To be 13amp or not to be 13amp

[smurf59]

Question for the experts. I made a recent post re electrical appliances. One of which is an Indesit hob. Product literature is rubbish 'ensure a fuse of the correct rating is fitted'. i know some basic physics. Amps = Watts/Volts. Max rating is 7000W (3 rings with 2x2kw and 2x1.5kw). So my basic maths tells me 29 amps.
However the Indesit technical gurus are saying 13 amps, with no real explanation other than that is what the technical specification states.
Am I missing something. Any reasons anyone can think of as to why this should be the case (each ring independent?)
I'm tempted to fit 30 amps, but any help appreciated. I have given up with the Indesit HelpDesk.
By the by, a radial circuit is already fitted with 10mm cable, so I can fit to that via an Isolating fuse.
Thanks in advance.

 

[smurf59]

sorry make that 4 rings

 

[andy]

use 230V for the calculation. its over 30A. use a 40A breaker

 

[brown-nought]

7kw -- 7000 /230 = 30.5A

Allowing for diversity 10 + (30% 20.5) = 16.2A

Therefore how they get to 13A is beyond me.

16.2A would require a 20A breaker and atleast 2.5 mm cable.

However:-

1) Are you sure about the rating?
2) Did it come with a cable fitted?
3) What model no is it?

Lots of other posts about how to connect in.

You need to ensure all cables used are protected by the fuse. If its 30a that means they must be 4mm+ throughout (edit) 6mm if using re-wireable fuses (/edit)

 

[smurf59]

Interesting difference of opinion 40 amp or 20 amp. I thought that was simple!
the model is a P604IX Hob. It has no cable fitted. The instructions (as I say rubbish) say 'connect cable' !) and yes its got 4 hob plates (better than calling them rings as previously I think) totalling 7kw
As I already have a 10mm cable pre-fitted (for what I guess was a 30/45amp cooker, its no longer connected to the consumer unit..work on that to follow once I have the circuits sorted out), I was intending to run the hob on a radial circuit (i.e. the existing 10mm), even though cable will be oversized, its already in place

 

[smurf59]

Ok, I have checked up on diversity now. What a great forum. I dont think this applies as this will be a single appliance on a radial circuit? i.e. if all 4 hob plates are on there will be 7kw. So I am with the 30 amp estimate at present.....

 

[brown-nought]

Difference comes from allowing for diversity. 30.5A is the max the hob can draw. However how often do you have all the plates on at once? Therefore the regs suggest allowances (called diversity) that can be applied.

For a cooker its 10A + 30% of the remaining load + 5 if you have a socket on the cooker connection unit.

Your hob must be wired into a cooker connection unit. It must be a dedicated cooker circuit. (It may be shared with another cooker (oven) but nothing else). 10mm is a good size back to the CU. If you can terminate the cable in the hob I'd wire it in 10mm2 too, if not use 6mm. You existing fuse 30a will protect the circuit fine.

With all the rings on at once a 30a fuse will be marginally overloaded, but it will not blow - this is within the regs as well.

Main things are

Ensure the hob and Cooker unit are within 2m of each other. The switch is accessible. The circuit is the first 30A circuit in your Cu (unless you have an elec. shower).

 

[smurf59]

brown-nought, great feedback and thanks. Just what I wanted. I might have to ask the wife about potential for using all plates (ha). I was going to run just the hob on this circuit (and your right, rather than isolating fuse, I meant a dedicated cooker connection unit.
I also have a cooker (13 amp) and an extractor (13 amp) and I was thinking of feeding off the 10mm cable already in place.
However in view of what you say, and as I already have another 2.5mm radial circuit in situ, I think I will stick to just the hob on the 10mm and connect the cooker and extractor to the additional radial.
I dont yet know enough to do all of this, but i least I forward planned when I did all the woodwork andput in (spare) radial circuit for the kitchen; not connected up yet so dont panic.
Again many thanks