Can I run a garage sub-main from a 50A MCB?

[rudeboy]

Hi. I Have a garage in need of power and an almost full CU on O/H TT earth, with remote 30mA RCD, between the supply and the CU (std REB fit). The garage is 25-30m from the house and another 5m from the CU.

There's an existing but faulty SWA cable to the garage, only point in mentioning this is in case fixing faults helps resolve any part P issues?
BTW, to avoid any future hassle I intend to run in the cabling, and get Mr part P to terminate and certify.

I was originally planning to split-load the CU, but Crabtree don't do a kit (for mine) and anyway I need more 'ways' for new ccts. I decided to leave the existing CU as it is and run a sub-main from a Henly block split, to a utility room on the outside wall of the house. Then fit a new CU, about 6-way, to supply the garage and 3 or 4 ccts for lighting, fridge/freezer, boiler, alarm etc, which would have otherwise been on the non RCD side of a split CU.

I've already been advised to use 16mm T&E for the sub-main and to fit a 100mA time-delay RCD incomer on the new CU - 63A I guess. This is where my sums start going wrong.

The garage will double up as workshop (when extended) and will have lights, ring, and shower-type water heater all on 30mA RCD.
Now I'm not planning on running an arc-welder or anything really juicy, and I can only use 1 power tool at a time , but a 63A RCD downline from a 45-50A MCB does seem a bit backwards?

Also earthing: I'm pretty sure a new TT earth will be required in the garage. I've checked with the REB (SEB) and no PME is available. Should I provide an additional spike for the sub-main CU as well? Is this likely to give me massive earth-loop headaches? - My stereo is VERY sensitive to earth loops, and VERY inportant for my sanity.

I don't really fancy parting with huge amounts of wedge for the armoured cable, but for the distance will 6mm SWA cable cut it? Any suggestion on what I will need?

Thanks, Rudy.

 

[Aardvark Avo]

unless the rcd is stated as 63amp with overload protection(rcbo):-

then the 63amp refers purely to the contact rating-no 63 amp overload protection.

Take a look at the discussion
here
//www.diynot.com/forums/viewtopic.php?t=99855

 

[luminaire]

The size of the cable will depend to a certain extent on your load.
If is is less than 9kW and assuming a max length of 35M then I guess 6mm SWA will be sufficient.

I would have thought that rather than having an extra CU why not change your existing one to a 100mA TD type CU, and run your cable off the 100mA side to a garage CU in the garage. You can then run your fridge and lights etc off the 100mA side and sockets etc off 30mA side.

 

[rudeboy]

Cheers Aardvark Avo, I had seen that thread but not the IET link which is particularly useful I will use RCBO

Luminaire, what you've suggested is pretty much what I originally wanted to do, although I didn't know about 100mA TD then. However, doing that involves buying a new CU anyway, and the location for my sub-main CU is along the run to the garage, also close to some of the equipment to be fed from it. Point taken though, I'll have to see how the costs balance out.

Cheers, Rudy