RCB for 10mm circuit

I want a cable run to an adjoining garage which is to be converted into a utility room and study. The actual work will be carried out professionally, but in order to feed the circuit from my existing consumer unit, I need to try and get hold of the MCB myself, most likely second-hand.
I envisage either a 6mm T&E or a 10mm T&E being fed out there. I'm more inclined to go for 10mm actually, just so as I know there will be plenty of capacity. There will be a couple of computers out there - possibly also an oil filled radiator in the conservatory, tumble dryer and iron, and also whatever else is running in the garden.
In the converted garage, there will be a new, smaller consumer unit, to split things up as the electrician sees fit. But what rating MCB should I be looking for to fit the existing consumer unit?
I see that 6mm T&E has max load of 40A, whilst 10mm T&E has max load of 53A. In those instances, would I be looking for MCBs of 32A and 50A respectively? For my consumer unit (GE Series E), MCBs were made (once upon a time) in 32A, 40A, 53A and 60A.
Thanks
John
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John Whitworth wrote:

Cable size is determined by 2 parameters. 1) The amount of power required which is another way of saying current required. You can use a MCB smaller than the max capability of the cable, so how much power do you want?
2) The distance or length of cable. The longer the cable the greater the voltage loss.
So if you want 8kW which is about 33 Amps the voltage drop over 15meters (30m there and back) would be about 6mm2 cable - 3.8 volts 10mm2 cable - 2.2 Volts
Get your sparky to check my calcs, they are not accurate.
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OK. Thanks - I guess I really need to start totting up all of the potential stuff that could be running out there.
John
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Chewbacca missed out the method of installation. If the cable passes through thermal insulation then that may derate T&Es maximum load by 50%.
What CU do you have in the house? I have a few old MCBs knocking about.
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The 6 or 10mm cable will only pass through about 5cm of Celotex insulation, as it leaves the house, and comes into the garage. However, all of the interior power cabling (which I envisage being 2.5mm ring) will pass in and around Celotex in it's entirety. Lighting cables will be laid on top of traditional loft insulation.
The Consumer Unit is a GE Series E.
John
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The supply to the garage is fine. No derating is needed.
You might need 4mm cable for the ring in the garage.
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OK. Thanks Adam. I'll keep 4mm cable in mind. It makes sense I guess.
I've found a new Type-C Series E MCB online, but the max rating they do is 40A. Would it be OK to use that 40A with 6mm T&E? Or should that cable really be protected with 32A?
Cheers
John
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...

As long as the 6mm is not grouped up with lots of other cables on its run then 40A will be fine.
Is it a long run from the CU the garage?
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wrote:

The run will be perhaps about 8m. That's an upper estimate - maybe more like 6m. The only area where it's going to be clumped with other cables will be immediately above the consumer unit, as they go through the hallway wall.
John
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