Interesting idea!

I think that you would want a 3-phase MCB for the distribution circuits. AFAIK, there is no obligation to connect circuits to all of the outgoing terminals. After all, at any one time, one out of three phases may be drawing no current.

I am not sure that the phases need to be balanced so long as there is a full-size neutral.

Thanks. 

The 10mm feed could be protected by a 50A 2 pole MCB, and within the new DB, the line conductors and neutral through a 3 pole main switch for complete isolation? 

https://www.cef.co.uk/catalogue/products/40889-50a-dp-10ka-type-b-mcb

A 3 pole switch in the DNO cupboard provides a single point of isolation for the new and existing DB. 

Why not a 3-phase DB for the house - then you'd have the option of 3-phase for other equipment too (e.g. the heat pump) or workshop equipment.

It might be worth talking to your local DNO about balancing. I gather one at least (WPD?) prefer all the "normal" single phase loads to be all on the same phase - which is normally counter-intuitive, but they reckon they can balance things better overall by rotating the phases between houses than by letting individual consumers try to balance things (as most of the loads within the installation don't really co-inside time-wise so don't cancel out reliably anyway).

   - Andy.

So if you put a 2 pole MCB in the 3 phase board, will you just blank off the 3rd phase? You might as well put in a 3 phase MCB.

Hi Andy, 

The meter and cut out is on the other side of the house. The current DB is fed via a 25mm 3 core armoured. The EV currently has a 10mm 4 core armoured, therefore it would be re-utilised and used as a 2 phase main for the second DB. 

Would prefer to install an actual 3P DB but no possible. 

Why not? 

Getting a new multicore cable for 3 phases + neutral + Earth from the meter cupboard to the main DB would be impossible without significant damage / long cable routes. 

The new DB would supply a hot tub and a single 16A socket only. 

You said “To balance the phases correctly, the current EV charger supply could be utilised to feed a new DB inside of the property. The cable is a 10mm2 4 core armoured.”.

Is it like this?

www.tlc-direct.co.uk/.../SW4slash4.html

A 25mm 3 core armoured runs from the meter cupboard to the current indoor DB (single phase). 

A 4 core (as above) runs from the current DB to the current EV charger, which is located next to the meter cupboard. 

Idea would be as follows: 

Current DB - connected into L1 of new supply 

New small DB located next to meter cupboard - connected into L1, L2, L3 

3ph EV charger connected into small DB next to meter cupboard 

Existing EV charger cable connected into small DB next to meter cupboard, with L2, L3, N and Earth to new indoor DB using the existing EV cable. 

This way, I would have 3 phases inside. L1 feeding the existing DB, and L2 & L3 feeding a new DB dedicated to the hot tub and 16A socket.