Cooker and hob...1 feed or 2?

[HuddsPhil]

Hi, I'm looking to get a new cooker (4.4kw) and induction hob (7.2kw) installed. I have been advised I'll need 2 feeds, one for each. I'm happy if this is correct, however I'd like to double check this first as I've had conflicting advice. The appliances will be on a 32a RCD fuse. My calculations were:
(4400+7200)/240 =48.33A.
30% of 38.33=11.5
11.5+10=21.5A, plus 5A for socket on isolator switch. My total is therefore 26.5A.

I have also calculated them separately, getting 28.5A.

I'm not an electrician and I may have made a mistake in my calculations. I'm just after a little bit of clarity and to know which advise I've had is good.

Thanks in advance.
Phil

 

[winston1]

Only one feed required as your calculation shows.

 

[Sureitsoff?]

did you mean 30% of 48.33 if so that comes to 14.5 + 10 + 5 = 29.5 so still OK

 

[HuddsPhil]

The calculation I found to use took 10a off the 48.3a I calculated (thus the 38.3), and than added it back in to get the 26.5a final value.

Thanks for checking my calculations.

 

[EFLImpudence]

(4400+7200)/240 =48.33A.
30% of 38.33=11.5
11.5+10=21.5A, plus 5A for socket on isolator switch. My total is therefore 26.5A.

Correct.

A standard cooker circuit - 32A - will be fine.

If it is new you don't have to have a socket on the switch, so even better

 

[EFLImpudence]

did you mean 30% of 48.33 if so that comes to 14.5 + 10 + 5 = 29.5 so still OK

No, he correctly deducted the 10A first without telling us.

 

[HuddsPhil]

Correct.

A standard cooker circuit - 32A - will be fine.

If it is new you don't have to have a socket on the switch, so even better

Thanks. It will be a new feed....an extra socket always comes in handy for something

 

[HuddsPhil]

No, he correctly deducted the 10A first without telling us.

Sorry for the confusion.

Am I also correct in thinking you can have one isolating switch for both appliances? Thus having the feed from the FB into an isolator which then goes to a splitter to power each appliance separately. The isolator will be 1m from both appliances. Again I've had conflicting info. Thx.

 

[EFLImpudence]

Am I also correct in thinking you can have one isolating switch for both appliances?

Yes, my view is that is better for emergencies - although few and far between.
I like to have the hood controlled by it as well.

Thus having the feed from the FB into an isolator which then goes to a splitter to power each appliance separately.

Yes, dual cooker outlet.
https://www.tlc-direct.co.uk/Products/AA45DCOP.html

The isolator will be 1m from both appliances. Again I've had conflicting info.

Well, the switch is not a requirement of the regulations at all so, obviously, there are no regulations regarding them.

A lot of people, for some reason, write recommendations - their opinions and hopefully common sense.

 

[HuddsPhil]

Yes, my view is that is better for emergencies - although few and far between.
I like to have the hood controlled by it as well.


Yes, dual cooker outlet.

Well, the switch is not a requirement of the regulations at all so, obviously, there are no regulations regarding them.

A lot of people, for some reason, write recommendations - their opinions and hopefully common sense.

Thanks. I was thinking of the dual cooker outlet, thus my confusion when it was suggested to have two isolating switches. I should be having another electrician in tomorrow...he comes with a strong recommendation so it will be interesting to see what he says. Thx.

 

[bsr]

My kitchen was similar. The electrician looked at the cable and put a 40A breaker on it. If you can do that it will give you more headroom for Xmas day. I think that's borderline for 6mm cable so will have to check routing (for cable capacity) and whether the earthing is adequate.

 

[HuddsPhil]

My kitchen was similar. The electrician looked at the cable and put a 40A breaker on it. If you can do that it will give you more headroom for Xmas day. I think that's borderline for 6mm cable so will have to check routing (for cable capacity) and whether the earthing is adequate.

That's interesting to know. I've been told now by a couple of electricians to have separate feeds. As the kitchen is still empty and I have easy access to the CU I'm going to have that done. The only shame is having 2 isolator switches, but I can live with that. Cheers.

 

[JohnW2]

That's interesting to know. I've been told now by a couple of electricians to have separate feeds. As the kitchen is still empty and I have easy access to the CU I'm going to have that done. The only shame is having 2 isolator switches, but I can live with that.

It's obviously up to you, but I would personally say that it isn't really necessary. As you've seen from your diversity calculations, a 32A circuit should be able to handle a total 'peak' load of about 83A (about 19 kW), well above your 11.6 kW. Don't forget that your entire house may well only have an "80A supply".

One hears an awful lot about 'Christmas Day' but I've never personally known of anyone experiencing that problem - and, even if they did (I assume very briefly), all they would have to do would be to, say, switch off one of the hob 'rings' and then reset the MCB.

I suppose that if you were really concerned, you could (like bsr) have a single 40A circuit (with diversity, allowing for a total of 110A / 25 kW), but I wouldn't personally bother even with that!

Kind Regards, John

 

[bsr]

John - I think part of the challenge now is the use of induction hobs with high peak loads. On my induction hob it's easy to max out demand. The hob has 2x "generators", each for two rings, so for example putting on two rings on maximum means the hob is instantly drawing max current and will continue to for some time. Therefore I think the historical diversity calculations above can be too conservative.

As an example if I get home, put on a pan of water to boil noodles, start a second pan for a stir fry, and turn on the oven to warm up desert or spring rolls - hey presto 100% of peak load for at least 10 minutes. How long would the MCB last? I believe it should thermal trip at about 10 seconds at 2x rated current?

 

[JohnW2]

John - I think part of the challenge now is the use of induction hobs with high peak loads. On my induction hob it's easy to max out demand. The hob has 2x "generators", each for two rings, so for example putting on two rings on maximum means the hob is instantly drawing max current and will continue to for some time. Therefore I think the historical diversity calculations above can be too conservative.

That might be at least partially true - I don't really know enough about the control/behaviour of induction hobs to be ablle to comment very intelligently. As I understand it, at least some (many? most?) have control systems which limit the peak instantaneous current demand - some to the extent that they come with 13A plugs.

Again, I have no personal experience, but would two (or more) rings really run at max current for an appreciable period of time (like the 10 mins you mention below) - one would not normally expect that of 'conventional' rings (which is one of the reasons why traditional diversity calcs work for them).

As an example if I get home, put on a pan of water to boil noodles, start a second pan for a stir fry, and turn on the oven to warm up desert or spring rolls - hey presto 100% of peak load for at least 10 minutes.

As above, would it really be "100% of peak load for at least 10 minutes"?

How long would the MCB last? I believe it should thermal trip at about 10 seconds at 2x rated current?

I think that's a bit optimistic/pessimistic (depending on how one looks at it!). A glance at the curves suggests that a B32 should take around 200 seconds to thermally trip - but, I agree, certainly less than 10 minutes.

Mind you, whilst that might be an argument for splitting the circuits (per OP), I don't think that changing from a B32 to a B40 (on a single circuit) would make a lot of difference to your argument.

Kind Regards, John