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Design Requirements on Sub-Mains Cables

Ciaran McMahon

Hi All,

I'm looking for some assistance. We are currently working on a project, and a separate contractor has installed a Sub-Mains supply to our control panel. The supply cable calculation has used 5% volt drop to our panel, leaving us with minimal scope for cables derived from our panel. They have also used a protective device that has a lower instantaneous fault level than some of our upstream devices, which is making it very difficult for us to achieve compliant Max Zs readings. 

Basically, the supply cables have been designed and installed to suit cost. 

We are trying to provide feedback to our client, and I am looking for assistance in finding which regulations they may have not considered in their design. 

Would greatly appreciate any assistance.  

Thanks in advance. C

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  • 0

    In terms of regulation, the bit where the design has to be suitable for its intended purpose sounds like it might be a good start. But, ignoring the sarky comment about why have 2 designers who do not communicate, what spec or other information about the final circuits and their loads was given to the sub-main designers ?


    As a general rule If the finals are long and thin, then the sub-main needs to be short or fat, and the voltage drop may be partitioned unevenly. Have they perhaps incorrectly assumed some diversity in the final circuits so the sub-main is not fully loaded?
    Mike.

  • 0 in reply to mapj1

    Hi Mapj,

    Thanks for your reply. 

    The control panel requires a 1250Amp supply and has several large motors on VSDs on it. The main issue on site is that the Transformer and Main switch room are quite far away from the control panel. (200Mtrs) Then we have circuits derived from our panel up to 80-90mtrs. 

    Its not that uncommon on larger projects for there to be multiple contractors working on seperate systems / subsystems. however,it is my understanding that the designer/installer of the Main power distribution is responsible to insure their design considers the requirments of the equipment upstream. 

    They would have been issued with a copy of panel drawings, load requirements prior to installation. 

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  • 0 in reply to mapj1

    Hi Mapj,

    Thanks for your reply. 

    The control panel requires a 1250Amp supply and has several large motors on VSDs on it. The main issue on site is that the Transformer and Main switch room are quite far away from the control panel. (200Mtrs) Then we have circuits derived from our panel up to 80-90mtrs. 

    Its not that uncommon on larger projects for there to be multiple contractors working on seperate systems / subsystems. however,it is my understanding that the designer/installer of the Main power distribution is responsible to insure their design considers the requirments of the equipment upstream. 

    They would have been issued with a copy of panel drawings, load requirements prior to installation. 

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  • 0 in reply to Ciaran McMahon
    Ciaran McMahon said:
    They would have been issued with a copy of panel drawings, load requirements prior to installation. 

    Would have have explicitly included max Zs and min v.d. requirements or any other design assumptions?

        - Andy.

  • 0 in reply to AJJewsbury

    Thanks Andy

    No, just the protective devices in our panel. 

  • 0 in reply to Ciaran McMahon

    That sounds like a fairly bosky set-up - and also sounds like it is at, or very near to, the planner's rule of thumb of one meter per volt of when it becomes worthwhile to move the transformer, as the cable sizes and losses on long runs start to become un-economic. Perhaps apart from raw cost, they also  wanted to avoid having space problems with terminating the cables if they were upsized or parallel singles. Before panicking, do all the motors start at once, or is it more staggered ? Do the VSDs worry if the voltage drop is a bit more than the usual 8%for a private transformer set-up?
    The other concern is that 5% VD translates into only a PSSC of full load x20 at your board - which means that you maybe in a bit of a squeeze for prompt disconnection for a far end fault and not tripping on start-up surges. Is their an earth fault relay for that sort of problem?
    Probably time to raise the concerns with that over-arching designer.

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