Diversity and induction hobs

[Pretty Mouth]

I recently had a discussion with another spark about induction hobs and diversity.

He pointed out that an induction hob may have, for example, 4 X 3.2kW cooking zones (total 12.8kW), but due to internal power management, the appliance could never draw more than say 7.2kW, that being shared among the zones in use. His view is that the limited maximum, in this example 7.2kW, should be the design current for the circuit, and no diversity can be applied as the maximum could easily be reached.

My conflicting view is that diversity can be applied, but should use the sum of all elements (in this example 12.8kW) for the calculation. My logic is as follows:

Take another hob, identical ratings, but this time not an induction hob, and with no power management to limit the maximum load. After diversity, it is expected to pull 24A although in theory it could pull 56A if all elements are on full simultaneously.
Compare this to the induction hob - again after diversity it would be expected to pull 24A, but due to power management could only pull a maximum of 31A, even if all zones are on full whack at the same time.

What are the views and experiences of other members of the forum on the subject?

 

[Markyd]

Depends what part you read
VERIFICATION OF ELECTRICAL INSTALLATIONS
134.1 Erection
Regulation 134.1.1Good workmanship by competent persons or persons under their supervision and proper materials shall be used in the erection of the electrical installation, Electrical equipment shall be installed in accordance with the instructions provided by the manufacturer of the equipment.

 

[Andy78]

Depends what part you read
VERIFICATION OF ELECTRICAL INSTALLATIONS
134.1 Erection
Regulation 134.1.1Good workmanship by competent persons or persons under their supervision and proper materials shall be used in the erection of the electrical installation, Electrical equipment shall be installed in accordance with the instructions provided by the manufacturer of the equipment.

My blue book says this, and so did my yellow one. You have quoted from an earlier version of the regs.

 

[Midwest]

Some interesting responses.

I have recently fitted a huge range cooker which had an electric hob. I worked out the max demand using the OSG diversity and it 'just' crept into using a 6mm cable. However the MI clearly said 50A supply with a 10mm cable. The 6mm on an appropriate MCB would have been fine but after being clearly told by the manufacturer that it needed a 10mm cable that was the right option.

Was that a Rangemaster? I had a similar thing.
[automerge]1583153796[/automerge]

My blue book says this, and so did my yellow one. You have quoted from an earlier version of the regs.

‘Twas in the green one that said, ‘shall be installed in accordance with’, I believe.

 

[HappyHippyDad]

My blue book says this, and so did my yellow one. You have quoted from an earlier version of the regs.

View attachment 56101

Yes he has, I read the new regulation (new with Amd 3 of the 17th) slightly different to the old. Now we have to take the MI into 'account' which I read as the electrician having more of a choice rather than sticking rigidly to the MI as in the older regs that @Markyd has quoted from.

Was that a Rangemaster? I had a similar thing.
[automerge]1583153796[/automerge]
‘Twas in the green one that said, ‘shall be installed in accordance with’, I believe.

Yes, I think it was a rangemaster, a beast! The first time I have had to use 10mm for a cooker!

 

[Markyd]

ok boys let me have it....didnt have my blue book with me so looked it up on IET forum...should have checked the date...2013.....big schoolboy error waiting for some stick

 

[picturevalve]

Do you have to take into account any failure of the hob power management?
Is any such failure "failsafe"?
Bet it isn't in the manufacturer literature.

 

[Pretty Mouth]

This is also my view. I have seen hobs that only ever pull though one element at a time and ones that can have all 4 on at once. Diversity has no place in connecting such appliances.

Do you think that is the case for all hobs, or just induction hobs?

 

[Wilko]

Do you think that is the case for all hobs, or just induction hobs?

My take is the induction hob has the potential to require a significant circuit upgrade if replacing a ceramic hob. To avoid adding a series buyer resistance (sorry) they’ve invested in the power management to bring it back to a “normal hob” maximum load.

 

[Andy78]

Do you think that is the case for all hobs, or just induction hobs?

I'd make the connection instructions my first reference on any electric hob ime.