DIY Doctor

Determining the amp rating of an MCB and circuit

Postby Alligator » Thu Nov 08, 2012 11:38 am

Hi All,

I'm hoping someone may be able to help me. We're thinking of replacing our existing gas only cooker with a dual fuel one. Before I do this I want to understand what KW the existing cooker circuit is capable of carrying. At the moment the cooker circuit appears to flow only to a double socket on the kitchen wall and has been used occasionally for small appliances.

The question I have is whether there is a way of determining the amp rating for the cooker circuit. The consumer unit clearly shows which one is the cooker circuit but it has no label on the MCB to confirm the amp rating. The only thing I can just about make out is that, just below the 'ON' marking there is a number 30 on the top of the MCB for the cooker, a 15 on the one marked for the immersion heater and a 6 on the one for the ground floor lighting and I'm wondering if these are the amp ratings. I do have a picture of this but I haven't been able to upload it here.

The specification for the cooker we're looking at says that it must be connected "to a suitable double - pole control unit with a minimum rating of 30A"

I understand that a 6mm cable is OK for this, but I'm hoping that we have what we need in place without the need for any new wiring and will only need to add the pole control unit in place of the existing socket.

Is there any way of confirming this from the consumer box? or would I need to isolate the circuit at the consumer unit and open up the socket in the kitchen to see what size cable feeds it?

If it's the latter, can anyone advise how I measure the cable to determine its size? I've read somewhere that the measurement should be the "cross section", however I'm not sure what this means. Does this include the outer sleeving on the whole cable, or do I look only at the size of the live, neutral or earth cables inside the outer sleeve? If so, do I only measure the core copper part or the sleeve as well?

Any guidance would be appreciated.

Thanks in advance.
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Postby ericmark » Fri Nov 09, 2012 6:14 pm

Oh dear what a tangled web we weave? In the main the consumer unit/fuse box should with MCB's or fuses stop us drawing over what the cables can take. At least in an ideal world.

So for example connecting a cooker to the 16A immersion heater spur should only result in the MCB/Fuse going open circuit. i.e. it fails safe.

In real terms this is not true and one has to be careful.

In the main cooker needs a 30A fuse or 32A MCB. With a fuse likely just 30 marked with MCB likely something like B32 marked. The B is important as the MCB is in two parts. A thermal part controls the long term amperage. But the magnet part controls the instant max. So a B32 means 5 x 32 for magnetic and 32 for thermal part of trip. C = times 10 and D = times 20.

To measure this a loop impedance meter is required. Which means in the main not a DIY job.

So being blunt although you could DIY likely cheaper to employ a scheme registered electrician. If you are intent on DIY then I will help. But be warned more expensive than getting some one in to do the work.
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Postby sparx » Sat Nov 17, 2012 6:07 pm

simple answer to a simple question would be nice......but!
CSA is cross sectional area which means measuring the diameter of each strand of the cable with a micrometer, times Pi D/2 times the number of strands!!!
OR looking at the cable size and comparing it with a cable coming from a 16A mcb which will look much smaller.
EG 6A lighting cable 1.5mm, 16a immersion circuit 2.5mm, 32A power ring circuit 2x 2.5mm's.
If not sure still call in a leckie!
regards Sparx (rtd).
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Postby Alligator » Sat Nov 24, 2012 5:36 pm

Hi Sparx,

Thanks for your advice, I've now been able to measure the existing cable based on the guidance you provided.
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