4mm cable, 32a breaker for 16.2kw Rangemaster installed by electrician

[Chris C300]

Hi, during kitchen refurbishment, the electrician installed a 4mm cable and 32a breaker for the new induction hob rangemaster oven that is rated at 16.2kw.

Same electrician couldn't connect hook up cooker, so I asked someone else to do it. The new electrician refused to connect up saying it needs a 10mm cable 45a breaker. Both professionals are in disagreement. Would be great to get a consensus here please. I want to use the oven without concern it may trip the breaker (5 induction hobs, 2 ovens and warming drawer)

Thanks

 

[timhoward]

The most important thing by far is to consult the manufacturers instructions. Do you have a link to the appliance please?

The backup plan is to do a diversity calculation according to the On Site Guide which basically works on the principle that unless you simultaneously turn all the hobs and ovens on together you won't draw the total rated power. Once on they all have their own thermostats and turn on and off when needed, so again you won't draw the maximum rated power. Some modern appliances have internal ways to limit this too.

My prediction is the instructions will agree with the 2nd electrician and the calculation will agree with the first electrician (regarding breaker size alone). The manufacturers instructions always take precedence though.

(There is a further concern with the 4mm cable and a 32A breaker but this might prove to be immaterial. )

 

[Rockingit]

I had this scenario years ago with Aga (it may even be a thread on here..) The customer was having a new Induction Range fitted directly by AGA, their installer was insisting on something ridiculous like a 16mm feed, customer called me in to fit one. Wasn't an option practically so began the dispute with Aga technical that actually they didn't understand diversity and that the existing 6mm/32A feed would suffice. Went on for days and days before Aga finally conceded that I was correct and to go ahead and fit on a 6/32. That was years ago - never heard back with a problem from the client!

I would say, however, that 4mm is probably cutting it fine once you've actually done the proper de-rating calcs.

 

[Chris C300]

The most important thing by far is to consult the manufacturers instructions. Do you have a link to the appliance please?

The backup plan is to do a diversity calculation according to the On Site Guide which basically works on the principle that unless you simultaneously turn all the hobs and ovens on together you won't draw the total rated power. Once on they all have their own thermostats and turn on and off when needed, so again you won't draw the maximum rated power. Some modern appliances have internal ways to limit this too.

My prediction is the instructions will agree with the 2nd electrician and the calculation will agree with the first electrician (regarding breaker size alone). The manufacturers instructions always take precedence though.

(There is a further concern with the 4mm cable and a 32A breaker but this might prove to be immaterial. )

The most important thing by far is to consult the manufacturers instructions. Do you have a link to the appliance please?

The backup plan is to do a diversity calculation according to the On Site Guide which basically works on the principle that unless you simultaneously turn all the hobs and ovens on together you won't draw the total rated power. Once on they all have their own thermostats and turn on and off when needed, so again you won't draw the maximum rated power. Some modern appliances have internal ways to limit this too.

My prediction is the instructions will agree with the 2nd electrician and the calculation will agree with the first electrician (regarding breaker size alone). The manufacturers instructions always take precedence though.

(There is a further concern with the 4mm cable and a 32A breaker but this might prove to be immaterial. )

Hi thanks for the response. Its a Rangemaster profesional delux induction hob. See link here:

https://www.rangemaster.co.uk/sites/default/files/user_guides_uploaded/U110890-01%20%5BGB%5D_0.pdf

Rangemaster say the following in an email following phone call:
START EMAIL
Good Afternoon,

Thanks for your earlier phone call,

We always recommend ensuring your fitter refer to the users guide before installation.

Ceramic and induction models may be connected to a suitable 6mm cable with a 32-amp breaker (providing the isolator switch, which turns the power on and off to the cooker, does not incorporate a 13 amp socket). Based on *domestic cooker diversity it would not be necessary to upgrade to a 10mm cable / 45 amp fuse, however we would recommend you do so, as it would be better to provide a greater load flexibility. We would recommend that induction cookers are also fitted to a RCBO fuse (Residual Current Breaker with Overload protection).

