13A Hob and 45A Oven isolators, confusing.

Hi Everyone,

Thank God winter is finished with for another trip around the sun.

I'm just wondering - usually, a cooker is fed with a 10mm or 6mm sq cable to a 45A FCU with a big bright red button or light. But these days with a separate oven or hob needing a 13A supply, how would these be arranged. Traditionally after using a hob or cookertop, people reach for the red switch. But in this instance, the 45A switch will knock both off instead of just the hob or oven.
To be able to isolate them individually, you'd need to split the incoming feed from the 10/6mm cable via a junction box, then into each respective switched isolator. Does that go against wiring regs I don't know.
I guess having one main 45A FCU controlling both is best, linked to a junction box which then splits the feed with one going back to a switched 13A FCU located right next to it above the worktop.

AttitudeAllStar said:

Hi Everyone,

Thank God winter is finished with for another trip around the sun.

I'm just wondering - usually, a cooker is fed with a 10mm or 6mm sq cable to a 45A FCU with a big bright red button or light. But these days with a separate oven or hob needing a 13A supply, how would these be arranged. Traditionally after using a hob or cookertop, people reach for the red switch. But in this instance, the 45A switch will knock both off instead of just the hob or oven.
To be able to isolate them individually, you'd need to split the incoming feed from the 10/6mm cable via a junction box, then into each respective switched isolator. Does that go against wiring regs I don't know.
I guess having one main 45A FCU controlling both is best, linked to a junction box which then splits the feed with one going back to a switched 13A FCU located right next to it above the worktop.

Click to expand...

I don't think isolators are confusing...

But possibly your question is confusing... What exactly are you asking?

Not 100% convinced your assumptions are accurate?
e.g. why would anybody reach for "The-Red-Switch" after using a hob or cooker-top?

Many modern single ovens can just plug into a 13A socket off a ring circuit...
No isolator.. No 6.0mm.. No 10.0mm.. Just a 13A pre-fitted moulded plug!

Most Ovens have integral clocks to allow for timed cooking operations..
But if you keep turning the power off the clock will reset to 0:00.. which is of no use at all!.

Suitable protective devices and cable sizes must be installed to supply any load..
And some isolation functions can also be incorporated in the protective device without need to additional local switch..

The majority of electrical design concepts have multiple solutions that can all meet BS7671,
And there are NO specific wiring regulations stating exactly how a cooker/oven circuit must be wired...

But a competent electrician will be able to assess the possible practical and cost effective solutions for any particular application....

Without seeing your exact property and circuit arrangements, any other advice is mostly guesswork!

Very,very few people ever turn off an oven or hob at the isolator, as Special days above, it means resetting clocks all the time.

The OP has it the wrong way round. It is the HOB that is usually the big consumer of power (unless you have bought one of those inadequate low power induction hobs that claim to run from a 13A plug) and a single oven os usually no more than 2.2kW and could run from a plug.

If your kitchen is wired with just one feed and you don't want too many alterations replace the cooker connection unit with one of the dual outlet ones. The big red switch will still turn off both hob and oven but how is that any different from an all in one cooker?

Wiring a new kitchen from scratch I put in a separate feed to hob and oven. They are not always together.

ProDave I haven't laughed so hard in a long time after reading your Plumber's van motto!

The hob here draws max 29A as is 5.8kw, the diagram on the back indicates 2.5mmsq wiring. The oven is a 4-5kw from memory. Conveerting from an old one-stop-shop cooker (with no fancy timer!) and a huge red switch that looks like it should be in a war room.

Edit: coffee seems to have kicked in, obviously a 5.8kw hob capable of drawing 29A should not be on a 13A FCU, rather as you suggest should share the load or have its own dedicated circuit. Still, if that 45A isolator switch is on a 32A breaker, it may be a problem. Maybe the 32A can be upgraded.

