I am planning to install an induction hob rating 7.75kw and separate oven rating 2 75kw.
I have an existing supply 6mm rated at 30amp
I assume I will need to install a separate 20amp connection for the oven back to the box?
Any advice

i'm afraid you will need a little bit more work than that. your existing supply is insufficient for the cooker as well.

My induction hob with two heat areas in use can draw 6.7kW not quite as big as yours. Using all four areas one can't use the boost so it uses less power. The oven in the stand alone unit however is much bigger allowing top, bottom, and back(fan) heating elements to be used together. Plus a oven grill again two elements for top oven. So oven is around the 6kW as well. But the stand alone unit still had a manufactures recommendation for a 32 amp supply.

How much is diversity and how much is internal circuitry not allowing all outputs to energise together I don't know as I have never stripped it down. And so you have to consider the risk.

So what happens if you put a hob and oven on a supply which is not big enough. Well as long as it still has the 30/32A Fuse/MCB then the protective device will open. So in theory it fails safe.

However there is one problem. Cooling. Most ovens and induction hobs have fans to ensure it does not over heat. Any power cut will mean those fans will fail. We hope even without fans it will not cause any damage but it could cause overheating which could damage the electronics of the induction hob or cause the oven housing to catch fire.

I would think it is very unlikely that damage will be caused as of course other failures could cause loss of supply and one would hope manufactures would not rely on fans.

So in all likely hood you can run both off the same supply and since the oven is under 3kW you can use a fused connection unit to limit the supply to the oven. So using a double cooker connection unit and FCU likely everything will work without a problem.

But there is a risk and you have to consider if it's worth the risk. Personally I would take the chance that under normal use it will not trip.

I find the time one can use full power on an induction hob is quite short. The biggest longest load must be using a pressure cooker as using boost with anything but plain water will just burn the food. So 1.7 litres will take about 3 minuets to boil since over boiling point likely you may have full power for 5 minutes after which point you will need to turn it down. So what is the likely hood that the oven and hob will both demand max power at the same time. In practice very slim. I rarely use the boost. And I now use a microwave pressure cooker as they are faster even than using one on an induction hob.

However in a commercial kitchen it could well be a different story. Although they still tend to use gas which is of course slower than an induction hob but instead of turning it off they will swap pans so it can run on full for a lot longer. But my boost I think has max time of 5 mins anyway.

Because removing a pan auto switches off hob it is near impossible to use the power for an extended time. And the auto boil then simmer function means you don't leave pans boiling longer than required. So although the peak power on an induction hob may be more than a non induction hob the average power is a lot lower and all heat goes into the pan not into the room so kitchen is cooler and you use less power.

immaterial of diversity, there is a chance he can run this to max. if he does, you are suggesting he tries putting a load of 33.7Amps on a 32Amp MCB, driving it hot, not a clever idea imho.

I would have thought the existing cooker circuit would be fine as you can apply diversity.

edit And I have just seen your post made 2 min before mine. The OSG suggests a 30 or 32A supply is OK for a 15kW load when cooking

i know it does, just working on the bare bones. i dislike the idea of gambling. we tend to have both our daughters round with their partners quite regularly on Sundays. i can guarantee you ours is all on at the same time. we have all 4 hobs on with both ovens. mine is 10.00mm on a 50A. my MCB doesn't run hot nor does my cable.

i prefer to be 100% safe than gamble.

I would be temped at design stage to use a 10mm cable, however for a swap like this one I would use the existing circuit (unless the customer wanted to spend their money). Some years ago when I did some council rewires all the customer got was a 6mm 32A supply and councils usually overspec their jobs!

sometimes it's hard for us as sparks to advise what is ok but we wouldn't do because it doesn't mean paying out for us to do it.

a did enough rewires for councils to know what a ballache they were. used to annoy me that their own lads were allowed to do cr4p jobs yet everything we did was scrutinised with a fine tooth comb. trust me, their lads really did do cr4p jobs too, they just didn't care.

I can't see how any domestic cooker can run flat out for more than 5 minutes. Looking at fuse chart a 30A fuse will give 50A for that short time. As I said only the pressure cooker can even run that long before it has to be taken off boost. So unlikely it will ever blow the 30A fuse.

However I will admit over time the fuse specially if not sand filled will weaken and after around 5 years it may blow likely on Christmas day when doing Christmas dinner and it would be prudent to have a spare.

Do remember it's an induction hob which means even if you lift pan to stop it boiling over the power will still be switched off. I am lucky mine has knobs so as quick to turn off as lift pan. Never understood why they use touch controls? They are too slow to operate needing multi-touch rather than one simple turn.

But having use an induction hob I would have no worries about the fuse/MCB opening.

quite obvious you don't do the cooking in your house then

i know ours does a lot of Sundays cooking for 7 people.

My 13.8Kw electric range cooker quite often get abused in the same way...running on 6mm sq from a 30A fuse..only a matter of time till it burns the whole house down....

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Verwood Handyman
I Know it says I'm On-line, but I might have just left myself logged in, whilst I'm out....

Normally only cook for two. When we do cook for more the pressure cooker normally is used. I will use both the steel pressure cooker and the plastic one in order to get the quantity required and that means the microwave is doing some of the work.

However it takes around 3 minutes to boil a kettle of water that's 1.7 litre and very little in a pan has more than 1.7 litre of water. Once boiled then the hob will be put on a lower setting so no longer will it take the full power. It would be very unusual to have all the pans and the oven filled and ready for the cooker to be switched on together. All items don't take the same time so some items will already be hot before all heat areas are in use. In a commercial kitchen one may juggle the pans around leaving the cooker on full but even that with an induction hob will mean the hob switches off every time a pan is changed.

I will admit if one put a clamp-on ammeter on the cable set to measure max it may record 50A as at some point all elements and heat areas may be on together but the time at 50A would be very short and unlikely to rupture the fuse.

In the same way I am sure you can overload the whole house supply by turning on everything. But again it's rare. What we must consider is if it's likely. If looking at for example the lights in a house then yes it's likely all lights will be used together. The same with showers no point in having two showers unless they will be used at the same time. But even with just 10 sockets it's highly unlikely one will ever try to draw 130A. Even if you did only harm would be a blown fuse.

So my only worry with overloading on a cooker is the cooling fans. Loss of power means all fans stop so heat could peculate to areas where it can cause damage. But unlikely to happen too quickly so as long as a spare fuse is available then unlikely to cause a problem.

eric, your whole point is floored because you don't know my missus

i have no hair for the sake of pulling it out. after telling and telling and telling her that water cannot cook any faster once it has already reached boiling point. the amount of times i go in to find everything still on max, to be honest, it pees me off that i now just walk in after 5 mins and turn everything down myself.

She must know my missus then. She turns the thermostat on the oven up to full to get the oven to heat up quicker

i can't say too much though because i know what her response is after hearing it a few times: -

"if you don't like how i cook you can do your own food"

i quickly shut up and learn my place