Replacing a cooker is 6mm cable enough?

Hello,

Fantastic forum guys! we recently moved house and are doing some changes to the kitchen, we are replacing a built-in oven and a steel plate hob for a slot-in hotpoint HUE61 cooker which according to the book has a max load of 10.7Kw the existing wire running to the cooker is 6mm2 and the circuit is protected by a 32amp MCB in the consumer unit.

Would this need to be changed to 10mm2 cable and a 45amp MCB? the length of cable from the CU to the cooker is approx 10 meters and runs under loft insulation.

The conduit running from the loft to the CU and from the loft to the cooker is only 20mm so wouldn't allow 10mm2 cable to run down it I would have to re track the walls etc to fit new conduit so if possible I'd like to stick to the 6mm2 cable.

I dont have any manuals for the existing built in oven or hob to compare against.

Advice please?

Because the cable runs through the loft insulation, this will have an effect on current capacity of the cable.
Taking everything in to consideration without allowing for any diversity, you would be looking at 16mm cable.
Without diversity your current load would be 46.5A and a derating factor needs to be applied for the method reference of the cable, as it is run through insulation.
Diversity would bring the current rating down to 21.1A(+ an extra 5A if a socket outlet is on you Cooker Control Unit) so 26.1A at worst case, so if your cable was run through insulation above 100mm thickness, you should still get away with a 6.0mm cable over the 10M distance.

Thanks PrenticeBoy, I'll stick to the 6mm2 cable and if I find the MCB is being tripped I will replace with 10mm2 and increase the MCB to a 40-45amp.

One other question - we recently replaced our old fuse box with a new CU and got our 9.5KW shower rewired with 10mm2 cable, the spark replaced the existing 6mm2 cable from the CU to the shower pull cord but said that he couldn't fit the 10mm2 cable down the conduit and that it would be fine to leave the 6mm2 cable running from the switch to the shower as it was such a short distance (1meter) I took his word for it but reading some of the shower topics on this forum has got me a little worried.

Burnside43 said:

One other question - we recently replaced our old fuse box with a new CU and got our 9.5KW shower rewired with 10mm2 cable, the spark replaced the existing 6mm2 cable from the CU to the shower pull cord but said that he couldn't fit the 10mm2 cable down the conduit and that it would be fine to leave the 6mm2 cable running from the switch to the shower as it was such a short distance (1meter) I took his word for it but reading some of the shower topics on this forum has got me a little worried.

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Except in relation to voltage drop (which is not an issue with such a short length), length of cable does not really come into it - if a cable is insufficiently large for the current it's carrying (hence at risk of overheating etc), that's true whether it is 1 metre or 50 metres in length.

6mm² cable in conduit is theoretically not quite big enough for a 9.5kW load (although it would be if the cable itself were clipped to the surface of the wall, or buried in the wall), so it theoretically ought to be 10mm².

Kind Regards, John.

Thanks John,

I think I already know the answer to this but I'll risk getting a telling off - could I strip the grey moulding and sleeve the earth and run these down the conduit and leave the grey moulding on for any wire running from the top of the conduit to the switch? or alternatively could I buy singles and run these through the conduit or would these simply not fit? I think the conduit is 25mm.

could I strip the grey moulding and sleeve the earth and run these down the conduit

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Is it 25 mm round conduit? 10mm² should fit down that. If it's oval, then it could well be next door to impossible, especially if it's been flattened or kinked at all.

Burnside43 said:

I think I already know the answer to this but I'll risk getting a telling off - could I strip the grey moulding and sleeve the earth and run these down the conduit and leave the grey moulding on for any wire running from the top of the conduit to the switch? or alternatively could I buy singles and run these through the conduit or would these simply not fit? I think the conduit is 25mm.

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As you knew we were going to say, I think you should forget the idea of manufacturing your own singhles in the way you describe, but using 'proper singles' would certainly be an option.

However, if the conduit really is 25mm, I would not have thought there would be any problem getting 10mm² T&E (and certainly not 10mm² singles) down it.

Kind Regards, John.

I wouldn't strip the the T&E as your E cable will not be insulated, you could indeed use single core cables though, single core cables do have different ratings to T&E.
But should carry 57A in conduit providing no other deratings to be calculated, ie insulation as this could bring it down to 46A. Although this should still be safe for a 9.5kW shower.