Two Phase CU

I don't really see a problem, but I'd like to know a bit more. You mention a "fuse board" and a "main CU". How are they connected please?


Provided that the neutral is the same size as the lines, and there are no substantial harmonics, it should handle the current. If a service head was connected to a 3-phase switch fuse and you took out one of the fuses, would it present a problem? (Other than de-energising anything downstream.) No.


It seems to be quite a creative solution. ?

Hi Chris.

The intake has...amongst other things...6 single phase meters.

3 are inked to the Lower ground 3 phase board...no problem there (apart from no RCDs in sight).

One of the other three feeds a flat.

The other two feed the Ground. floor board.

The current loading shouldn't be an issue.

It was returning a substantially unbalanced load back via the neutral that concerned me.

That is an inherent problem with single-phase loads on a 3 phase supply. It is no different to any 3 phase board, and a full size (as phase conductors) neutral is quite adequate. It MUST not be coded and is not a problem! Whether RCDs are required at this point depends on the installation but would only code a C3.

It is not clear to me if you mean the incoming supply is 3 phase, but only 2 of the phases are used, or really it is a single phase supply, fed as if it was 3 phase  that has twice as much cable cross-section in the live path as the neutral.

Does the neutral show signs of distress (like over heating) ? Unless it does it is probably a non-problem.


Edit, if loads are mostly resistive, then when only 2 phases are loaded up but broadly balanced, then completing the triangle shows that the current in neutral is equal to that whcih would have flowed in the missing phase (as if the 3rd phase was present and loads more or less balanced, the neutral current would be near zero). If there is a lot of waveform distortion to the current from electronic loads, that is not the case, but that is not lilkley to be a large part of a residential load anyway.

Hi Dave,


I understand that you are always going to get imbalances with single phase circuits. But that is through usage, not by design.

The only code I considered was, possibly, a shared neutral. 


There are no RCDs on any of the sockets, all bar a few sentry cleaners sockets, or any of the cables run down walls...metal stud works.

Memshield 2 board as well...not much joy in obtaining RCBOs for that.

Hi Map,

It is a three phase mains supply.

The area is used as office space, with LED lighting installed.

If the two phases isolate together at the same place then it is not really a shared neutral between two seperate circuits any more than the neutral is shared when you have a 3 phse supply to a row of light switches. Clear labelling about where things turn off and isolate may be needed  if it is not clear. I asume the 2 phases and neutral are in the same multicore cable ?

regards Mike.

No that is not a shared neutral. It is a 3 phase supply. A shared neutral, in reality, only ever happens on a final circuit, almost always domestic.

Thanks Dave,

That is why I asked the question.

I must admit that I`m struggling to see a problem unless I`ve missunderstood it.

Forgeting harmonics for now then a 3 phase supply running 1 or 2 or 3 single phase loads would only cause phase imbalance to the supplier. (I`m assuming the N tail is same size as the phases). If everbody had say single phaes loads on three phase supplies and the were all on RED phase (sorry Brown) example then yes the supplier might not be happy but if the supplies were balanced to a degree then no problem