Size of electric cable for kitchen induction hob

[Oddbod1]

We are thinking of ordering a new induction hob for our existing kitchen. The manufacturer's web site states that it is 11kw power usage. I would like to know if our existing electrical cable (which is not on the same ring circuit as the rest of the kitchen sockets) will be capable of taking the load of the induction hob to replace an old ceramic one.

What cable thickness should this be? Would 6mm be sufficient?

 

[EFLImpudence]

Yes, a standard cooker circuit of 6mm² cable and 32A MCB is good for up to 15kW cooking appliances (possibly more).

 

[Oddbod1]

Thanks for the quick reply. Just for my lay understanding of circuit designs etc, is it normal for kitchen appliances like oven etc to be wired to their own circuit outside of the kitchen ring main?

Also when installing wet underfloor heating, is considered acceptable to have the cable laid under 100mm of kingspan/celotex boards with UFH pipes on top? I'm assuming the weight of the screed and general heat above the celotex will not be an issue or damage the cable?

 

[EFLImpudence]

Just for my lay understanding of circuit designs etc, is it normal for kitchen appliances like oven etc to be wired to their own circuit outside of the kitchen ring main?

Yes, essential.

Also when installing wet underfloor heating, is considered acceptable to have the cable laid under 100mm of kingspan/celotex boards with UFH pipes on top? I'm assuming the weight of the screed and general heat above the celotex will not be an issue or damage the cable?

If you mean the cooker circuit cable, no, not without increasing the size of the cable.
Excessive heating of the cable (by limited dissipation of its own heat; not from the water pipes) would be the problem; not the weight.

Is this how it has been done?

 

[Oddbod1]

Hi

Yes, essential.
If you mean the cooker circuit cable, no, not without increasing the size of the cable.
Excessive heating of the cable (by limited dissipation of its own heat; not from the water pipes) would be the problem; not the weight.

Is this how it has been done?

What I meant was, as a general concept, is it ok for any electrical cable to be placed under 100mm Celotex/Kingspan insulation board. The compressive forces of the screed, tiles etc and underfloor heating pipes won't damage it?

Our existing ceramic hob I know is on its own circuit so I'm not worried about that, I guess I was just wondering whether 6mm cable was sufficient because I know on some forums or on Amazon people are talking about 10mm cable for 10kw electric showers.

How does one calculate the size of the cable given the wattage of electricity to be used on the circuit?

 

[EFLImpudence]

Generally - Watts / Volts = Amps.

A 10kW shower is either on or off therefore the design current is 10000/240=41.67A.

However, cooking appliances have several heating units, making up the total wattage, which cycle on and off with thermostats so we may use the formula:
((Total Amps - 10) x 0.3) + 10 = design current.
Your 11kW hob is 45.83A, so
45.83 - 10 = 35.83 x 0.3 = 10.75 + 10 = 20.75A
There could also be an oven connected to the same circuit.

The cable rating varies with the installation method so you would choose the next size up which could carry the current safely and cope with any fault current.

 

[stillp]

A 10kW shower is either on or off therefore the design current is 10000/240=41.67A.

Don't you mean 10 000/230 = 43.48 ?

 

[EFLImpudence]

No, not really.

It will likely be quoted at 240 - even though this was just an example.
Converting to 230 will make it less and probably confuse the OP.

 

[stillp]

Converting to 230 will make it less

?

 

[EFLImpudence]

What?

10,000W @ 240V (41.67A) will be 9,180W @ 230V (39.9A)

 

[ban-all-sheds]

Converting to 230 will make it less and probably confuse the OP.

?

Well yes.

For a fixed impedance, P∝ V², so if you reduce V you reduce P.

 

[stillp]

Ah OK, you're talking about the power being less, which of course it will be, at the lower voltage. I was thinking that BS7671 requires the voltage used to calculate the circuit current to be taken as 230V.

 

[Oddbod1]

http://www.wickes.co.uk/Wickes-Twin-and-Earth-Cable-6-0mm-x-50m-6242YH/p/156256

In the above link, it says max rating 38 Amps for this 6mm cable. Can the same size/thickness of cable have different ampage?

 

[DetlefSchmitz]

Maximum rating is defined by the BS7671 regulations, in tables which require you to specify the installation method. These are the only specifications which matter. Random statements by cable vendors should be ignored.

 

[bernardgreen]

For a fixed impedance, P∝ V², so if you reduce V you reduce P.

I was thinking that BS7671 requires the voltage used to calculate the circuit current to be taken as 230V.

Calculating the current using 230 for the voltages will in some cases give a calculated current that is lower than the actual current taken by the appliance when the supply is 240 volts or 230 +10% ( 253 volts ) which is not uncommon in the UK. 253 volts is becoming more common with the increasing amount of micro generation feeding back into the local network.

Maximum current ratings for cables are generous and even when the actual current is 10% greater than the calculated current at 230 volts the calculated current can, on most cases, still be used for cable selection without the actual current exceeding the safe carrying capacity of the cable.

It would be sensible and more accurate with heaters and other resistive loads to

1) calculate the resistance of the device using the power and voltage rating from the manufacturer
2) calculate the maximum possible current using the resistance and the maximum voltage of 230+10%