Simple electric cooker questions

[Ian Chard]

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?

1. 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

[ARWadsworth]

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

[David Hansen]

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

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 , 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.

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.

[John Rumm]

Diversity comes to your rescue. The general formula for cookers in domestic settings is 10A + 30% of the remaining actual load, and an additional 5A if there is a socket on the cooker point.

So for a 7.5kW cooker that is a nominal 7500/230 = 32.6A full load (which in itself will not trip a 32A type B breaker).

So you take 10 + 22.6 * 0.3 = 16.8 A with diversity.

If they fit you could, Some single ovens plug in rather than require hard wiring. So you may need to feed a socket and a cooker flex outlet.

[John Rumm]

There is a better diagram here:

[Michael Chare]

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.

[David Hansen]

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

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.