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I ran a separate supply for the induction hob

At the time I asked the Kitchen designer for the loading of the hob that he was supplying 

He came back to me with 4kw 

Our electrician said 4 mil will be fine on its own Supply 

and easier for me to get through the ducting

Now it’s been pointed out to me that the hob is actually 7kw 

It will be very disruptive to get a 6 mil in now

My electrician friend has said don’t panic find out the loading and well sort it 

Kicking myself 

 

what are are my options 

 

 

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Isn't a 4mm cable rated at 37A, if ungrouped? That's more than 8kW, surely?

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Different hob option? 

Find out from the manufacturer what the max load for each zone is and how to stay below 4kw? Risky though if someone puts on a full load on all zones

Look at running 6mm - where can you chase cable / remove a unit to make a hole? 

Maybe the 4mm is actually ok....(no idea myself)

An idiot suggestion - Move the hob out to below the distribution board...?

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Posted (edited)

You could apply diversity...

 

The allowance here according to the OSG for a cooking appliance is 10A + 30% of the full load in excess of 10A of the connected appliance. (Add 5A if the cooker switch has a socket outlet on it).

 

So full load...7kW at 230V is 30.43A (@240V, 29.1A).

 

30% of 20.43A  (30.43-10) is 6.129A. Add the 10A. Gives you 16.129A

 

Not sure how your 4mm is run? That'll dictate the Reference Method. 4mm T&E is good for between 17.5 and 37A depending on how the cable is run. Enclosed in conduit in an insulated wall for instance is 26A.

 

Edit: Corrected

 

 

Edited by Onoff

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No way to pull the 4mm wire out with a string attached and then pull the bigger one through......

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Pull the old T&E out and pull thru 3 singles?

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My 7.*kW induction hob is run on a 30A (yes 30) breaker in clipped direct 6mm T&E then a bit of HO5 4mm butyl for the last meter. 

 

I always worry about the Christmas Dinner Effect with diversity considering modern eating habits. On ours, rather than one pot there'll be a chilli on one, pancakes in another and French fries on the third. Saying that it sort of pulses to "simmer" things so I imagine they all switch on / off at random keeping the overall draw down. Maybe I'll chuck a clamp meter on one day. 

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3 hours ago, Cpd said:

No way to pull the 4mm wire out with a string attached and then pull the bigger one through......

No it’s all boarded in and clipped 

it’s runs from the consumer unit about 15 mtrs directly to the hob 

A week ago I could have taken some boards down

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SACK the electrician. Any electrician that puts in a 4mm for a hob lacks experience, very minimum 6mm, 10mm for some.  In fact 4mm is such an odd cable size I don't normally use it, there are very few reasons to use it rather than 6mm.

 

Read the manual. Some hobs have the ability to limit maximum power by a sequence of button presses when first powered.

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I believe some  (Bosch?) induction hobs are intelligent in that that they limit the draw to circa 13A so you can plug them into a normal socket but you can have all 4 "rings" going at once.

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8 minutes ago, Onoff said:

I believe some  (Bosch?) induction hobs are intelligent in that that they limit the draw to circa 13A so you can plug them into a normal socket but you can have all 4 "rings" going at once.

That’s helpful

ill look into that

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2 hours ago, ProDave said:

SACK the electrician. Any electrician that puts in a 4mm for a hob lacks experience, very minimum 6mm, 10mm for some.  In fact 4mm is such an odd cable size I don't normally use it, there are very few reasons to use it rather than 6mm.

 

Read the manual. Some hobs have the ability to limit maximum power by a sequence of button presses when first powered.

Thanks Dave

No it my mistake 

The Electrian is real belt and braces I’d put Some conduit in the screed for an igniter for a gas hob

Not knowing that my wife would be talked into an induction hob

Neil the electrician said I would struggle to get 6 mill through and to find out the loading

As he had no experience of induction hobs

They emailed me back with 4 kw 

Instead of the 7 kw 

In your opinion do I need to replace the cable

Onoff's suggestion seems like a bit of a lifeline 

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Is it just the bit of conduit in the wall you are worried about?  If you have access to the top and bottom of the conduit then 6mm singles should fit and connect to a t&e above that for the run back to the consumer unit.

