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[JohnW2]

Would 6 supply onto single phase be too much for one incoming cable? Overloading?

Depends on the size of the cable. Do you think there are ever more than 6 houses on one phase from a substation, i.e. do you think that we have 1 substation for every 18 houses?

To be fair, he was asking about the service cable, not the main from the substation, and I doubt that it is common for a service cable to supply very many houses. However, as I've said, applying the sort of extreme diversity that DNOs necessarily apply, I would suspect that each property is not 'counted' as much more than a 10A-20A load - I may be wrong about the figure, but I think I'm right about the concept!

Kind Regards, John

 

[ban-all-sheds]

The point is that it is clearly possible for one of the standard cables used by DNOs to support a load of 6 flats, so why would they use one which is too small?

 

[JohnW2]

The point is that it is clearly possible for one of the standard cables used by DNOs to support a load of 6 flats, so why would they use one which is too small?

Indeed - you are giving the 'bottom line', whereas I was attempting to provide a rationale for it.

Having said that, I'm less certain that it is common for a DNO to use a 'standard cable' (or, indeed, any cable) to provide a single-phase supply (assuming that's what it is) to service 6 flats!

Kind Regards, John

 

[333rocky333]

interesting johnw, that you at first thought single phase supply, that never even crossed my mind.
maybe it was diverted from that old disused single phase meter then.
This lots in a restaurant cupboard, so who knows where they go, I assume flats above but I quess the total floorplan of them would be the same as the restaurant, so may be quite cosy.
Whats the cable called, it feels like it has a metal coating under the orange, but i dont think its pyro, looking at the lumps under the heatshrink im guessing its a type of concentric

 

[333rocky333]

The reason I say single phase is: -
1/ The incoming cable and the loop are too small to be 3 phase.
2/ I have never known anyone do a 3 phase loop as there is insufficient space in the crutch area of that type of cut-out to do it safely
3/ The loop cable is installed well under the bending radius of a 3 phase cable, so no way could one be installed in that situation.

As for load the cable will be rated at about 147A

Voice of experience, cant learn that at college

 

[JohnW2]

interesting johnw, that you at first thought single phase supply, that never even crossed my mind.

It was just my 'first impression', and I may be wrong. I think that, knowing how obsessive these people are about the phase-identification of cables, I suppose I was probably very much influenced by the absence of any such identification.

Kind Regards, John

 

[AdrianUK]

IF the supply is single phase & its been looped to all six fuses .... then that 'Henley' block that commons together 3 of those neutrals and connects to the neutral terminal of the first head by a piece of 25mm2 (??) tail would be protected by 3 x 60A fuses! Surely that is a classic mistake... that single combined neutral tail would be rather distressed long before those fuses opened.

However, if the supply IS 3 phase then the resultant current in the common neutral tail would be fine.... BUT the old Central Networks design manual specifically prohibited the looping of three phase cutouts.

Just my thoughts...

 

[AndyPRK]

Yes I did wonder about that poor neutral tail !

 

[JohnW2]

IF the supply is single phase & its been looped to all six fuses .... then that 'Henley' block that commons together 3 of those neutrals and connects to the neutral terminal of the first head by a piece of 25mm2 (??) tail would be protected by 3 x 60A fuses! Surely that is a classic mistake... that single combined neutral tail would be rather distressed long before those fuses opened.

That's a good point - but, to be realistic, (and assuming that it is single-phase) I doubt whether the CSA of that "single combined neutral tail" is any less than that of the neutral conductor in the incoming supply cable, which would have to carry the same neutral current.

However, if the supply IS 3 phase then the resultant current in the common neutral tail would be fine....

Indeed. In fact that's essentially what I've got ...

Untitled

• JohnW2
• 13 Aug 2011
• 5

The three phase tails are ~16mm² and, I suppose as some sort of gesture, the neutral one ~25mm² (and note the red insulation of the incoming neutral!). Whoever was responsible for that 'gesture' presumably didn't understand that the total neutral current can never be higher than that in one of the phase ones!

Edit: well, that didn't work - how am I meant to embed a pic from my album?!
Edit2: Aha, I've got it to work, thanks to EFLI - thanks!

Kind Regards, John

 

[EFLImpudence]

Click on the camera icon

 

[333rocky333]

 

[JohnW2]

Click on the camera icon

Thanks! ... now edited.

Kind Regards, John

 

[plugwash]

the total neutral current can never be higher than that in one of the phase ones!

That is not strictly true.

 

[westie101]

More correctly, the total current cannot be higher that in the incoming cable which with DNO interpretations of current capacity is acceptable.

That the incoming supply could be overloaded is possible but the same could be said of most incoming cables

 

[JohnW2]

the total neutral current can never be higher than that in one of the phase ones!

That is not strictly true.

Are you perhaps thinking of power factor issues? Otherwise, unless my mental trigonometry is failing me (which it may well be!), if the currents are all at 120° (all will be the case if all PFs are 1), it shouldn't be possible for the neutral current to be greater than the highest current in any of the phase conductors. The worst case of 'phase imbalance' arises when only one phase is loaded, in which case the neutral current will obviously be equal to the current in that one loaded phase conductor.

Kind Regards, John