A + 32A MCBs in the kitchen mounted switch),

This statement confirms that you don't know enough about electricity
to do this electrical work yourself.

What should I do?

Get someone who knows what they are doing to do the work.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Dear David,

Thanks for your help.

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.

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?

One hopes so. However, it would be worth checking the size of the
cable.

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.

But if the cooker supply is actually
48A (as implied by the 16A + 32A MCBs in the kitchen mounted switch),

This statement confirms that you don't know enough about electricity
to do this electrical work yourself.

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.


What should I do?

Get someone who knows what they are doing to do the work.

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,

Duncan

--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents mehttp://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

On 27 Mar 2007 07:28:56 -0700 someone who may be "Duncan"
wrote this:-

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.

Have you asked the manufacturer and / or supplier?

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?

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.

But if the cooker supply is actually
48A (as implied by the 16A + 32A MCBs in the kitchen mounted switch),

This statement confirms that you don't know enough about electricity
to do this electrical work yourself.

Could you explain why?

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).

Anyway, my understanding is that the MCBs are meant to trip if an
appliance is faulty such that it draws more current than usual,

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.

or if the current exceeds the safe limit for the cable.

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.

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,

To some extent this depends. Some bits of equipment have even higher
starting currents than is typical and need special consideration.

and below (of course) the rating for the cable.

Some way below in the case of some protective devices.

The oven requires a 13A supply (it says so) and
its cable is protected with the 16A MCB.

What is the rating of the oven? I would take a guess it is
equivalent to around 10A. That is before diversity calculations.

If the total cooker supply is
rated at 32A, this only leaves 19A for the old hob,

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.

and if this is all
that it needed then why would the installer have put a 32 MCB on it?

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.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

"Duncan" wrote in message
oups.com...
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.

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.

Hi David,

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.

What is the rating of the oven? I would take a guess it is
equivalent to around 10A. That is before diversity calculations.

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?


and if this is all
that it needed then why would the installer have put a 32 MCB on it?

Pass. A 45A double pole switch would have been fine by the sound of
it.

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.


BTW they failed to provide discrimination between the two (32A)
MCBs.

I don't understand this comment.

Thanks for your patience...

Duncan


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents mehttp://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

On Mar 27, 12:24 pm, "Duncan" wrote:
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.

Hi

It's 7.4kW according to the instruction manual of the identical Bosch
model - this is just a shade over 32A at 230V.

You may be able to use discrimination on the basis that you would
rarely have all four zones on simultaneously at full power.

Regards,

Steve

My Neff induction hob is on a 32A MCB'ed 6mm2 radial all to itself. I think
installation instructions call for a separate radial.

That's what I anticipate. So I, or the electrician, will have to
decide how to run the oven if it can't go on the same radial.

My Neff cooker and microwave are also on a 32A MCB'ed 6mm2 radial as well.

Well the oven (which is rated at 10A) only asks for a 13A fused double
pole switch, so in principle this could come from the kitchen ring
main.

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.

Yes. But my kitchen has its own ring, separate from the ring that
serves the rest of the flat, and another radial dedicated to the
washing machine.

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.

that's good to hear!

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..

Nick

Duncan wrote:

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.

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)

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.

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.

So how does one use a diversity calculation give a figure for the
actual current requirement of these actual and virtual appliances? And

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

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.

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.

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?

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.

BTW they failed to provide discrimination between the two (32A)
MCBs.

I don't understand this comment.

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:

http://www.amazon.co.uk/exec/obidos/...ternodeltdcomp

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Tue, 27 Mar 2007 16:18:22 +0100 someone who may be "Ian_m"
wrote this:-

My Neff induction hob is on a 32A MCB'ed 6mm2 radial all to itself. I think
installation instructions call for a separate radial.

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.




--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

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 £17.

Cheers,
Duncan

On Wed, 28 Mar 2007 10:54:22 +0100, David Hansen
mused:

On Tue, 27 Mar 2007 16:18:22 +0100 someone who may be "Ian_m"
wrote this:-

My Neff induction hob is on a 32A MCB'ed 6mm2 radial all to itself. I think
installation instructions call for a separate radial.

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.

How is a 45A cooker supply traditional? Only 45A cooker supplies I've
ever seen have been ones I've installed myself for all electric
ranges.
--
Regards,
Stuart.

On Wed, 28 Mar 2007 20:26:54 +0100 someone who may be Lurch
wrote this:-

How is a 45A cooker supply traditional? Only 45A cooker supplies I've
ever seen have been ones I've installed myself for all electric
ranges.

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.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

On Thu, 29 Mar 2007 08:41:01 +0100, David Hansen
mused:

On Wed, 28 Mar 2007 20:26:54 +0100 someone who may be Lurch
wrote this:-

How is a 45A cooker supply traditional? Only 45A cooker supplies I've
ever seen have been ones I've installed myself for all electric
ranges.

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.

Can't say I've ever seen anything of that era, most 'old wiring' that
I have ripped out has been 70's, some maybe 60's.

Since the 1970s or 80s it has been common to fit 30A protective
devices to cooker circuits.

Fairynuff then.
--
Regards,
Stuart.