Induction Hob

[symphonysteve]

I have to fit an Induction Hob in my kitchen which has a rating of 6.8kw. I have also got a double electric oven with a rating of 6.2kw. Can anyone tell me if I can connect from the oven to the hob and just use the 45amp rcd for both. cheers

 

[New2Game]

Its not as easy as that. What size cable does the exsisting supply have?

 

[symphonysteve]

The existing cable is 6.mm t & e.

 

[App Spark]

Hi, as i understand from the regs for calculating the load for a cooker circuit you should get the total demand current which in your case is about 55 amp, You can then work out what is likely to be used out of that 55 amps.
You take the first 10 amps from the 55 amps and 30% of the remainder which leaves 13.5amps, Plus the two together to give you 23.5 amps, You also add 5 amps if the control unit (cooker isolation swith) includes a socket which would give you 28.5 amps total load for the circuit. Also take into account the correction factors for how the cable is run.
So i think what you need to do is change your protective device from a 45amp to a 30amp or 32amp type B depending on your board and take two 6mm2 from your cooker isolation switch, one to the hob and one to the cooker. Hope this is helpful

 

[olly_k]

App Spark where do you get 10 Amps from? Do you have a lookup table? I mean what if your oven is 20A?
Just curious that's all!

 

[JohnD]

App Spark, I don't agree with that, even though the rules do say 10A + 30% of remainder plus 5A for a socket, I think that's a bit mean.

Consider the cooking of a christmas dinner.

Turkey in oven one; roast potatoes and stuffing in oven 2, sprouts on ring 1, pudding on ring 2, gravy on ring 3, parsnips on ring 4. With a 2kw kettle plugged in to the socket (if there is one) and the canapes under the grill (if there is one).

Although each of the heaters will be cycling on and off by its thermostat, the load won't really be as low as the chart suggests. I'd be looking more for a circuit that is capable of maintaining the max load, give or take "a bit"

The fairer use of diversity is to calculate the probable load on the meter tails, as it is less likely that, at the same time as the Christmas dinner is being cooked, the electric shower and the convector heaters will all be going full blast.

 

[Taylortwocities]

I dont have my OSG to hand but the whole diversity game is a guide and its down to the designer to decide if teh diversity percentages apply to any particular installation.

Especially with two appliances (like an oven and induction hob) I would be extra careful. Certainly I would at least use the 10A plus 30% for each and then add them together as a minimum. My gut feel for your installation is that you should be looking at 10mm².

Tayla

PS I make sure my mobile is OFF on Xmas Day - I just know that, somewhere, it will not just be the turkey that's cooking - could be the supply fuse, lets hope its not the cable..

 

[nozspark]

Surely when designing circuits you are not meant to use diversity except to calculate the maximum demand of the installation. I read it that you should use the full rating of the appliance, then select a suitable fuse / MCB, then select a suitable cable.

As JohnD said
"The fairer use of diversity is to calculate the probable load on the meter tails,....."

The way I see it this should read "The proper use of diversity......."

Back to the books me thinks..

 

[Taylortwocities]

You use diversity to calculate the ASSUMED max demand of a single circuit. Its based on the fact that not all of the devices in a house are on at the same time and most of those have thermostats etc so that you hob may not have all the rings blasting away at 100% useage (except on xmas day.)

OK so you've used diversity to estimate the assumed max demand and thats your design current
Then you select the protective device (fuse/cb)
Then you look at the environment
Then you select the cable size
Then you check the voltage drop over the cable distance

LATER
You can then add all of the assumed max demands together and get the total. (Hope its less than 100A).
Thats about the time to start thinking about size of tails - you might need to get three phases in to cope with the demand.
In design you start with the load and work back to the supply. In an existing installation you have to look at what the existing installation and supply is and judge what effect the extra load will have.

 

[nozspark]

Went straight back to my books and found it to be as Taylor said..

Me thinks I should read my books a bit more...

 

[App Spark]

I do agree with Taylortwocities and it does say in the john whitfield electricians guide which i got the information from that at christmas there would be a danger of triggering the protective device so be generous with cable.
I was just working with the fact that symphonysteve already had a 6mm in that would cope fine day to day and i was also under the assumption that he is a DIYer so changing the 6mm to a 10mm might have been bit of a problem with part P and all.