Running two mini consumer units from main one, as sub-circuits

[sandypants]

Hi,

Brand new to the forum, just after some expert advice/opinions:

Amongst other circuits, I have a radial circuit installed on my main home CU: 6mm^2 cable on a 32A MCB (with RCD).  This was installed to supply power to an outdoor brick building, and then onwards to a few extra areas beyond.

I'd like to put a consumer unit on this radial circuit (in the brick outhouse) - and then another consumer unit beyond that (in a 18m distant shed) to distribute the circuits and provide protection.  Unsure whether I am overloading my original domestic 32A radial circuit by having a successive 32A circuit from the outhouse CU supplying the next CU (again, as a 6mm^2 radial).  In addition to the 2nd radial described, there will be a 16A ring main for plug sockets, a 3A lighting circuit and a 20A ring main (for washer/dryer/deep freeze) in the brick outhouse.

I wish the original radial circuit had been a 10mm^2 64A circuit - but it's too late now.

Please see the Link for a diagram of what I am intending (I doubt the dropbox link will work but I'm trying anyway, also an alternative Link).  Is this generally sound? Or should circuit 1F be a lower rating?  I would post an image of the diagram but I'm not allowed to yet.

None of these circuits will hopefully ever be drawing the maximum possible loads - far from it.  And there is plenty of protection.

I'm not doing this work myself, I'm just at en early stage in the building/rendering/damp-proofing and I want to get the conduits into the walls and the spec of the cable runs sorted before bringing in a professional sparky to install and sign off.

Thanks in advance

 

[binky]

Looks a pretty sound system, as far as overloading goes, your original 6mm is protected by a suitable MCB. Depending on total cable run distances, it may be worth extending in 10mm? You also need to consider exporting of the earthing system from the house, and possibly changing the house RCD to an MCB and installing RCD protection at the Mini-CUs for greater reliability.

 

[steptoe]

Maybe  a 100mA TD RCD at house end,? 

 

[Essex1]

You need to at the least see if you can get rid of the RCD at the house end. This will dependant on the SWA installation method and wether it is suitable earthed. This will become a nuisance especially if you have fridge/freezers which it appears you will have. 

If your outbuildings have anything that requires bonding such as a water supply or a gas supply then the whole installation probably will not comply. 

You will not achieve diversity with your installation as you will have multiple 60898 MCBs feeding distribution circuits but to be honest Indo not think this will be the end of the world in your situation but you may want to consider putting the supply to the shed CU on a 20A OCPD so you will at least achieve overload diversity. 

 

[sandypants]

Thanks mate, that's really helpful.  I should have guessed the earthing may need improving.  

Changing the house RCD to a MCB is simply a case of moving the 6mm2 radial to a half of the CU not covered by the RCD?

I can't change the 6mm2 supply for the first mini-CU but I could probably stretch to supplying the 2nd min-CU with 10mm2, but for cost the radial circuit for the x4 outdoor plug sockets I may reduce this to 2.5mm and make it a ring main.

 

[Rob_the_rich]

Not much point running ring circuits, just run radials with 2.5 t&e with 16 or 20A MCBs, save a bit of cable.

Think about using RCBOs for final circuits.

I would have fridge freezer on its own circuit, preferably with an rcbo or without RCD protection on a fused connection unit.

Is the original 6mm cable SWA or T&E? If SWA, is it 2-core or 3-core?

Does it run through insulation or hot areas? Am thinking it might need derating.

Is it buried in a wall less than 50mm deep? Am thinking you may not be able to remove the front end RCD protection.

If you have a washing machine, I assume there is water, so might have to be bonded if metal pipes.

 

[leesparkykent]

what's the distance to the first outhouse? realistically what's the design current of the 1st outbuilding and the second out building? 

 

[sandypants]

what's the distance to the first outhouse? realistically what's the design current of the 1st outbuilding and the second out building? 

It's 3m to the first building (bit of a brick shack that is being turned into a usable utility room) and then a further 18m to the shed at the bottom of the garden. 

 

[sandypants]

Not much point running ring circuits, just run radials with 2.5 t&e with 16 or 20A MCBs, save a bit of cable.

Think about using RCBOs for final circuits.

I would have fridge freezer on its own circuit, preferably with an rcbo or without RCD protection on a fused connection unit.

Is the original 6mm cable SWA or T&E? If SWA, is it 2-core or 3-core?

Does it run through insulation or hot areas? Am thinking it might need derating.

Is it buried in a wall less than 50mm deep? Am thinking you may not be able to remove the front end RCD protection.

If you have a washing machine, I assume there is water, so might have to be bonded if metal pipes.

I see what you mean, yes the 32A 6mm2 is T&E.  It is running behind some timber boxing and then penetrates through to the outer skin of the house.  It will then run down to the ground in a metal conduit before heading beneath ground to supply the first building.  Doesn't actually run through any insulation or hot areas so hopefully it won't need derating.

The plumbing is all going to be plastic.  I can sink a new earth spike to improve the earthing and get rid of the RCD.

 

[Sidewinder]

...

The plumbing is all going to be plastic.  I can sink a new earth spike to improve the earthing and get rid of the RCD.

...

Not necessarily.