joining two feeds into one consumer unit.

[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.

 

[brianmoooore]

Yes, of course, eddy currents.
My excuse is that I've been working all day up in the air in my cherry picker, with the sun beating down. Brain more than a little frazzled.

 

[jouleflow]

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.

Could this happen anyway with 4 core?
Only one cable will be carrying current in one direction with the other cable carrying the neutral and earth the opposite direction.

 

[James]

Yes, of course, eddy currents.
My excuse is that I've been working all day up in the air in my cherry picker, with the sun beating down. Brain more than a little frazzled.

Find a cold beer, it will cool your faulty brain and make things much clearer!!

 

[James]

Could this happen anyway with 4 core?
Only one cable will be carrying current in one direction with the other cable carrying the neutral and earth the opposite direction.

In a 4 core you would have 3 phases and a N conductor.
the current will balance out to be equal.
outer armour provides the earth and there should be no current or just a negligible amount.

 

[jouleflow]

Will I need 5 core swa for 3 phase? or is the armour always used for the earth?