Cooker radial

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Hi all,

I live in a newesh built small house (7 years old) and recently had a gas leak on the pipe running under the kitchen floor (concrete) to the gas hob.

The easiest option was to isolate that supply and go for electric. My kitchen has a ccu with socket to the left of the hob and protected by a 32amp MCB at the consumer unit. This currently supplies the oven (single oven max 3kw)

To the right of the hob is another CCU without socket is a 32amp mk double pole switch marked 'HOB', runs on it's own radial back to the consumer unit and also protected by a 32amp MCB.

Without assuming anything, after removal of the gas hob and electric oven, I checked the cable diameter at both CCU's and at the consumer unit to see if they are the same thickness and is either 4mm or 6mm T&E.

The connection plate behind the oven is a typcial cooker cable connection plate, however the hob outlet also behind the oven is a 13amp unswitched single socket, without seeing the original specs or speaking to the electrician who wired the house, I assume that as this supply outlet has a single socket attached is because it is only for the gas hob ignition, which has a 13amp plug moulded on with a 3amp fuse, and therefore needs much lower protection than the 32amp supply. I can easily change the single unswitched socket to a Cooker cable outlet plate for the hob. The cable thickness from the 'HOB' 32amp DP switch to the single socket is also the same as that used for the circuit back to the consumer unit and the same as that used for the oven radial.

I wouldn't normally be this nervous, but after a gas leak, you do get a tad concerned about safety.

I consulted a retired electrician friend who said it was either 4 or 6mm.

The query I have, is the hob is rated at 6.5kw and if my sums are correct means that at full load, is rated at 28-29amps, so in my mind either circuit will handle this just fine, however, I cannot actually see what the cable really is, as there is no visible length anywhere to see the writing on it.

All I can say is that it has 7 copper strands for each conductor and the earth is single stranded, which lends me to believe that it is either 4mm or 6mm.

I looked at TLC's cable sizes for overall dimensions and the cable I have size wise, is in between the 4mm and the 6mm at about 12.5mm wide.

So, if in the worst case scenario, it is 4mm T&E, (which I would be very surprised at as it has been used for both oven and hob supply) would the circuit take the load for the hob?

The distance between the hob and the consumer unit is max 9 metres.


Thanks
 
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Before any advice is given you will need to confirm what size cable you have.

Isolate the hob supply from the CU and take the socket off. if there is enough slack available cut a small piece off it go to B & Q and check what size it is from there.

As you say the installation is approx 7 yrs old, are there no certificates for it? If you have some then on what is called the schedule of results, it's like a XL sheet, should be each individual circuit and that will give you the cable size to that circuit.
 
Hi and thankyou for your reply.

Well I've tried the house builders and they no longer have the records as they have changed hands over the years!

There was nothing with the house when I bought it as I have checked all the paperwork.

The cables are installed so precisely that there just isn't any spare anywhere to take a sample.

There is no mention on the consumer unit who the installation was made by.

I will go and get some samples for comparison though.
 
2.5mm² 10.3mm x 6mm 1.78mm
4mm² 11.9mm x 6.25mm 2.26mm
6mm² 13.5mm x 7mm 2.76mm
10mm² 17.1mm x 10mm 3.57mm


Above are cable sizes from the diynot wiki. last figure in each line is core size.
 
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Hi all

Well I've been to the hardware shop and they very kindly gave me samples of both 4mm and 6mm, both pieces in the new colours.

Although I can't compare the overall dimensions of the outer cover, I have compared the size of earth core and conductors and it appears to be 6mm afterall.

However, if it was 4mm T&E could you still run a 6.5kw electric hob on a radial?


Thanks again for your replies. :)
 
However, if it was 4mm T&E could you still run a 6.5kw electric hob on a radial?
Your calculations are correct and it could probably run on a 32Amp MCB, though that is with reference method C - clipped direct and without knowing the length of the cable for voltage drop purposes.

You would be best guided by your manufacturers instructions , since compliance with them seems to be linked to the guarantee they give.
 
Hi all

Well the maximum cable length would be 9m and runs between plasterboard and breezeblock up the wall in metal conduit then across ceiling between ground and 1st floor and down into consumer unit under stairs, the 9m includes the cable going up the wall from CCU and down to CU.
 
Sorry about missing the 9m earlier. There shouldn't be an issue with voltage drop it is less than 11.5v for 4mm or 6mm cable.
The introduction of conduit does change the current carrying capacity - ie using Reference method B instead of C the current carrying capacity for 4mm drops to 30Amps and 6mm to 38Amps.

The key thing is that the MCB should have a lower rating than the cable.

This makes the 4mm borderline without taking into account diversity.

Trust your judgement regarding the size of the cable - if as you believe it is 6mm then no problem - if you believe it to be 4mm then given the length of conduit I'm of the opinion it too would be no problem.
 

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