Connect Oven to a Cooker Outlet

[diyer56]

I have a 6kW induction hob connected to a cooker outlet in my kitchen. The cooker outlet is fed by a 32A MCB via a cooker isolator. The circuit is protected by a RCD.

I also have a 2.3kW oven which I would like to connect to the cooker outlet.

If I replace the cooker outlet with a dual outlet can I just directly connect the oven to this or will it need to be fused down?

I have checked the manual for the oven and it does not state that the oven must be protected by a fuse, only that it must have a dual pole isolator with at least a 3mm gap (this will be provided by the existing isolator).

On a second note, there is no mains cable supplied with the oven but the manual states a replacement cable (despite there not being one already) should be 3 x 1.5mm. If I can connect directly to the output without down fusing should I use 3 x 4mm cable due to the 32A MCB?

Thanks in anticipation of your help.

 

[EFLImpudence]

You can and

I If I can connect directly to the output without down fusing should I use 3 x 4mm cable due to the 32A MCB?

Yes.

 

[diyer56]

Thanks very much for your help.

 

[Risteard]

If I can connect directly to the output without down fusing should I use 3 x 4mm cable due to the 32A MCB?

You only require fault protection so 4mm^2 cable will be OTT.

 

[EFLImpudence]

Well, I thought we had agreed that although very likely true it was unwise to advise a DIYer who will not be able to determine if that is the case.

For the cost of two metres of 4mm² flex ...

 

[JohnW2]

Well, I thought we had agreed that although very likely true it was unwise to advise a DIYer who will not be able to determine if that is the case. For the cost of two metres of 4mm² flex ...

That is, indeed, what was suggested, and, in strict terms, I suppose one cannot deny that it's the correct advice to give someone who cannot confirm that fault protection is adequate.

However, in this case, I suppose that some might think that 'pragmatism' might suggest that 4mm² is a bit 'OTT' for a metre or two of cable supplying a 9-10A load.

However, as I said, strictly speaking you are correct.

Kind Regards, John

 

[diyer56]

As a DIYer that would like to learn what would you take into consideration to determine if the fault protection is adequate.

I understand that the load of the oven is low at 2.3kW. The length of cable from the outlet to the cooker is only around 1-2m.

Please understand this isn't me trying to scrimp on the price of cable. I would just like to learn what factors you would take into consideration.

For example the hob came with a length of cable which had 5 cores. They were labelled L1, L2, N1, N2 and E. At one end there were plugs which connected to the hob, at the other the lives were crimped together and the neutrals were crimped together. With this said the individual cores seemed to be about 1.5mm² each - so each pair was 3mm² - and this is supplying a 6kW appliance.

 

[JohnW2]

As a DIYer that would like to learn what would you take into consideration to determine if the fault protection is adequate.

It's not really a matter of 'taking things into consideration' - strictly speaking, the adequacy of fault protection has to be determined by measurement (of the 'loop impedance') which requires equipment that very few DIYers would have, and calculations.

I understand that the load of the oven is low at 2.3kW. The length of cable from the outlet to the cooker is only around 1-2m.

The 'fault' we are talking about (in terms of the adequacy of the 'fault protection) is a 'dead short' - although your 2.3kW oven would only take a current of 9A-10A, with the sort of fault I described, the resultant current would be hundreds of amps, and the magnitude of that potential current current can be assessed from the loop impedance measurement.

Kind Regards, John

 

[EFLImpudence]

The cable has to be able to cope with a fault to earth of negligible impedance (dead short) without being damaged before the overcurrent device (MCB) operates.

This is determined by measuring the Earth Fault Loop Impedance and thus the Fault Current.

As has been said, it is very unlikely that the cable would not be adequate but from here it is not possible to know anything which may affect the outcome.

 

[diyer56]

Thanks very much John. I thought it may have been based on the type of appliance. Following your explanation I now understand that this isn't the consideration but the 'worst case' which would be the dead short.

I will take a look at loop impedance testing to try to understand more.

What factors would increase/decrease the loop impedance of a circuit please?

 

[EFLImpudence]

Fundamentally it is the length of the circuit from the sub-station to the oven.

The nearer the sub-station you are and the shorter your circuit from CU to oven and larger your circuit conductors the lower will be the EFLI and higher the fault current.

 

[diyer56]

Thanks for being so helpful.

So as a professional you would measure at the cooker outlet and then consider if the addition of a smaller sized cable to the appliance would increase the total loop impedance to an unacceptable level?

If the impedance was increased too much (by the smaller cable) you would use the larger cable for the appliance?

Either way you would then take a measurement at the end of the flex to confirm it was acceptable and would meet the disconnect time?

 

[EFLImpudence]

So as a professional you would measure at the cooker outlet and then consider if the addition of a smaller sized cable to the appliance would increase the total loop impedance to an unacceptable level?

Mmm sort of.
It's to determine the fault current won't damage the cable because it is not big enough.

If the impedance was increased too much (by the smaller cable) you would use the larger cable for the appliance?

It won't be increased too much by the smaller cable.
It's to determine what the cooker circuit already is plus the short length of flex.

Either way you would then take a measurement at the end of the flex to confirm it was acceptable and would meet the disconnect time?

That will be done as well.
In this context it is primarily about ascertaining that the flex can cope with the fault current.

As above, it is extremely unlikely the smaller cable would not be adequate but what if ...

 

[diyer56]

And the reason the flex is unlikely to effect it is because the short length of the flex (1-2m) and the difference in diameter (2.5mm² vs 4mm²) is likely to make little difference to the EFLI compared to the 'fixed' part of the installation at 6mm²/4mm² which runs the distance from the CU to the cooker outlet?

 

[EFLImpudence]

And the reason the flex is unlikely to effect it is because the short length of the flex (1-2m) and the difference in diameter (2.5mm² vs 4mm²) is likely to make little difference to the EFLI compared to the 'fixed' part of the installation at 6mm²/4mm² which runs the distance from the CU to the cooker outlet?

Yes that is true but, although unlikely, the smaller flex (1.5mm² although you say there are two) may not be able to handle the current involved whereas the 4mm² would be.

For example, the resistance of 4m. (L and E 2m. each) of two 1.5mm² is 0.024Ω.
This is not going to make much difference to the overall EFLI but with a very low EFLI on the circuit already it may result in a fault current which may be too much for a small flex.

Say the overall EFLI is 0.2Ω, a fault of negligible impedance will result in a current of 240/0.2 = 1,200A.
EFLI of 0.4 it would be 600A but the MCB will take longer to open.
There is a formula to work out the minimum CPC (earth wire) which can cope with the current and your flex, as was stated, would probably be perfectly adequate.

However, without knowing I think it inadvisable to say on a DIY forum that it definitely will be.