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Consumer unit MCB selection

Discuss Consumer unit MCB selection in the UK Electrical Forum area at ElectriciansForums.net

Spud1975

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Hi , looking for some advise please . So we had new house house rewired a year ago and had foresight to ask sparky to add MCB for garage with wire terminated in enclosure on outside of house in anticipation of extending it to detached garage at a later date once the internal renovation was completed. A year later and come to light that the spark has put in a 2.5mm cable on a 16a MCB. Feel this is a bit light would have preferred a 4mm cable and larger MCB , spark not to blame as I did not specify and although had foresight to connect it I did not have the foresight to check it. The cable is well buried and upgrading is not an option . So we are where we are. I have buried 12m x 2.5 mm. Armoured cable 3 feet deep across garden with electrical hazard tape above. Fitted a 4 foot earthing rod and am planning on fitting 2x surface mounted double sockets with conduit+ ferules + and a couple of LED strip lights all attached to a mini garage consumer unit which comes pre loaded with 63a , 30ma RCD + 16a +6a mcbs . So 2.5mm radial circuit = 16a max load but typically consumer unit has 2 separate breakers 1 for lights and 1sockets. So my question is as I have limited amps and want to maximise sockets is it ok to just have the one MCB at 16a and run both lights and sockets from it or would this be frowned upon? Alternatively is it ok to plug led strip lights into sockets and just use sockets as switches . Or lastly is there another combination of mcbs or an alternative solution to maximise my consumer unit. For reference usage in garage will be worse case scenario 1x small pillar drill or pressure washer or bench grinder at a time + freezer + led lights . Heat is supplied by oil burner. Any help would be greatly appreciated.
 
I'm intrigued to know why you want to feed the lights from a 16A MCB when your CU has a 6A one fitted?
So in my unedumacated little brain , if the 16a breaker is fully loaded and a partial short occurs on the 6a breaker the current draw on the combined could exceed the 16a rating on the cable ? But suppose I'm covered by the breaker on the main board ?
 
You don't need either the consumer unit or the earth rod (unless you've got metal service pipework entering the shed from underground).

If the feed from the house CU has RCD protection then you will not need RCD protection at the shed, if not then you will need RCD protection but this can be a standalone device in an enclosure.

You can just wire it as a radial circuit and use a switched fused spur or a fused spur to feed the lighting.

You may be able to change the 16A MCB for a 20A MCB but this will be subject to confirmation of the earth fault loop impedance at the shed.

Installing a consumer unit in the shed will be pointless as the MCBs in it will.not discriminate with the 16A MCB feeding it and faults will cause both to trip.
 
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