joining two feeds into one consumer unit.

[jouleflow]

I have two 40A swa cables from my main house consumer unit. This consumer unit serves the whole house and the two swa cables serve two consumer units in an extension. One is upstairs and has very little demand and one is downstairs with a higher demand.
My question is:
Can I add the upstairs swa into the downstairs consumer unit allowing more demand and an appropriate t+e cable back to feed the upstairs cu.
Basically I want to join the two feeds together either in the downstairs cu or another way.

This will be done by a qualified sparky but I just want to know if it is feasable before I hire him.

Thanks

Greg

 

[James]

parallel cables are allowed in the regs, however there are some pre requisites that need to be met.
it is worth asking a spark to quote but it may not be worth the effort and you might not save anything in the end.

 

[jouleflow]

parallel cables are allowed in the regs, however there are some pre requisites that need to be met.
it is worth asking a spark to quote but it may not be worth the effort and you might not save anything in the end.

Thanks James

Could the two incoming cables go into the same main isolator?

 

[pc1966]

As above, it is possible but has significant further implications for cable protection, isolation, and selectivity between protective devices if a board fed from another board.

Probably you would be better to look at having the SWA for the highly loaded board replaced with something bigger.

 

[James]

That is one of the many issues you may have.
the terminals at both ends will have to be suitable to accept both cables.
the cable run should be the same length, the same size and ideally the same route.

it is not quite a simple as you might imagine.

 

[brianmoooore]

Why is there a CU on each floor? Is it some kind of flat arrangement?

 

[jouleflow]

Thankyou all for your help, it seems that it is not such a good idea.
Can I ask if another solution is viable.
The downstairs newish metal cu has about 16 ways. It has one 100A main isolator and two double rcd's - one 80A & the other 63A.
Can this cu be used as two individual ones if I add another 100A isolator?
So the original feed supplies one half of the board and then splice into the upstairs feed to independly supply the other half. I could then balance the loads accross the board.

 

[James]

When you say you have 2 off swa cables that are 40A
what size are the cables?
i.e. 3 core 4 square mm?

 

[jouleflow]

When you say you have 2 off swa cables that are 40A
what size are the cables?
i.e. 3 core 4 square mm?

They are 3 core with an outside diameter of 20mm. I assume they are 10mm sq.

 

[pc1966]

They are 3 core with an outside diameter of 20mm. I assume they are 10mm sq.

If they are 10mm then they would usually be rated at least 60A (depends on thermal environment, etc), however, there may be other factors limiting the capacity such as the voltage drop if very long, or fault disconnection time depending on how it is powered (MCB or fuse, your supply earth loop impedance, etc).

It would be worth getting a professional electrician in to asses the origin supply characteristics, the cable & route, and sub-main protection, as it might be simply a case of changing the feed to a 60A switched-fuse arrangement for the board needing more power.

 

[jouleflow]

I'm now considering 3 phase installation from the grid.

Could I use the 2 buried existing 3 core -10mm sq swa cables in combination for the 3 phase supply from the new meter/cu/fuses to a 3 phase distribution board about 6 metres away in a new outbuilding?

i.e. one cable to carry 3 live & the other to carry the neutral and earth.

 

[James]

No you can’t
You would need a 4 core cable as a minimum.

 

[Rockingit]

The requirements for conductors in parallel, which is what you're proposing here, insist that the cables must be of equal length (so that the resistance of each is the same otherwise you don't get equal current sharing). As I'm guessing this won't be the case, by the time you've bashed about making them just so, I strongly suspect it would have been quicker and cheaper to just upgrade the existing cable.

 

[brianmoooore]

i.e. one cable to carry 3 live & the other to carry the neutral and earth.

No you can’t

Devil's advocate mood.
Why not? Neutral will only carry 3 phase out of balance current, so sized to suit the supply fuse rating of one phase, and the other conductor, paralleled with 2 x SWA wire armour should more than cover the earthing requirements.

 

[James]

Worst case is that one phase is fully loaded and others are carrying no current

so you now have 2 steel armoured cables carrying the current in one direction each.
the eddy currents will be horrible, it is for this reason that you can’t get single core swa cable.
edit,
single core aluminium wire armour is available and is ok because there is no inductive coupling.