Hob and cooker diversity

[adsmith]

hi guys

I’m a qualified industrial maintenance elec although not been on the tools for a couple of years. I’m looking on rewiring my home I’ve just moved into. I have been reading up a lot on the regs etc as domestic isn’t my field. Ideally I want to keep the kitchen as it is as it the Only room in the house that doesn’t require decoration therefore kitchen electrics ideally could stay the same. however I noticed the separate hob and oven are both fed off the same 32amp breaker.

I was under the impression these would ideally be separate circuits? It is fed via a 6mm cable from consumer unit to one isolator in cupboard and then 2 x 6mm feeds for both hob and oven.
The hob is rated at 6.4kw and the oven is 4.8kw so overall 11.2kw which equates to 46amp. Correct me if I’m wrong but with diversity would this be overall 20.8amp? (10 + (30% of 36amp) =20.8.
Or is diversity for each appliance seperatly ie HOB with diversity 15 + OVEN with diversity =16 so hob and oven with diversity =31amps.

So Therefore 6mm cable off a 32amp breaker is perfectly fine am I right? Or shall I bite the bullet and put in separate circuits? Just means messing up the kitchen.

So Sorry for the long winded question!!!

 

[buzzlightyear]

how far is the hob and the oven?

 

[Andy78]

I think the diversity calc was originally intended for all in one cookers, not multiple separate appliances, however it works to the same end.

Why not investigate with a clamp meter and see what the realistic current draw is ?
You could also refer to the manufacturer's connection instructions for each appliance.

My spidey senses are telling me that an additional circuit would not be necessary though.

 

[Octopus]

If I were you I would run a spare cable from the fuseboard area to under the kitchen floor.

Then it's there if you need it later

 

[adsmith]

It’s approx 6m from CU. And the oven and hob are approx 2m from eachother

 

[telectrix]

it's hobson's choice.it will be fine as it is, but a second cable (as murdoch says) is installed it gives a bit of latitude if in the future you want additional load/s.

 

[naylorpd]

s diversity for each appliance seperatly ie HOB with diversity 15 + OVEN with diversity =16 so hob and oven with diversity =31amps

I'm not an electrician but I understand a bit of maths...

When you apply the principle of diversity, you are really looking for a justification to downgrade the cable to less than the sum of all the peak loads. So it's not really a mechanical formula of 70% of this and 70% of that, but more what is the probability that in real life usage conditions the cable rating will not be (dangerously) exceeded.

If you have a circuit where there are a large number of varying loads (e.g. ring main) then the likelihood of enough individual loads to max out the circuit being on at the same time is quite low, so it's reasonable to average down.

But in a kitchen, if you are preparing a meal then the likelihood of you using the oven and the hob at the same time is rather high, perhaps almost 100%. Also you don't have a diverse range of appliances, you have only two - which, when they are on, both pull rather large loads. Thus, instead of a general formula you should look at the likelihood of the specific loads being used.

So I would be thinking along the lines:
1) What does my oven have to be doing to be drawing its full load. Basically probably the oven and the grill on together, or both ovens (if a double oven). What's the likelihood? Oven and grill together - probably quite low. If only one, I'd guess the load is about 50% but the manual may tell you.
2) For the hob, full load will clearly be when all the rings are on full. Likelihood? I can't recall ever cooking like that. At best probably two on full and maybe two simmering.
3) What's the likelihood of a typical oven load plus simultaneous hob load exceeding the cable rating? Probably quite low.

Another thought - my single oven is rated only 2.8kW so you clearly have a less efficient oven. Perhaps it's cheaper to change the oven than to put in a new cable...

But my 5-ring hob is 7.9kW and if I recall induction hobs are even higher. So if you want to also plan for the future it may be best to put the new cable in and be done with it.

A further thought on hobs - due to the large load, many hobs these days are designed to connect to 2 phases of a 3-phase supply. So if you go for installing cable it would be worthwhile putting in a 3-phase cable even if you won't ever connect it. It might save a big job for someone else in future at low incremental cost for you.

So sorry for the long-winded answer!!!