6mm supply cooker question....

[r6deano]

Evening all!

Fitting a kitchen for parents next week.

There new home was built approx 12yrs ago, has a plastic CU with spare ways. There's a 6mm supply going to cooker point.

Ditching the gas hob and ordered up is a 7.2kwh induction hob, and a 5.3kwh double oven.

The electrical shop we've been going to for lights is run by a spark, when ordering the new sockets and switches he asked what switches we need for cooker/hob/hood etc. Upon explaining, he feels the current 6mm should be used for the hob, and a separate radial run for the double oven. But for a spark to work on the current CU and add a circuit, because it's plastic, we would need to replace the CU with current regs metal one. Any new breaker wouldn't be covered by the existing one RCD (although why they can't be shunted down the buzz bar I don't know).

Now my spark I usually call in when I need works done, hasn't heard of not being able to add a circuit to a plastic board.

I though you guys and girls might be able to offer some advice.

Having looked for a different hob (non induction) they're still beefy on the draw so need the existing circuit. Likewise, with the ovens hard to find a double that's 3.2kwn or under. Only other option is they contact magnet kitchens and try and swap the double oven tower for a single oven tower.

Appreciate any advice.

Tia

Deano

 

[JohnW2]

.. There new home was built approx 12yrs ago, has a plastic CU with spare ways. There's a 6mm supply going to cooker point. ... Ditching the gas hob and ordered up is a 7.2kwh induction hob, and a 5.3kwh double oven. .... Upon explaining, he feels the current 6mm should be used for the hob, and a separate radial run for the double oven. ...

A 6mm² (in fact, 4mm² is usually adequate) cable on a 32A breaker is good for cooking appliance(s) up to a total of about 19 kW, so more than enough for your two items.

But for a spark to work on the current CU and add a circuit, because it's plastic, we would need to replace the CU with current regs metal one. Any new breaker wouldn't be covered by the existing one RCD (although why they can't be shunted down the buzz bar I don't know). .... Now my spark I usually call in when I need works done, hasn't heard of not being able to add a circuit to a plastic board.

As you say, as far as I'm concerned (I suppose others might disagree), it's rubbish.

Kind Regards, John

 

[EFLImpudence]

Agree. It is rubbish.

 

[plugwash]

Ditching the gas hob and ordered up is a 7.2kwh induction hob, and a 5.3kwh double oven.

When designing electrics it would ,in most cases, be ridiculous to assume that everything would be operating on full power at once. The regs therefore contain "diversity" guidelines which provide guidance as to what we should actually provision for.

The diversity guidelines for cookers in domestic premises say you should allow.

The first 10A of total cooking load.
30% of the remaining cooking load.
5A if there is a socket.

Working backwards, from a 32A supply that would allow up to about 19KW of cooking load if a plain switch is used and up to about 15KW if a cooker control unit with socket is used.

Some electricians however belive that diversity should not be allowed for induction hobs. I'm not at all convinced there is any rational basis for this belief.

But for a spark to work on the current CU and add a circuit, because it's plastic, we would need to replace the CU with current regs metal one.

Current versions of BS7671 require that consumer units in domestic premisis are made of "non-combustible material" (which practically is interpreted to mean metal).

Afaik BS7671 gives little guidance as to what extent an existing installation should be brought up to current standards when working on it. So different electricians may have differing opinions. Ultimately what matters is the opinion of the people doing/certifying the work.

Any new breaker wouldn't be covered by the existing one RCD (although why they can't be shunted down the buzz bar I don't know).

Can you post a picture of the CU?

When a CU is installed the bussbars are generally cut to accommodate the arrangement of devices that are installed. If the board was configured with more ways on the RCD side than the non-RCD side then it may be possible to swap the two busbars. If the CU is from a range that is still current it may be possible to buy a new busbar. I've heard of some sparkies using wires to make up for a busbar that is too short, but opinions are likely to vary on whether that is acceptable practice.

 

[r6deano]

Attached is the only photo I have ATM.

Having seen there's no insulation between floors , I'm pretty certain the 6mm takes a fairly direct route through the joists approx 10m in total.

From my (basic) understanding,( not knowing about the earthing set up etc), I was hoping people on here (and subsequently my spark hoping he can visit tomorrow /Friday) can confirm that 7.2kwh hob + 5.3kwh oven (no socket), giving a total max loading of 12.5.....using diversity to get 23.3A..... that the existing set up should be adequate.
So the 45a cooker plate and its existing 6mm t&e supplying a flex outlet for old cooker, could now supply the induction hob, and this flex outlet could spur off and be clipped behind service void of new kitchen to supply a final flex outlet for the double oven to be hard wired into.

Appreciate opinions. If the customer is told it's a new CU/additional radial, then it's likely they'll bail on the oven order only to potentially find out in a few days that a spark concurs on the above diversity applying....in which case would be great to keep them with their double oven tower and appliances that are on order to arrive Tuesday!

Argh!

 

[plugwash]

I can't recognise the brand of the CU at that distance (maybe others can).

Assuming the busbars match the labels it seems like you have free spaces on both the RCD and non-RCD sides of the CU. Since there are only two breakers on the non-rcd side I would guess everything except the lights are currently on the RCD.

 

[AndyPRK]

Is the hob and oven next to each other.
You will probably find one of these useful.

https://www.ebay.co.uk/itm/173876127186

it maybe a legrand Cu
And if you believe the green labelling, there are some spare ways off rcd

however 6mm should be fine for all

 

[r6deano]

I would guess everything except the lights are currently on the RCD.

Yes correct.

Is the hob and oven next to each other.

There is X1 600 unit Inbetween the hob unit and the oven tower.

it maybe a legrand Cu

Correct, it is.

Hoping to get confirmation today from someone local whom will connect the hob and oven and confirm the max 12.5kwh combo can safely be installed on the existing 6mm. I don't think customer wants to go through extra costs and making good of getting another radial to pass thru x4 areas, they'd rather downgrade the oven .

Thanks to all, and appreciate any further comments.