Induction Hob - what is the maximum rated power hob I can install in conjunction with a with a 3.5KW oven on a 32A cooker circuit?

[plugwash]

The IET is the authority for Wiring Regulations/BS7671:2018, including inspection and certification etc.

That is true to some extent, but even BS7671s "authority" comes more from "custom and practice" than from anything legal. Though it is mentioned in some official government guidance, and now in the recent "electrical standards in the private rented sector" law.

It is also publishes the Onsite Guide (OSG)

And "guides" are another step down authority wise.

Appendix H4 covers standard cooker circuits, but Appendix A TableA2 clearly specifies a diversity allowance for cooking appliances in household installations. Hence, the OSG's diversity allowance would allow my proposed induction hob and oven on the existing 32A cooker circuit.

Yup. IIRC the guidance is first 10 amps + 30% of the remainder +5A for a socket. If you work that backwards you get a maximum of (((32 - 10 - 5) / 0.3 )+ 10) * 230 = 15333W with a socket or (((32 - 10) / 0.3 )+ 10) * 230 = 19166W without a socket on a 32 amp circuit. It's been this way for a very long time, since before induction hobs or active load control existed.

So why is an electrician permitted to ignore the IET's guidance and sell a customer an over-inflated solution "just to be safe" and "cover his own arse" (my words)?

Ultimately the IET can't force anyone to do anything.

As an aircraft design engineer I had a multitude of regulations to meet....the important word here being "meet" not "exceed". For instance, if I proposed to fit a heavier gauge wire than required, "to be extra safe" it would be rejected. Extra wire is extra weight and extra cost and reduced performance.

The aviation industry is very sensitive to weight, because increasing the weight of an aircraft has many adverse effects. They also produce a large number of planes to the same design, so the any descision is scaled by that number of planes. It's worth their while to spend huge amounts of man hours optimising their soloutions.

The electrical installatoin industry on the other hand is driven by either one man bands or small teams doing one off installs. They can be a superstitious bunch. Overkill wiring increases the upfront cost, but doesn't really incur any ongoing cost the way it would in an aircraft design.

On the other hand, doing an installation that turns out to be inadequate and getting called back can turn into a very expensive mess.

Some electricans seem to have got the idea that induction hobs are "worse" loads than conventional cooking appliances of the same rated power and/or that the diversity guidelines in the OSG are over-optimistic. I think it may have started with some smaller hobs having built in "load control", allowing them to have a lower declared maximum power for the same typical power.

Others seem happy to continue doing installs following the same old guidelines they have followed for decades.

 

[ericmark]

I would say a 32 amp supply is considered enough for a cooker with a resistive hob and oven, and an induction hob uses less power as a resistive hob, so there should be no problem using a cooker with an induction hob on a 32 amp supply, and worse case, it trips the 32 amp overload.

My cooker double oven and induction hob has been used in the last house and this one now for around 10 years, the maximum draw is around 60 amp, but it has never tripped the 32 amp MCB (last house) RCBO (this house) and unlikely it ever will. Yes, in theory one heat area can draw 3.7 kW but in practice this would burn anything but water in the pan, so it is never used at 3.7 kW likely no more than 1 kW, so in the real world no problem having an oven and induction hob from one 32 amp supply.

 

[Taylortwocities]

Actually, I found him on Checkatrade.

Please understand that these sorts of trades directories are just a list of names. Have a look at the joining requirments for Checkatrade. There are several areas that are checked before a tradesperson is added to the directory. See them here https://www.checkatrade.com/blog/trade/checkatrade-approved/

There are 11 checks. All of them are checking that you are who you say you are, dont have any CCJs etc. There is no, zero, test of the competence of the applicant. Except for the rigorous "Customer experience check". For this they may check reviews on third-party websites. And we all know how reliable those are. Just get your parents and a couple of friends to big you up and that's you, qualified.

Oh I forgot the final hurdle to join Checkatrade, or should I say Chequeatrade? Yes, pay an annual fee.
No wonder you got bad advice.

Next time go here. these are all trained, competent electricians who have their membership of the competent person schemes assessed annually. https://electricalcompetentperson.co.uk/

PS This is not imply that persons on registers like this are not capable. Some, I am sure, are excellent at what they do. But, as Forrest Gump said

 

[P drop]

Taylortwocities
Thanks for the link. I will try it

 

[securespark]

I have found that a load of latter day sparks know nothing (or choose to ignore) diversity re cooker circuits.

I've had arguments with them where they insist you cannot "overload" a 32A circuit. Crazy.

The other common argument I have is fire-rated down lights.
Watch your typical sparky short on YT re EICRs and they find down lights that are not fire rated.

I've argued with them on sites. "What's the issue in this single dwelling?" (Lights in lounge)

"You'll get fire spreading up to the bedroom."

(Moves into the hall) "There's a whacking great hole in the ceiling there..." (Points to staircase...)

 

[P drop]

I have researched this more.