 

 

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You should always try and future proof. I've put in 25mm galv conduit in the wall to my cooker. It's just got the 6mm T&E in there at the mo:

 

Sam_1773a.jpg.8ab32350d88341038de46e3c7d5cb8f2.jpg

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1 hour ago, ProDave said:

Is it just the bit of conduit in the wall you are worried about?  If you have access to the top and bottom of the conduit then 6mm singles should fit and connect to a t&e above that for the run back to the consumer unit.

 

 

No id put less than two metres in the screed from the intended island to the wall and run the 4 mil up the wall across the plant room cieling and study down the wall into the consumer unit

Tge annoying thing is that I could have chipped some screed up and run a six mil in the same way

The ceilings are now skinned It wouldn’t be good to get a 6 mil cable in now But much worse once the kitchen and flooring is in 

So bows the time to replace If I’m going to have to doi it 

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What is the conduit buried in?

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It’s buried in screed 

 

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Posted (edited)
9 hours ago, nod said:

It’s buried in screed 

 

Maybe a small or irrelevant note, but is there value in protecting this from the cooker end?

 

eg by fitting an MCB or trip of some sort for if it risks going over the cable limit. Then you just turn one off and continue. If it only happens at Christmas or a couple of times a year, that may be viable.

 

Fixing the cable is the better way, but if that is very difficult...

Edited by Ferdinand
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I think this is a case of do it again.  Who knows what will happen in the future when you come to replace the hob.

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What's the make and model of the hob? so we can look at the instructions.

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I'm far from an expert practitioner, but I still don't see the problem. As I see it, your mitigating factors are:

  • A metal conduit in screed isn't going to insulate a warming cable, so a 37 A rating at 30 degrees will be conservative, no?
  • 7 kW is less than 30 A at 240 V, assuming a resistive load
  • The 7 kW spec is probably conservative anyway
  • The 37 A rating will be continuous, and even at Christmas you won't draw 30 A for very long (induction hobs modulate by switching the load off and on fairly fast)
  • @Ferdinand's suggestion of a breaker at the hob end seems like good belt 'n' braces if you're worried (32 A?)

I am not an electrician, this is not advice.

 

See also this remarkably similar discussion: https://www.electriciansforums.co.uk/threads/28a-in-4mm-cable-6-5kw-hob.122707/

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4mm cable is 37A in free air.  In a conduit in a wall (closest regs have to buried in a floor) is method B where it drops to 32A.  It it's an insulated wall (floor) then it's method A and drops to 26A

 

At 240V (which lets be honest most supplies are, 230V being a fictitious number) your hob will be a max of 32A

 

The only problem might be if you put 4 large pans of water on to boil all at the same time, Otherwise diversity will take care of it.

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21 minutes ago, ProDave said:

4mm cable is 37A in free air.  In a conduit in a wall (closest regs have to buried in a floor) is method B where it drops to 32A.  It it's an insulated wall (floor) then it's method A and drops to 26A

 

At 240V (which lets be honest most supplies are, 230V being a fictitious number) your bob will be a max of 32A

 

The only problem might be if you put 4 large pans of water on to boil all at the same time, Otherwise diversity will take care of it.

Thanks for all the advise

ive squeezed a 6 mil in the conduit 

I’ve had to hack a bit of screed up to remove the bend against the wall

and have used the down light holes to fish the cable to the un skimed wall by the mains

Only taken me five hours

Lesson learned 

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2 hours ago, nod said:

Only taken me five hours

Lesson learned 

But you can sleep easy and cook a better breakfast now.

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3 hours ago, nod said:

Only taken me five hours

Lesson learned 

 

Deffo the best outcome or you would always have wondered if you should have done it. Induction is great - enjoy.  And be sure to put those 4 pans on at full whack, just because you can :)

 

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