Please note: These are guidelines and other factors may alter the cable/breaker power. Therefore we cannot provide electrical information as the qualified electrician has to make calculations based on what is around the cooker e.g. boilers etc.
END EMAIL

the electrician is adamant he has done calculation and 4mm is enough.

Thanks again

 

[Chris C300]

I had this scenario years ago with Aga (it may even be a thread on here..) The customer was having a new Induction Range fitted directly by AGA, their installer was insisting on something ridiculous like a 16mm feed, customer called me in to fit one. Wasn't an option practically so began the dispute with Aga technical that actually they didn't understand diversity and that the existing 6mm/32A feed would suffice. Went on for days and days before Aga finally conceded that I was correct and to go ahead and fit on a 6/32. That was years ago - never heard back with a problem from the client!

I would say, however, that 4mm is probably cutting it fine once you've actually done the proper de-rating calcs.

Thanks, good to know. But there is nowhere I can see confirming 4mm will be enough

 

[Rockingit]

Thanks, good to know. But there is nowhere I can see confirming 4mm will be enough

In the most perfect of conditions, what I assume to be 6242Y (standard twin and earth) at 4mm is rated for 36A. However, at around lets say a 15m run from your CU to where it's actually running to, you'll lose roughly 6v running at 32A, plus you're also asking the cable to run at the very extreme end of it's 70C rating - it will probably feel very hot to touch and as a consequence probably degrade fairly quickly by comparison to 6mm which is 46A in the same scenario.

 

[timhoward]

The calculation comes out at 28A if ordinary isolator and 33A if there's a socket on the isolator. So the email makes sense to me

Thanks, good to know. But there is nowhere I can see confirming 4mm will be enough

Sorry, we can't give you a definite yes over the internet.

The cable rating depends on it's route, length, what is passes through, and how it's attached. Clipped direct to a surface it's higher rated than floating in middle of insulation or in conduit. 4mm rating can be as low as 17.5 amps and as high as 36 amps.

The entire circuit design has to be sufficient for the load (32A is ok for 28A load but more headroom is definitely better).
The breaker has to rated at less than the max cable rating (32A is only < cable rating in the scenario it is clipped direct; 32A > cable rating in every other scenario).
The max cable rating has be be sufficient for the load (definitely at upper limit in most of the scenarios, insufficient in other scenarios)

So it's certainly all a bit tight. But we can't say what is there is definitely wrong.
(My biased arrogant opinion is that 4mm was a slightly daft and inflexible choice - but that doesn't help you.)

 

[Rockingit]

(My biased arrogant opinion is that 4mm was a slightly daft and inflexible choice - but that doesn't help you.)

Agreed!! Given the pence difference it would have been. I think if I went back to doing domestic new-build again [shudder], I'd be laying in 10mm for cookers and showers as a default.

 

[brianmoooore]

In this particular case, it looks like both electricians might be wrong. 4mm2 is pushing the limits one way, and 10mm2 with 45A fusing is over the top the other.
6mm2 on 32A would be fine (installation method dependant), but as Rockingit wrote above, if I were changing it, I'd lay in 10mm2.

 

[Chris C300]

In this particular case, it looks like both electricians might be wrong. 4mm2 is pushing the limits one way, and 10mm2 with 45A fusing is over the top the other.
6mm2 on 32A would be fine (installation method dependant), but as Rockingit wrote above, if I were changing it, I'd lay in 10mm2.

Hi, thanks everyone for your comments

In this particular case, it looks like both electricians might be wrong. 4mm2 is pushing the limits one way, and 10mm2 with 45A fusing is over the top the other.
6mm2 on 32A would be fine (installation method dependant), but as Rockingit wrote above, if I were changing it, I'd lay in 10mm2.

Thanks all, here are the calcs from installer aimed to reassure me. Final thoughts on this would be really appreciated. At this point looks like I will be asking him to change to 10mm, may as well if changing from 4mm as you say. But there will no doubt be a difference of opinion. Thanks again

INSTALLER CALCS:
Your range is rated at 16kw, this equates to 69.57a (16000/230).

