Splitting and armoured supply to two consumer units.

[quattro64]

I have a job comming up where I need to run power from the house to an outbuilding into a new consumer unit and then from this building to another where there will be a smaller consumer unit. I have completed my calculations and the cable leaving the house will need to be 25mm and then 16mm on to the second smaller con unit.
Where the armoured supply comes into the first building, I was going to split it using a 3 way Pratley box with one line going to the first consumer unit and the other going to the second via a fused switch. Could someone confirm if this is a sensible way of doing it or suggest a better / easier alternative?
Many thanks in advance.

 

[Guitarist]

What power is actually required at the 1st outbuilding CU?
Is it an option to power the 2nd outbuilding CU from a breaker on the first?
Is it a PME supply, and do you have any extraneous parts in either of the outbuildings, or are you TT'ing them?

Just a few thoughts before offering any advice

 

[quattro64]

Thank you for replying Guitarist,

Requirements at first outbuilding will be 55A, 2nd outbuilding 40A and yes it is a PME supply. 2nd building will be running some outside garden lights, pool pump and air source heat pump.
Thanks

 

[Guitarist]

Rather than splitting to a switched fuse for your 16mm to the 2nd outbuilding, I would probably use an MCB in the 1st outbuilding CU. Just make sure the main switch/RCD in this CU is rated at what you are going to be drawing, if you go down this route. Just seems a neater way of doing it to me. Of course, the drawback with that would be losing power to the 2nd CU if you had to isolate the first. Anyway, food for thought

 

[quattro64]

Thanks Guitarist,
I see your point about it being neater but I was thinking about maintaining supply to the 2nd consumer unit in all eventualities. My main issue concerns exporting the earth from the house. I was planning to use 3 core swa but maybe it would be better to TT at each location, i cant seem to find a definitive opinion concerning this.
Cheers

 

[Guitarist]

You wouldn't be "exporting" the earth, but rather "extending" it.
You are right that it is an area surrounded by a lot of confusion, and there are several circumstances where it is not advisable to do so.
TT'ing is of course an option, but I wouldn't dismiss using the house earth straight away. Have a good read of the "sticky" regarding this, and maybe talk to your scheme provider, explaining the setup in detail.
At least that way, you will be able to carry out the work without worrying.