Hi,

I'm going to replace my horrible old electric cooker shortly, and I have
a couple of questions.

The cooker circuit has a 30A breaker, and the switch for the cooker
doesn't have a 13A socket in it (it's just a switch). I don't know the
rating of the cooker but the breaker has never popped.

1. Lots of the cookers I've looked at have ratings above 7.5kW.
Presumably this would only be a problem if the whole thing (4 rings + 2
ovens) was switched on at the same time. Is it legal to install a
cooker rated higher than the circuit?

2. If I replace it with standalone units (oven + hob), can I use the
same wall terminals for both, or would I need to get a second set of
terminals fitted?

Thanks
- Ian

Ian Chard wrote:
Hi,

I'm going to replace my horrible old electric cooker shortly, and I
have a couple of questions.

The cooker circuit has a 30A breaker, and the switch for the cooker
doesn't have a 13A socket in it (it's just a switch). I don't know
the rating of the cooker but the breaker has never popped.

1. Lots of the cookers I've looked at have ratings above 7.5kW.
Presumably this would only be a problem if the whole thing (4 rings +
2 ovens) was switched on at the same time. Is it legal to install a
cooker rated higher than the circuit?

2. If I replace it with standalone units (oven + hob), can I use the
same wall terminals for both, or would I need to get a second set of
terminals fitted?

Thanks
- Ian

The general rule for cookers is that a 30 or 32 A supply is OK for a
household cooker up to 15kW due to diversity (ie all the rings are not all
on at the same time for long).

You can supply two seperate cooking appliances (oven and hob in your case)
from the same switch (and therefore the same wall terminals) if their
combined load is less than 15kW AND the switch is within 2m of both
appliances.

Cheers

--
Adam

On Thu, 16 Sep 2010 16:12:55 +0100 someone who may be Ian Chard
wrote this:-

1. Lots of the cookers I've looked at have ratings above 7.5kW.
Presumably this would only be a problem if the whole thing (4 rings + 2
ovens) was switched on at the same time. Is it legal to install a
cooker rated higher than the circuit?

Think about how an electric cooker is used and works. Turn the grill
on at breakfast and perhaps another ring. Nothing else. Sunday roast
involves turning the oven on for a couple of hours. At first it is
on constantly, but after perhaps 15 minutes the element starts
cycling on and off. After a while the first ring is turned on, again
this cycles on and off, the same with other rings. It is unlikely
that everything is on at once and even if it is on at once this
doesn't last for long.

A 30A MCB will not operate instantly at 31A, in fact it won't
operate at all. It has to carry a lot more than the rated current
before it will operate even fairly quickly. Look at some curves at
http://www.tlc-direct.co.uk/Figures/3.18b.gif, note the
logarithmic scales. The current is along the bottom, the operating
time on the side. To operate in 90s, a minute and a half, a 32A MCB
needs to be passing a current of something like 70A. The cables can
take this sort of short duration overload, they warm up a little and
cool down when the load reduces.

2. If I replace it with standalone units (oven + hob), can I use the
same wall terminals for both,

Provided the switch is within 2m of both of them that is fine. You
can run two cables from the switch. Alternatively you could run a
cable to the first one and then one from there to the second. Just
check that the terminals can physically take the cables.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000...#pt3-pb3-l1g54

"David Hansen" wrote in message
...
On Thu, 16 Sep 2010 16:12:55 +0100 someone who may be Ian Chard
wrote this:-

1. Lots of the cookers I've looked at have ratings above 7.5kW.
Presumably this would only be a problem if the whole thing (4 rings + 2
ovens) was switched on at the same time. Is it legal to install a
cooker rated higher than the circuit?

Think about how an electric cooker is used and works. Turn the grill
on at breakfast and perhaps another ring. Nothing else. Sunday roast
involves turning the oven on for a couple of hours. At first it is
on constantly, but after perhaps 15 minutes the element starts
cycling on and off. After a while the first ring is turned on, again
this cycles on and off, the same with other rings. It is unlikely
that everything is on at once and even if it is on at once this
doesn't last for long.


In our house, Christmas dinner is probably the greatest load. Both parts of
oven on for several hours, and all four rings of hob in use.

--
Michael Chare

On Thu, 16 Sep 2010 20:33:28 +0100 someone who may be "Michael
Chare" wrote this:-

In our house, Christmas dinner is probably the greatest load. Both parts of
oven on for several hours, and all four rings of hob in use.

It is in many houses. However, the ovens and rings are not taking
current all the time.

Electrical distribution equipment can generally sustain an overload
of double the rated current for an hour or two [1]. If one bolts a
large fan on to the radiator of a transformer than that can double
the rating too, so the thing can take up to four times its rated
current. The fan possibly only runs on Christmas Day and a few other
days, but it is a lot cheaper than providing a new transformer.



[1] traditionally. Some modern equipment and systems are designed
far closer to the limits than was traditional. If there is not
enough time for equipment and cables to cool down from an overload
the end result is ever rising temperatures. That possibility led to
reinforcement of railway electrification systems in SE England
around a decade ago.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000...#pt3-pb3-l1g54