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

ive had a good look and cant see anything that says no.

:

indeed a 3 core SWA

Regs wise 521.5 - the steel armour counts as a ferromagnetic enclosure - there's an exception for additional c.p.c.s (3rd para of 521.5.1) but not for neutrals.

  - Andy.

Dont disagree with the answers already given, but given you have to install 120M of "Neutral" conductor between A and B plus treminate correctly that single conductor and be happy design and install is compliant with BS7671. Surely the labour and install costs of a 4mm or 6mm SWA for that new load will be similar if not less expensive, you design a dedicated final circuit and issue a EIC then both you and client know there is full compliance with regs and leave existing as is??

GTB

thanks for your input guys. 

i cant see how it would be different from installing 4 singles i.e. separate cable for each phase and neutral. only that in this case the 3 phase are enclosed in a single SWA.

I've had a look at the circuits now. seems like the existing cable feeds a section board (3 phase only) no loads currently on the board. they want to feed from the section board 2 125amp 3PH N supplies to 2 local distribution boards. these boards have mainly lighting and sockets plus 2 40amp ELV chargers.

im not against replacing the cable, but only want to if nessesary

thanks Andy

reading up on that regulation, i think its clear i will need to install a new cable. 

100% will need to be a new cable. thanks for pointing me in the right direction

i cant see how it would be different from installing 4 singles i.e. separate cable for each phase and neutral. only that in this case the 3 phase are enclosed in a single SWA.

Because the currents inside the steel armour do not add up to zero. and so the steel is being repeatedly magnetised and demagnetised with the difference current.

As I said in my 1st reply 3 singles without armour, or a 3 core cable with a non magnetic armour would be just fine.

SWA or for that matter un-split steel conduit, isn't OK - flow and return currents in those situations really do need to sum to essentially zero .

Mike.

Thanks Mike

yes, ive finally wrapped my head around it. all in single and the Armouring would all be aluminium. but with those cores being in steel armouring it wont work. im going to install a new supply 4 core cable. cant be wrong then