Electrical advice, please....

[Cozzer]

It was time for a new oven.
Our 31 year old (wow!) Neff job was beginning to cause cooking problems, so we splashed out on a new Smeg number. Paid for delivery and fitting, but when it came to it, we were told that someone had bodged the wiring on the Neff and nearby 4 ring Beko ceramic hob, by cabling them together.
We had a stark choice : yes, the installer could do the same, but it would invalidate the Smeg warranty, or he could cable the hob and oven separately, but that would mean getting an electrician to add a cable back to the fuse box...not handy in our case, courtesy of a concrete floor in between. (Yes, the same concrete floor that is involved in our boiler F22 error farce! See other thread!)
He then states that the new oven is "too powerful" to connect in the same bad/old way, so leaves us with the new oven connected, but the hob not.
We have an electrician neighbour, who eventually called round to have a gander....
I don't understand about electricity, so I'll give you some numbers in an effort to get to grips with this, if I may.
We have an old bridge-type fuse box. The cooker fuse wire is rated at 30A.
The new Smeg features a sticker claiming 3mW.
The old (but currently unconnected) Beko hob boasts 5.8mW.
Both the oven installer and sparky neighbour have said we have to replace the hob and search for a "13amp" replacement, either ceramic or induction.
Spent last evening searching...these beasts are few and far between, it seems. I'm loathe to spend mega hundreds on something to replace the current hob which works (when connected!) absolutely fine!
What surprises me is that the original setup worked well for years - unfortunately the old Neff manual doesn't reveal the power situation, but has worked with 2 or 3 replacement hobs over the years without any problems.
Can anyone explain the "13amp" business to me, in Noddy language please!
Cheers....

 

[flying haggis]

A separate oven and hob can be connected as your previous set up was as it can be thought of as no different to a free standing cooker. Most oven/hob combos will be on a 30a fuse/mcb.
You do not have to have a circuit that can handle the combined max power of both appliances as a thing called diversity comes in to play ie the oven element/hob element will not be at full power all the time as it with switch off when it reaches temperature then will cycle on and off to maintain that temperature.
What are the actual wattages of each appliance?

 

[flying haggis]

Using the diversity rules your cct only needs to be rated to about 18A so well within the 30A of the supply.

The diversity rule is 10A plus 30% of the combined load with another 5A added if there is a socket on the cooker switch.

 

[guineafowl21]

I get the same as FH. Assuming the cable to the oven/hob is big enough (either 6 or 4 mm2 depending on how it’s installed), the sparky might be hung up on the requirement to install to manufacturer’s specs.

13 amp means a plug-in hob. It’s kW, not mW.

 

[Cozzer]

I get the same as FH. Assuming the cable to the oven/hob is big enough (either 6 or 4 mm2 depending on how it’s installed), the sparky might be hung up on the requirement to install to manufacturer’s specs.

13 amp means a plug-in hob. It’s kW, not mW.

Yes, my fat-finger mistake.
5.8kW....

That's the rating on the existing (unconnected as I write) hob.
The new oven claims 3kW.

If -if - my schoolboy maths/memory is correct, does that mean that if the oven and all 4 hob rings were on, the total would be 8.8kW.
Again, memory, would that mean dividing 8800 by 240, giving 36.6amps? Wouldn't that simply blow the fuse wire, or am I being too simplistic?!
(Thanks for your help here, boys...truly grateful...)

 

[Sideways]

FH is right that electricians use the idea of "diversity" in designing circuits. This is an assumption that everything won't be running full power all the time so the real current will be less than max.
Turn everything on, on your oven and hob and it will pull 36A for 10 or 15 minutes while the oven comes up to temp, the current will then drop by 13A when the oven thermostat turns off. It'll jump up again periodically while you roast that turkey or swap things in and out of the oven.
Meanwhile if you turn the rings down a bit to simmer rather than having them all on full for boiling, the hob current could be lower too.
There are rules of thumb that have been written down as standard practice but diversity is a guessing game.
Measurement is better than rules of thumb because no one knows if your use will be the same as "average". Are you someone who uses the oven to slow cook a casserole or do you cook multi course meals for the extended family.
Catering for the worst case is safest which is what your two local sparkies have been thinking.

It is a thing that properly designed wiring includes an allowance for small overloads of long duration. These are the nastiest ones to deal with because breakers let through more than their nominal currents without tripping, the wires in the wall get too hot and over time thermal damage occurs.
Because of all this, the fixed wiring is sized to handle about 40% overload for hours on end. It will get hot and the insulation will soften at this, but it shouldn't fail. It has to be robust enough to cope while the breaker warms up and eventually decides to trip.
If Christmas is a small affair and you don't normally work the cooker hard, then diversity means you should get away with it, and the oversizing designed into everyone's household wiring will let you get away with it on the odd times you do push it too hard.

 

[Spectric]

Things have changed in recent times and many modern ovens require a 16 amp supply as they are a european design so you have no option but to run it as an independant radial from the board, you cannot have any other loads connected on this such as hobs.

With the original cooker that has both oven, hob and grill diversity was allowed and mostly a 6mm cable on a 32amp protective device was used, now we have even bigger cookers so a 10mm cable on a 40 amp protective device is used which covers anything a customer may want.

It all comes down to correct design and doing the calculations but at the end of the day the customer should have what they want so

Both the oven installer and sparky neighbour have said we have to replace the hob and search for a "13amp" replacement,

that is not an approach I would take, they want the hob they have brought so change the wiring and whilst you are at it fit a modern distribution board that has modern protective devices and not wired fuses !

The cooker fuse wire is rated at 30A.

If this board has no RCD protection then you need to change it, I would not be happy undertaking any work for a customer knowing that they have only short circuit protection and nothing to protect people.

 

[MikeJhn]

If changing the CU go for a full RCBO one, that's a combined Residue Current breaker and Overload in one device it will protect all your circuits individually and not trip out the whole house for a fault if only one RCD on the board.

 

[Spectric]

RCBO's are the best way forward, unlike an RCD which takes out multiple circuits these only take out the single circuit they are protecting so you don't lose a freezer if an external light bulb blows.

 

[deema]

I was about to say the only problem with RCBO’s is the eye watering price…..and then I googled, and wow, they appear to have dropped like the preverbal stone in price compared to what I recall them being…..is that the case or am I delusional? They now appear to be listed at what I used to expect to pay for a decent MCB.

 

[MikeJhn]

Not delusional the price of RCBO's has dropped dramatically over the last couple of years to the point it is not worth considering MCB's even as replacements for existing faulty MCB's.

 

[Cozzer]

Morning, chaps...
Intrigued how our old Neff oven had worked with a series of hobs over the years without any obvious problems, we asked Neff's HQ about the power rating of the original oven.
They eventually found the model's details in their archives - having asked for serial numbers etc. - and confirmed it was rated at 2.8kW....not much different to the Smeg's 3.0.
Our neighbour sparky fitted the hob using a 60A junction box and bearing in mind the "diversity model" mentioned above, all seems well.

Cheers, boys...