Oh dear... time for school!

[RF Lighting]

As far as I'm aware the neutral always has to be as large as the phase conductor when a neutral is required.

I have seen reduced CSA neutrals to TP&N boards, but this was on installations from 60 or 70 years ago, and is only possible if the circuit is fed in singles in containment!

 

[JohnW2]

As far as I'm aware the neutral always has to be as large as the phase conductor when a neutral is required.

As I said/implied, that's what I would have expected, at least with a circuit feeding a socket. I suppose it might be different if it were a dedicated circuit for a single hard-wired (and fairly balanced) 3-phase load - but, even then, there could well be concerns that the load might in the future be changed to something less (or not at all!) balanced.

Kind Regards, John

 

[Risteard]

I have seen reduced CSA neutrals to TP&N boards, but this was on installations from 60 or 70 years ago, and is only possible if the circuit is fed in singles in containment!

It's more recent than that, and there were 3 1/2 core cables made for it so it's not just singles.

 

[mfarrow]

Some would argue a distribution board supplying 30A balanced three phase load, and 30A single-phase loads, only needs an incoming neutral sized for 30A.

 

[bernardgreen]

Some would argue that a Neutral is not needed in a feeder when the loads on the three phases are all equal ( such as a delta connected motor ) and there is no current in the neutral

But having no Neutral in the feed plays havoc when the loads on the phases become un-balanced and the "neutral" current wants a path back to the tranny supplying the feeder.

So 32 amps on all the phases would not require a 32 amp neutral conductor. 32 amps on only one phase would require a 32 amp neutral conductor. What would the current in the neutral conductor be if there was a 32 amp load on each of two of the phases ( wired as two 32 amp 230 volt supplies each between a phase and the Neutral ).

Could it be that the Neutral would need to carry more than 32 amps in this un-balance situation

Some one else can do the phase / vector calculations.

 

[Adam_151]

I have seen reduced CSA neutrals to TP&N boards, but this was on installations from 60 or 70 years ago, and is only possible if the circuit is fed in singles in containment!

As another poster alludes to, there were certainly SWA cables made with a half size neutral. Though I can only bring to mind seeing such a situation once. Though the age is probably not far off what you suggest, I'd guess it was probably late 50s early 60's (Huge 12way bill 100A rewireable board with 450A incomming supply and feeding all the boards in one side of a factory that used to be a foundry)

These days I suppose we are getting closer to needing Double neutrals due to the amount of harmonic distortion from electronic loads

 

[JohnW2]

So 32 amps on all the phases would not require a 32 amp neutral conductor. 32 amps on only one phase would require a 32 amp neutral conductor. What would the current in the neutral conductor be if there was a 32 amp load on each of two of the phases ( wired as two 32 amp 230 volt supplies each between a phase and the Neutral ). ... Could it be that the Neutral would need to carry more than 32 amps in this un-balance situation ... Some one else can do the phase / vector calculations.

No - and one doesn't even have to do any trig to discover that. If you draw the vectors, you'll see that you simply end up with an equilateral triangle - so the vector sum of two 32A currents at 120° is 32A. If there were any current in the third phase, that would reduce that figure. Hence, at least in terms of these simple calculations (ignoring harmonics etc. - and I havn't yet thought about the effects of PF), the maximum neutral current that could ever arise in a 3 x 32A 3-phase circuit is 32A, no matter how unbalanced the load.

Kind Regards, John

 

[JohnW2]

Some would argue a distribution board supplying 30A balanced three phase load, and 30A single-phase loads, only needs an incoming neutral sized for 30A.

As I've just written, if the currents in the three phases are 120° apart, and if one forgets complications like harmonics, it's not a case of something that "some would argue", it is AFAICS actually an inevitable truth that if the maximum current in any one phase is 30A, then the neutral current cannot be greater than 30A.

Kind Regards, John

 

[RF Lighting]

I have seen reduced CSA neutrals to TP&N boards, but this was on installations from 60 or 70 years ago, and is only possible if the circuit is fed in singles in containment!

As another poster alludes to, there were certainly SWA cables made with a half size neutral. Though I can only bring to mind seeing such a situation once. Though the age is probably not far off what you suggest, I'd guess it was probably late 50s early 60's (Huge 12way bill 100A rewireable board with 450A incomming supply and feeding all the boards in one side of a factory that used to be a foundry)

These days I suppose we are getting closer to needing Double neutrals due to the amount of harmonic distortion from electronic loads

Here's a 400A TP supply for some dimmers we installed. It's 2x 120mm² 4 core SWAs run in parallell for the supply to oversize the neutral due to the harmonic currents.

