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Discuss volt drop or not in the Electrical Wiring, Theories and Regulations area at ElectrciansForums.co.uk.

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  1. shocked
    Offline

    shocked Regular EF Member

    Hi all
    My question is this
    If a submain is run off the No.1 C/U to a No. 2 C/U, when we calculate the cable sizes for the C/U2 final circuits do we use the volt drop value of the sub main or do we use the 230v value ???
    thanks for any help
    john
     
  2. telectrix
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    telectrix Scouser and Proud of It Respected Member

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    i would take the worst scenario and go from the main CU.
     
  3. BigBreakfast
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    BigBreakfast Guest

    Hi shocked,

    requirements for BS7671 are satisfied when volt drop is measured form the origin of the installation (telectrix beat me to it!). :)
     
  4. MFHEng
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    MFHEng Guest

    The volt drop is calculated from the supply origin (CU1) to the final circuits (CU2's final circuits)
     
  5. shocked
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    shocked Regular EF Member

    thanks all for the replies
    so just so its all clear in my head how would i calculate the final circuits for C/U 2???
     
  6. J@mes
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    [email protected] Regular EF Member

    Location:
    Bomber County
    Best thing to do is make sure the sub main supplying the final circuit has a very low volt drop, so it may need to be a fair bit bigger than expected even for it's current carrying capacity.

    I spec my submains so that there is no more than 1% volt drop between the origin and the end of the submain.
     
    • Like Like x 1
  7. Murdoch
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    Murdoch Electrician's Arms

    Location:
    Woking
    This may sound a daft question could you take a "reading" at the first CU and the 2nd CU to assess the actual volt drop?

    The reason I ask is that I've just been to see a job which has 2 CU's in place and the customer wants at least 3 new circuits added to the 2nd CU. All the cabling is underground and existing and "stated" to be good enough!

    Thanks
     
  8. telectrix
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    telectrix Scouser and Proud of It Respected Member

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    you can only measure volt drop with a full load on the circuit/s. yo will have to calculate your loading and factor in the cable from CU1 to CU2. and then calculate cable size for a max. drop of 11.5v
     
  9. Murdoch
    Offline

    Murdoch Electrician's Arms

    Location:
    Woking
    As I thought - the 2nd CU has very little (currently) connected so the "load" would be negligible - he wants a ring circuit added, along with a lighting circuit.

    Main problem is he is a "typical" customer who simply doesn't understand the requirements we have to abide with and add cost!
     
    Last edited: Nov 20, 2010
  10. malcolmsanford
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    malcolmsanford Electrician's Arms

    If your putting lights on the CU then you have to use 3%. To be honest if you have 230 volts at the 2nd CU and it's existing cable then I wouldn't worry to much about it.

    If you were getting a volt drop taking you down below 205 volts then you should be concerned. If you have 230volts at that CU then a single lighting and ring final circuit should not cause you a problem
     
  11. pushrod
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    pushrod Electrician's Arms

    Location:
    leicestershire
    I would calculate the volt drop for supply to the submain and add it to the volt drop for the individual circuits supplied by the submain. Using Vd =(Ib x mV/A/M x L) / 1000

    The %s are always worked out against 230V even if it is greater (which it usually is) as 230V (+10% or-6%?) is all that is guaranteed by the supplier.
     
  12. shocked
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    shocked Regular EF Member

    thanks all
    like with MURDOCK if the submain is existing how would you calculate whether you can add extra circuits and still comply with voltdrop ???
    john
     
  13. shocked
    Offline

    shocked Regular EF Member

    Hi all
    Also what if you wanted to fit a further sub main to another C/U ???
     
  14. pushrod
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    pushrod Electrician's Arms

    Location:
    leicestershire
    think i just answered those 2 questions in post #11.
     
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