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Ovens and 30A fuse/feed

Hello, I googled and found some info in the old IET forums, which has generated this post.

We need to replace our double oven. For the last 20 years, a 30A trip fuse has provided power to a dual wall socket (kettle 2.2kw, microwave 1.29kw) and the oven with a 5.1kw max rated power consumption. I've discovered this by tripping the 30A fuses in the fuse box to see which appliances were still live.

It appears that when the kitchen was fitted (20 years ago), the original standalone cooker on/off master switch was converted to a dual wall socket and the then a spur used for the double oven.

According to my maths this is a total of 35.79A @ 240v which is more than 30A, although I'm guessing the times all three appliances have been on at the same time would be infrequent.

Appliance	Load rating (kw)	Amps (at 240v)
Current oven	5.1	21.25
Kettle	2.2	9.166666667
Microwave	1.29	5.375
total	8.59	35.79166667

Why has the 30A RCD at the fusebox not tripped when they have been on at the same time?

The new oven has a load rating of 6.2kw. This increases the total amps to just over 40A.

Appliance	Load rating (kw)	Amps (at 240v)
New oven	6.2	25.83333333
Kettle	2.2	9.166666667
Microwave	1.29	5.375
total	9.69	40.375

I'm guessing this may cause some issues with the 30A feed. Would a more modern 32A be better?

I suppose I could always move the kettle and microwave to other sockets in the kitchen which are on a different 30A circuit.

Please could I have your comments?

Thank you

Paul

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0 AJJewsbury over 3 years ago

Am I correct in thinking a kettle wouldn't have a diversity factor applied?
No it still could. The amount of diversity would vary with the situation, but the main thing about a kettle is that it's only on for a relatively short length of time - a couple of minutes rather hours - so the cables don't really get the time to warm up (as David mentioned). The heating effect is smaller with larger cables - so the 5A allowance for a socket that (traditionally) fed a 3kW kettle on say a 30A circuit is entirely reasonable. Also as you consider more “things” there's generally less chance of things being on at the same time, which also increases the amount of diversity you can apply.
It's a similar situation with motors - we usually size their circuits for their normal running current - yet they can take several times that current during starting (which can last several seconds) - with no ill effects.
As an extreme example, DNOs consider a typical house to have a maximum load of about 2kW - when averaged across many houses.
- Andy.
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0 AJJewsbury over 3 years ago

Am I correct in thinking a kettle wouldn't have a diversity factor applied?
No it still could. The amount of diversity would vary with the situation, but the main thing about a kettle is that it's only on for a relatively short length of time - a couple of minutes rather hours - so the cables don't really get the time to warm up (as David mentioned). The heating effect is smaller with larger cables - so the 5A allowance for a socket that (traditionally) fed a 3kW kettle on say a 30A circuit is entirely reasonable. Also as you consider more “things” there's generally less chance of things being on at the same time, which also increases the amount of diversity you can apply.
It's a similar situation with motors - we usually size their circuits for their normal running current - yet they can take several times that current during starting (which can last several seconds) - with no ill effects.
As an extreme example, DNOs consider a typical house to have a maximum load of about 2kW - when averaged across many houses.
- Andy.
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Vote Up 0 Vote Down

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Ovens and 30A fuse/feed

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