Diversity rules OK

Thanks Murdoch, I'm thinking the same. Would I be right in thinking should both hob and oven draw their maximum power, the 6mm cable will tolerate it ok all the way back to the MCB; and at their maximum, the 45A FCU shouldn't be affected (unless there's a fault), however, if attempting to draw above a total of 32 Amps, the 32A MCB would trip? So with diversity in mind, all should be ok. And if in the future there is nuisance tripping, I could ask the electrician to up it to a 40 or 45A MCB or probably RCBO.
Can't believe I thought a 13A FCU was needed for the hob.

if the hob is rated at 13A it does need fusing down

The 32a MCB is to protect the cable from overload.

Yes, so a 6mm cable is protected by the 32A MCB, and downstream the 6mm oven cable would also be protected by that 32A MCB, but the hob on 2.5mm might be in trouble I think. The 45A isolator wouldn't really do much if the power can never be drawn over 32A... that's my understanding...

The hob states 220-240V~/380-415V 2N~,50Hz
Class I
Hob Total Load 5.8kW
Type: C4

I'll put a picture of the wiring. If the 13A FCU would limit the hob to only 3kW (13A), then why would they make it a 5.8kW hob? I would imagine this means it is rated higher, so maybe it should have a 25A FCU? Or a 20A, especially considering the 2.5mm cable would be safe.

AttitudeAllStar said:

Yes, so a 6mm cable is protected by the 32A MCB, and downstream the 6mm oven cable would also be protected by that 32A MCB, but the hob on 2.5mm might be in trouble I think. The 45A isolator wouldn't really do much if the power can never be drawn over 32A... that's my understanding...

The hob states 220-240V~/380-415V 2N~,50Hz
Class I
Hob Total Load 5.8kW
Type: C4

I'll put a picture of the wiring. If the 13A FCU would limit the hob to only 3kW (13A), then why would they make it a 5.8kW hob? I would imagine this means it is rated higher, so maybe it should have a 25A FCU? Or a 20A, especially considering the 2.5mm cable would be safe.

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I am always amazed when you open them up and the cable size is so small!!

Is the gob prewired? 2.5 heat resistant flex is good for around 25 Amps. If the terminals are big enough,I would use 4mm or 6mm, eradicating any need to fuse down.

AttitudeAllStar said:

Yes, so a 6mm cable is protected by the 32A MCB, ..... but the hob on 2.5mm might be in trouble I think

Click to expand...

why? if the hob has a max rating of 13a then the cable cannot be overloaded. and in almost all circumstances, 32a fault protection on a 2.5 is also good, so why would it be in trouble? what about every 2.5 spur from a 32a MCB? are they in trouble too?

Andy™ said:

why? if the hob has a max rating of 13a then the cable cannot be overloaded. and in almost all circumstances, 32a fault protection on a 2.5 is also good, so why would it be in trouble? what about every 2.5 spur from a 32a MCB? are they in trouble too?

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Click to expand...

Thanks Andy - I am presuming it is down to the draw on the cable... if a 2.5mmsq cable is drawing 31.9 amps and the breaker doesn't trip, then the cable is at risk of melting isn't it? It's not designed to take 31.9? Maybe a ring main works differently. I'm not an electrician

binky said:

Is the gob prewired? 2.5 heat resistant flex is good for around 25 Amps. If the terminals are big enough,I would use 4mm or 6mm, eradicating any need to fuse down.

Click to expand...

Very clever Binky! That is absolutely the solution. The cable that is on it is a flexible 2.5mm I think, pretty strong, maybe it is 4mm, there are no markings on it, I think it's best to just replace it with 6mm and wire it exactly as it states using the 2.5mm diagram. I'll need the oven and hob to share the load, so maybe using a dual connection plate - would I be correct in saying it is more or less the same as a surface mounted junction box with three 45A-60A terminal strips.

Think this a good example of the manufacturers information being open to interpretation ......... which isn't helpful

Make and model of the hob ?

Standard 4 ring hobb, Beko Model BIC62X. I googled it and struggled !

Edit: After searching a bit further, I found a similar hob, looks the same, same brand, and same power per ring. Here is the technical info from the manual: