correct cable and fuse for cooker install

[candyman45]

Cooker circuit is protected at consumer unit via a 40a fuse and has isolation switch within 2m and i have a outlet plate behind the cooker..Cooker will be fitted within 2m of isolation switch.



Cooker i am putting in is rated at 7.9kw

manual states min 25a fuse and cable to be 3x 2.5 mm2 or equivalent .

Trouble is i can get some heat resistant cable 2.5mm2 but it's rated at 25amps so would you use 4.mm2 which is rated at 35a ? The thing that bothers me is that the fuse is there for protecting the cable but none of the above are higher than my fuse in CU . I like to protect the cable from a possible over load if there were a fault with the appliance and the appliances internal protection did not work and the cable over heated ,so could i use 6mm2 as this is rated past my 40a fuse or would you consider replacing the cu fuse with a smaller one ?

I know about applying diversity but just wondering about the cable and cu fuse rating.

 

[EFLImpudence]

With 40A fuse/mcb you will need 6mm² cable, or

Reduce to 30A fuse/32Amcb and use 4mm² cable.

You say the instructions state 3 x 2.5 - This will be for European application.
There will likely be an instruction for British use showing how to connect one cable with links to the other L terminals.

 

[candyman45]

With 40A fuse/mcb you will need 6mm² cable, or

Reduce to 30A fuse/32Amcb and use 4mm² cable.

You say the instructions state 3 x 2.5 - This will be for European application.
There will likely be an instruction for British use showing how to connect one cable with links to the other L terminals.

Thank you for your response .. i think the 3x 2.5mm2 mentioned is for the uk i guess as there is no other info stating anything else.Am i right in saying that the 3x2.5mm2 is just 3 core x 2.5mm2 heat resistant cable that they are referring to ? I think it's too small anyway.

Anyway i think i will go with the 6mm2 as you mentioned.One last question if say i did change the 40a fuse would an electrician have to do it ? not gonna change it but just wondered about fuses in CU and who is allowed to change them.

Thanks again

 

[PrenticeBoyofDerry]

You can change the fuse from 40A to 30/32A fuse, this is not something your require an electrician to do, providing you feel confident and competent enough to do this. Make sure you follow safe isolation procedures and prove the system is dead prior to taking on this task.
You will also require a device that is type tested to the board, so same type for brand model of unit and more than likely a B type device.
Depending on your board type/model, you may find it difficult to actually find a 25A protective device that fits your unit.

So the option 4.0mm2 heat resistant flex would be a option on 30/32A.

You say that the CCU is behind the appliance, but the isolator is within 2 metres ( I guess you mean by CCU the outlet plate and by Isolator you mean CCU/cooker control unit) could please elaborate on this.
Because having a CCU/cooker control unit behind the appliance, whether below or above worktop height is not wise. The reasons being accessibility and dangers of burns and combustion.

 

[candyman45]

Hi sorry ,yes you are correct ccu is within 2m and outlet plate is behind cooker (i have edited my op so not to confuse anyone ).I will just be using 6mm2 t&e and leaving the 40a fuse as is.. just wanted to know the answer about fuse changing if i wanted to at consumer unit which you have answered .

Thanks.

 

[PrenticeBoyofDerry]

just wanted to know the answer about fuse changing if i wanted to at consumer unit which you have answered .

Yes, strangely it seems that "schedule 4" allows protective devices to be changed without the need for any notification to building controls.
So legally you can change the fuse/MCB/RCBO without notification, providing you do not create an unsafe situation.

 

[SaladFingers]

just wanted to know the answer about fuse changing if i wanted to at consumer unit which you have answered .

Strangely? Why strange?[/b]

 

[bernardgreen]

If the connections in the cooker have more than one Live terminal then is likely to internally wired to operate on more than one phase. Many houses in Europe have 3 phase available.

For use in the UK on a single phase supply it will be necessary to link the Live terminals in the cooker, ( often a metal link is provided for this )

7.9 Kw at 230 volts is 34 amps on a single phase.

Diversity in the cooker means the 34 amps full load is unlikely to be taken for a prolonged period but may happen if every element is turned on at the same and then last until one or more of the thermostats turns off some of the elements. This short duration overload will probably not trip a 25 Amp MCB as they are designed to be slow acting on that level of over load. But it is sensible design cable size with that overload in mind , expecially the cable behind the cooker where temperature derating of current carrying capacity should be applied. Hence the 40 Amp rating of MCB and all cable is sensible when on a single phase supply.

 

[JohnD]

I saw this 4mm heat-resistant flexible cable recently, which will be a bit more expensive, but I thought very suitable for an electric cooker or oven

http://www.tlc-direct.co.uk/Products/CA4TQ3slash50.html

 

[candyman45]

I saw this 4mm heat-resistant flexible cable recently, which will be a bit more expensive, but I thought very suitable for an electric cooker or oven

http://www.tlc-direct.co.uk/Products/CA4TQ3slash50.html[/QUOTE] Seen this cable before ..4mm2 is rated at 35a .Question to you all ..Do you choose the cable size after applying diversity or do you choose it at the rating of the fuse.I have always gone with size of fuse in CU incase of an overload as the fuse is there to protect the cable so i would not use a cable smaller than the rated fuse as a precution .Thoughts please.

 

[JohnD]

fuse has to matched to cable.

both must be matched to max load

if you were relying on diversity you would reduce the fuse rating.

 

[bernardgreen]

No question in my mind. Cable rating must be greater than fuse rating.

The ratings for cables and fuses and MCBs ( apparently ) take into account that a cable can be severly overload by a current several times its rated maximum for very short duration. Safety is assured because the fuse / MCB rating will ( it is said ) ensure that the fuse / MCB will detect the overload and operate and stop the current before the wires in the cable get hot enough to melt.

 

[EFLImpudence]

7.9 Kw at 230 volts is 34 amps on a single phase.

Just to point out that the manufacturer will probably have quoted 7.9kW @ 240V.
This means a current of 32.92A.

However, as the nominal voltage for our calculations is 230V it is necessary to convert this.

At 230V the power is 7.27kW giving a current of 31.6A meaning (even without diversity) a 32A (or two 16A) opd is perfectly adequate and acceptable.

Probably why the appliance was so designed.

 

[bernardgreen]

Just to point out that the manufacturer will probably have quoted 7.9kW @ 240V.
This means a current of 32.92A.

True.. but if it is quoted at 230 volt the absolute worse case current will be 37.65 Amps if the mains voltage goes to maximum of 230 +10% (253 volts)

Probably why the appliance was so designed.

Or designed and rated for mainland Europe where 230 volt is the normal and is often 5 or 10 % lower which led to domestic 3 phase supplies becoming common in new buildings.

 

[EFLImpudence]

True.. but if it is quoted at 230 volt the absolute worse case current will be 37.65 Amps if the mains voltage goes to maximum of 230 +10% (253 volts)

We've been here before.

The nominal voltage (that used for calculations) IS 230V and therefore allowances must have been made for higher voltage situations especially as it is extremely unlikely that the voltage is ever actually 230.

Alternatively the regulations do not, when stating that the nominal voltage is 230V, add the proviso that you had better allow for 253V just in case.
If they did then the nominal voltage would be 253.

You cannot have it both ways.