I note BS7671 Chap31 Para 311.1 states "For economic and reliable design of an installation within thermal limits and admissible voltage drop the maximum demand shall be determined. In determining the maximum demand of an installation or part thereof, diversity may be taken into account"

Hence, from the start, BS7671 acknowledges that an installation should be economic and reliable as long as safety is achieved (re thermal and voltage drop). Hence, an electrician should install an economic solution ....ie if 6mm TE meets the maximum demand calculation, using 10mmTE , would be "uneconomic".

Note also the "part thereof".....ie the cooker circuit is a part of the whole domestic installation.

"diversity may be taken into account"......this appears to give discretion to use, or not use, diversity. However, the word "may" implies permission not discretion.

BS7671 makes no further mention of diversity. However, the Onsite Guide (Appendix A) gives fine detail of how the maximum demand is to be calculated. Given that Appendix A is entitled "Maximum demand and diversity allowance" would imply that using diversity should be part and parcel of maximum demand.

Finally, the IET state that the OSG is "an essential guide to BS7671". Essential implies the OSG is part of BS7671, not optional. The word "guide" should be taken as perhaps a quick reference. The word guide does not mean it is optional or of lesser authority. For example, Para 331.1 instructs that maximum demand should (ie must) be determined, and the OSG Appendix A gives the procedure to calculate it

In conclusion, I suggest a competent electrician should include diversity allowance, where permitted, to provide the customer the most economic solution compliant with BS7671.

 

[morqthana]

Finally, the IET state that the OSG is "an essential guide to BS7671".

Of course they do. Just like if you pick up a recipe card in Tescos it will tell you to use Tesco eggs, Tesco flour, etc.

 

[davelx]

P drop,

Not sure you've grasped the fact that, with the exception of rented properties covered by Landlord regulations, from a legal point of view, almost nobody has to comply with what BS7671 or the IET say.

Get several quotes, go with the one you prefer.

 

[EFLImpudence]

I note BS7671 Chap31 Para 311.1 states "For economic and reliable design of an installation within thermal limits and admissible voltage drop the maximum demand shall be determined. In determining the maximum demand of an installation or part thereof, diversity may be taken into account"

...because that's how it works.

Hence, from the start, BS7671 acknowledges that an installation should be economic and reliable as long as safety is achieved (re thermal and voltage drop). Hence, an electrician should install an economic solution ....ie if 6mm TE meets the maximum demand calculation, using 10mmTE , would be "uneconomic".

Note also the "part thereof".....ie the cooker circuit is a part of the whole domestic installation.

4mm² is adequate for a 32A cooker circuit (method C of course).
People still use 6mm² because that was required with rewireable fuses and old habits never die.

"diversity may be taken into account"......this appears to give discretion to use, or not use, diversity. However, the word "may" implies permission not discretion.

No - "may" means "is allowed to".

BS7671 makes no further mention of diversity. However, the Onsite Guide (Appendix A) gives fine detail of how the maximum demand is to be calculated. Given that Appendix A is entitled "Maximum demand and diversity allowance" would imply that using diversity should be part and parcel of maximum demand.

Finally, the IET state that the OSG is "an essential guide to BS7671". Essential implies the OSG is part of BS7671, not optional. The word "guide" should be taken as perhaps a quick reference. The word guide does not mean it is optional or of lesser authority. For example, Para 331.1 instructs that maximum demand should (ie must) be determined, and the OSG Appendix A gives the procedure to calculate it

The OSG is a very poor publication; discard it.
One has to be very experienced to use the OSG so that the mistakes in it are apparent.

In conclusion, I suggest a competent electrician should include diversity allowance, where permitted, to provide the customer the most economic solution compliant with BS7671.

Ok. Thank you.

 

[ebee]

almost nobody has to comply with what BS7671 or the IET say

Unless they are legally contracted to of course (plus the bit you mentioned about landlords law)

 

[davelx]

Unless they are legally contracted to of course (plus the bit you mentioned about landlords law)

Except that they have the choice of entering into such a contract, or not. So they are not compelled to comply.

 

[ebee]

4mm² is adequate for a 32A cooker circuit (method C of course).
People still use 6mm² because that was required with rewireable fuses and old habits never die.

Yes I`m one of those old habiters I admit.
1.0, 2.5,6.0 and sometimes 10.0 used as my bank of common sizes to keep in stock at all times, like said from the days of rewireable fuses, it meant I kept stocks costs down whilst still keeping plenty in . I avoided 4.0 because I seldom if ever actually need it and 10.0 only for higher powered electric showers really. Cookers and lassoo circuits were well served by 6.0 and a lot of the time 4.0 cost difference was not much anyway.

 

[ebee]

Except that they have the choice of entering into such a contract, or not. So they are not compelled to comply

Agreed, I was meaning once they had accepted a contract they would be obliged to but you are quite correct that they have no need, legally, to accept a contract in the first place

 

[EFLImpudence]

Registered electricians have to comply with BS7671 - and also must have an OSG unfortunately (if still the same as when I was - ten years ago now.

 

[davelx]

Registered electricians have to comply with BS7671 - and also must have an OSG unfortunately (if still the same as when I was - ten years ago now.

That's a condition of their registration with a trade body. At their choice.