This figure isn't the end of the matter however.

BS7671 states we are to apply a principle called diversity. Your range oven even with everything on won't pull 69.57 amps as it's not a purely resistive load. -Elements actually switch on/off to keep temperature.
Diversity law states you calculate your actual cooker load as 10 amps plus 30% of the remainder. (In your case 10 amps plus 30% of 59.57 amps or 17.87 amps.

This equates to 27.87 amps.

Now we know the actual load we must design the circuit so the safety device (mcb) exceeds this, (yours is a 32a device); and the cable current carrying capacity exceeds the safety device. I chose 4mm cable which can carry 37a when it's clipped direct to wood, and not grouped with other circuits ie in your type of installation.

Basically the maths is to ensure the cooker can pull its load without the breaker going.
If something goes wrong, the breaker will go before the cable reaches its maximum load
In your case load 27.87a < 32a mcb < 37a cable.

 

[UNG]

Hi thanks for the response. Its a Rangemaster profesional delux induction hob. See link here:

https://www.rangemaster.co.uk/sites/default/files/user_guides_uploaded/U110890-01%20%5BGB%5D_0.pdf

Rangemaster say the following in an email following phone call:
START EMAIL
Good Afternoon,

Thanks for your earlier phone call,

We always recommend ensuring your fitter refer to the users guide before installation.

Ceramic and induction models may be connected to a suitable 6mm cable with a 32-amp breaker (providing the isolator switch, which turns the power on and off to the cooker, does not incorporate a 13 amp socket). Based on *domestic cooker diversity it would not be necessary to upgrade to a 10mm cable / 45 amp fuse, however we would recommend you do so, as it would be better to provide a greater load flexibility. We would recommend that induction cookers are also fitted to a RCBO fuse (Residual Current Breaker with Overload protection).

Please note: These are guidelines and other factors may alter the cable/breaker power. Therefore we cannot provide electrical information as the qualified electrician has to make calculations based on what is around the cooker e.g. boilers etc.
END EMAIL

the electrician is adamant he has done calculation and 4mm is enough.

Thanks again

Looks like you have had a definitive answer from the manufacturer here as others have said for a new installation I would err towards 10mm² cable but wouldn't have a problem connecting to a 6mm² if that was already fitted 4mm² is just a bit too thin for this load

 

[Electro-tech]

This link may be useful to the OP.

 

[davesparks]

In the most perfect of conditions, what I assume to be 6242Y (standard twin and earth) at 4mm is rated for 36A. However, at around lets say a 15m run from your CU to where it's actually running to, you'll lose roughly 6v running at 32A, plus you're also asking the cable to run at the very extreme end of it's 70C rating - it will probably feel very hot to touch and as a consequence probably degrade fairly quickly by comparison to 6mm which is 46A in the same scenario.

A 6V drop is well within what is allowed by the regulations.

The cable is designed to work safely at a conductor operating temperature of 70C, all necessary safety margins have already been allowed for in the tabulated values.

I always find it odd that people will happily install almost any other 32A circuit in 4mm but for cookers they refuse to accept anything less than 6mm

 

[davesparks]

I don't have a book to hand but it doesn't look like anyone has considered rating factors in this thread, bearing in mind they can increase cable ratings in some circumstances.

 

[Rockingit]

A 6V drop is well within what is allowed by the regulations.

The cable is designed to work safely at a conductor operating temperature of 70C, all necessary safety margins have already been allowed for in the tabulated values.

I always find it odd that people will happily install almost any other 32A circuit in 4mm but for cookers they refuse to accept anything less than 6mm

The principle difference is simply time. It's reasonabl to assume that a 32A radial circuit instead of a 32A RFC (for example) will peak/trough all over the place and average out at only a minor % whereas something like an oven will be drawing full whack for some time until it gets to temperature.

I don't have a book to hand but it doesn't look like anyone has considered rating factors in this thread, bearing in mind they can increase cable ratings in some circumstances.

Like this, you mean? "I would say, however, that 4mm is probably cutting it fine once you've actually done the proper de-rating calcs"