What sort of cable and route is used for the 3 phases ? for BS 7671 a neutral needs all the insulation and consideration of a 'live conductor' but that does not mean that an insulated and sheathed or insulated and aluminium  armoured cable could not be used. (on the non BS7671 side the earth probably would be the neutral anyway)

It needs to be run along side the lives - both to be obvious and to avoid current loop EMC type issues by having flow and return currents widely separated - more or less as you would do trefoil or quadrafoil with singles on tray or ladder rack.

If the existing 3 lives are in an armoured cable,  there is a potential problem if  the armour wires are ferrous, as they form a magnetic enclosure, and the neutral current is uncancelled.  Even if it is the problem  may be avoidable -- how much neutral current is expected - how well balanced will be the load ?

If it is one small 230V thing and a large 3 phase thing, it may be easier to bring in an auxiliary single phase supply, or look at a having a transformer either as 400V-230 step-down, or to regenerate a centre point of the 3 phases.

but yes, quite often  the side-along cable can be done, with caveats.

Mike.

Hi Mike. thankyou for the reply. the 3 existing cores are indeed a 3 core SWA with the armouring used as earth. loading is likely to be reasonably light. lighting sockets etc. i can cover eddy currents as the gland plates are already non ferrus. as far as i can tell the loads should be reasonably balanced. within a couple of amps anyway. 

the runs about 120meters on ladder rack. so quite a jump in cable if i have to replace the lot

Ah!

The problem is not eddy currents,  It is magnetic  hysteresis - much like cutting holes in steel boxes - if the flow and return currents do not match inside the magnetic enclose then the  material alternately magnetisies and de-magnetises in little loops surrounding the currents  on every half cycle.

Now in many versions of SWA the steel wires do not really form an unbroken circle about the current carrying conductors, but the regs do not allow much latitude and you may be on a sticky wicket. Alu armour or copper armour still has the eddy current problem so gland plates etc,  but not the magnetic one.

At this point some circuit layout info is needed. If this 250A board is really 220A of 3 wire loads and one 32A single phase lights and sockets one, you may be better to actually run out some 4 or 6mm SWA for that and not actually use the same board.

Hang in and see what others say.

Mike

Ah!

The problem is not eddy currents,  It is magnetic  hysteresis - much like cutting holes in steel boxes - if the flow and return currents do not match inside the magnetic enclose then the  material alternately magnetisies and de-magnetises in little loops surrounding the currents  on every half cycle.

Now in many versions of SWA the steel wires do not really form an unbroken circle about the current carrying conductors, but the regs do not allow much latitude and you may be on a sticky wicket. Alu armour or copper armour still has the eddy current problem so gland plates etc,  but not the magnetic one.

At this point some circuit layout info is needed. If this 250A board is really 220A of 3 wire loads and one 32A single phase lights and sockets one, you may be better to actually run out some 4 or 6mm SWA for that and not actually use the same board.

Hang in and see what others say.

Mike