On 15/11/16 12:46, Michael Chare wrote:
I am installing a new 4 ring Bosch induction hob to replace a Neff
ceramic hob where one of the rings had failed. I was very pleased to
find that it fits in the same size hole in the work top. (I had checked
to ensure that there would be no large gap.)

I did not look at the electrical specification very closely before I
made the purchase, but the manual says that the rings are 2500, 2500,
1800 and 3700 watts when boost function is used, so I was surprised when
I found at label saying that the maximum current is 20 amps. Presumably
the electronics makes sure that the rings are used in turn to conform to
this limit.

Yes - that is indeed what happens.

2 things:

Cookers have a diversity factor applied - that means the short term
*average* total load is not assumed to be the sum of all the elements.
It might peak above 20A with a dumb hob, but cables and fuses/breakers
are quite happy with short term overloads. Damage occurs when cables and
devices overheat, and that will not happen for 10 minutes of significant
overload. Indeed, a Type B breaker (the most common) has a tripping
characteristic designed to accommodate this.

Even if you jack all rings to full power, within a few minutes things
are boiling and the full power of the ring is no longer required.



The other factor, and the one that probably applies he this is a
smart hob and the electronics can and probably will regulate the overall
load. Some are even programmable so that they will run off lower powered
circuits, right down to a 13A plug in some cases of twin hobs I've seen.

You can in general wire these hobs up to either a 20A or 32A (more
usual) hob/cooker circuit. And for a 20A hob, no reason it cannot share
a common 32A/40A single cooker circuit with an oven. Again, ovens heat
up then start duty cycling on the thermostat fairly quickly.


The previous hob was connect by a grey flat twin and earth that goes to
the cooker switch. The new hob has its own round cable. Ideally, I would
like to connect the cables together using a junction box which I could
mount on the wall and which has a support for the cooker cable. Can
anyone suggest a suitable product?

Cooker flex outlet is the usual method:

https://www.tlc-direct.co.uk/Products/CB4506.html

Can be surface mounted if required (and there is space)

(Disconnecting the grey cable from the cooker switch might require
taking more of the kitchen apart than I would like as the cable runs
behind some tiles.)

Needless to say two of the pans that we have that I hoped might work,
actually don't work!