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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 Chris Pearson over 3 years ago

robopj:
Am I correct in thinking a kettle wouldn't have a diversity factor applied? From my understanding - the current is on from switch on to boiling point.
What about microwaves? Is there a diversity factor? I'm guessing they would pulse the waves - just like an oven thermostat?
Yes, a kettle will go flat out until the water boils and then it will turn itself off.
A microwave on anything less than full heat does pulse - you can often hear a relay clicking on and off.
FWIW, I don't think that the installation charge is excessive because the oven has to be delivered; it has to be unpacked; I would expect that the old one will slide out and the new one will fit without too much bother; there will be two operatives to carry it on ‘elf ’n' safety grounds; and most importantly, they take away the old one and may well have to pay to get rid of it - hopefully by recycling and not straight into land fill.
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0 Chris Pearson over 3 years ago

robopj:
Am I correct in thinking a kettle wouldn't have a diversity factor applied? From my understanding - the current is on from switch on to boiling point.
What about microwaves? Is there a diversity factor? I'm guessing they would pulse the waves - just like an oven thermostat?
Yes, a kettle will go flat out until the water boils and then it will turn itself off.
A microwave on anything less than full heat does pulse - you can often hear a relay clicking on and off.
FWIW, I don't think that the installation charge is excessive because the oven has to be delivered; it has to be unpacked; I would expect that the old one will slide out and the new one will fit without too much bother; there will be two operatives to carry it on ‘elf ’n' safety grounds; and most importantly, they take away the old one and may well have to pay to get rid of it - hopefully by recycling and not straight into land fill.
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Ovens and 30A fuse/feed

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