installing a separate neutral conductor

Question

Hello 
 i have a customer who currently has a 250Amp section board with no neutral conductor. (a 3 core cable installed for 3 phase and a separate earth) some new equipment is being installed that requires a neutral. I've been asked to get a neutral into the box. 
 is there any reason why i cant just add a separate neutral conductor. id argue its not best practice, but is it against regulation. ive had a good look and cant see anything that says no. 
 
 your thoughts and input are appreciated

mapj1 · Accepted Answer

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.

mapj1 · Answer

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

AJJewsbury · Answer

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.

GTB · Answer

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

installing a separate neutral conductor

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