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150A Ring Final Circuit

Neil Annetts

I am after some help in calculating the curent carrying capacity of a ring final circuit wired in 16mm Tri rated singles!

From the book 16mm tri rated will take 100A. circuit no more than 10 meters long, all in steel trunking / DB enclosures etc.

Supplied from a Schnieder NSX160F with a 100 - 160A TMD unit.

I know there is a way of calculating this but for the life of me i cannot remember how to do it and no amount of Googling is being productive!

Some background on the reason - 8-row schneider Prisma panel with 6x 80A Multiclip bus bars. cannot wire each back seperatley without having 6 cables per phase at the MCCB so the thought was to wire a ring around the lot.

Any help would be greatly appreciated

 

  • 0

    In general, I would say that given some diversity in the busbar loadings you are on fairly safe ground. With 160 A maximum load overall and 100A cable the only way you could overload it would probably be to load the 2 bars very close to one end at 80A each and then nothing on the rest. To work this out you need the exact cable lengths and loadings, then a fairly simple network problem, but there is not a generic solution. If you are stuck send me the details and I will give you a calculation.

    Regards

    David

  • 0

    Assuming the cable is the same cross-section all the way round, the thing to realize is that the current from the origin to any point on the ring divides round the two paths in the ratio of those path resistances - which are in effect path lengths.

    So from half way round the path lengths are equal and the current splits perfectly 50/50 . In this case with 100A cable you could have a 200A total  load, so long as the load was positioned symmetrically.

    Now if you had your load ¾ of the way round then the path lengths and current division would be in the ratio of 1:3 , with clearly the larger share going down the shorter path. Now if there were no other loads you could have a total load of 133A, with 33A going the long way and 100A the short way.

    The 160% thing works for13A sockets, becasue they are spread around the ring, and are not all fully loaded. This probably deserves a better analysis

    In general you want to avoid the situation where most of the load is near one end, adding dead length to the shorter path is actually worthwhile, not in terms of VD, but maximum current.

    If you can sketch the layout with the real distances, then we can chat you through a proper analysis.

    Mike.

     

     

  • 0

    Older versions of BS 7671 used to specify a general rule for ring circuits that the conductors had to have a current carrying capacity of at least 0.67x the circuit rating. (It was only later fudged to 20A for a 32A circuit to take account of 2.5mm2 T&E in thermally insulated walls and the move from 30A to 32A protective devices).

    But (as above) some consideration of your specific circumstances would be very sensible.

        - Andy.

  • 0
    Thanks all for your replies, very helpful.


    making sure the load is ballanced Symmetrically is not something I considered but it is obvious really. 


    Cheers


    Neil

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