[/quote]

 

[ericmark]

So you need to resolve the problem without upsetting the client. To get single phase from a three phase supply will require a transformer with zig zag wired secondary I have seen it done with generators but not transformers at a quick guess I would say a 110 volt three phase output could be wired to give a 230 volt single phase supply. It could also be done with an inverter but looking at around the £1000 for the unit.

So it would likely be cheaper if the neutral will not allow the full 32A to be drawn to install a new neutral rather than buy the converter unit.

Since the cost would be passed to the university you might suggest it would cheaper to install an new neutral than use other methods to get around the lack of single phase capacity on the socket and would also ensure in the future no errors were made when supplying other equipment.

Yes I realise the guy has no idea but to tell the university that could upset them and cause you to lose the contract. Also you want to give the electrician a get out clause and once he realises his error he can claim he's fitted a new neutral so no lost of face.

 

[JohnW2]

So it would likely be cheaper if the neutral will not allow the full 32A to be drawn to install a new neutral rather than buy the converter unit.

As discussed, in terms of recent times (rather than 50+ years) ago, it is surely almost certain that the neutral conductor will be the same size as the phase ones, hence suitable for a 32A single-phase load (IF it is a 32A circuit). As I've recently written, if the currents are 120° apart and on doesn't consider harmonics, then AFAICS if the maximum current flowing in any of the three phases were 32A, then the neutral current could not possibly be greater than 32A, regardless of how unbalanced the load was.

The question which the OP has got to resolve with the university (and its electrician) is whether the 32A socket in question is fed from a 10A or 32A 3-phase circuit. If it transpires that it is a 32A circuit, then it's hard to see what 'get out clause' one could offer the uni's electrician!

Kind Regards, John

 

[AdrianUK]

So you need to resolve the problem without upsetting the client. To get single phase from a three phase supply will require a transformer with zig zag wired secondary I have seen it done with generators but not transformers at a quick guess

Yes it could be done with an open-delta transformer configured as 400/127 BUT I reckon the line currents at 400V would be 8.8A/17.6A/8.8A to get 230V @ 28A (since its not possible to use a transformer to drive a single phase load from a three phase source and get balanced phase currents ... one current will always be twice the other two) so it still wouldn't be possible with a 10A supply.

 

[AdrianUK]

The question which the OP has got to resolve with the university (and its electrician) is whether the 32A socket in question is fed from a 10A or 32A 3-phase circuit. If it transpires that it is a 32A circuit, then it's hard to see what 'get out clause' one could offer the uni's electrician!

Indeed ... and we still await the response of the Unis' electrician to the question of the OCPD.

As to a diplomatic way forward ..... I think I'm gonna let the Salesman who closed the deal earn his money

 

[JohnW2]

The question which the OP has got to resolve with the university (and its electrician) is whether the 32A socket in question is fed from a 10A or 32A 3-phase circuit. If it transpires that it is a 32A circuit, then it's hard to see what 'get out clause' one could offer the uni's electrician!

Indeed ... and we still await the response of the Unis' electrician to the question of the OCPD. .... As to a diplomatic way forward ..... I think I'm gonna let the Salesman who closed the deal earn his money

I didn't know that salesmen were necessarily known for their diplomatic skills ... but, as I said, if it transpires that it is a 32A circuit, then even the best diplomat in the world might have some difficulty in helping the electrician to save his face!

Kind Regards, John

 

[AdrianUK]

So it would likely be cheaper if the neutral will not allow the full 32A to be drawn to install a new neutral rather than buy the converter unit.

Since the cost would be passed to the university you might suggest it would cheaper to install an new neutral than use other methods to get around the lack of single phase capacity on the socket and would also ensure in the future no errors were made when supplying other equipment.

I'm still trying to find the old reg but I'm certain that its never been allowed to use a reduced size neutral on a general purpose socket outlet - only on a distribution circuit to a fuseboard (hence rely on cancellation from a variety of loads). The load placed on a general purpose outlet could never be predicted and the use of a single phase load is not unreasonable.

Reduced neutral SWA was made in days gone by. Some of the distribution circuits in our factory clearly have a smaller sized black conductor than the other three ...... but the other three are RED, WHITE & BLUE ...so that dates the age of the cable!