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Discuss Voltage drop and it's effect? in the Electrical Forum area at

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  1. happyhippydad

    happyhippydad Member Electrician's Arms

    Hello all, just been thinking over Voltage drop!!

    1. Where does the 3% and 5% figures for voltage drop (VD) come from? Appendix 4 (BS7671) gives the figures but why the different values between lights and sockets?

    2. Also permitted voltage levels are -6% and +10% from 230V giving a min of 216V and max of 253V. If these permitted figures allow a voltage as low as 216V why can voltage drop only be 3% for lights (i.e bringing it down to 223.1V)?

  2. plugsandsparks

    plugsandsparks Electrician's Arms

    3% lower than 216V, eventually discharge lights will drop out
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  3. Lucien Nunes

    Lucien Nunes Mercury Arc Rectifier Respected Member

    The ESQCR percentage tolerances allow for the supply to reach your consumers terminals. You then have to ensure that the additional voltage drop in the installation is within limits.

    Typically, lighting is more seriously affected by voltage drop. Fliament lamps especially, as the voltage falls not only does the light output decrease very rapidly but also the efficiency. So if you fit more / bigger lamps to counteract the loss of output, the consumption is increased. Compare electric heaters, where although the heat output reduces with the square of voltage, the efficiency remains constant, so under thermostatic control the element is just on for longer. Switched-mode power supplies will use the same amount of power regardless, as will some motors. And as mentioned above, discharge / fluo lighting is often one of the first things to fail altogether at the voltage falls, if it cannot strike.
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    SPARTYKUS Electrician's Arms


    Yep seen it on my road, neighbour has two MH Lights in garden (green and magenta), he rang me because they were failing, plugged MFT In and he only had 216. Knackered a tv and his fridge.

    they fixed it, not sure how, maybe theres a big dial on the transformer!
  5. Guest55

    Guest55 Guest

    Although bs7671 states 230v as the nominal single phase voltage for design calcs , no way does the dno use 230v as their baseline figure for distribution.
    hence 9 times out of 10 the voltage at the terminals of a domestic cut-out is 240 or higher.

    and the regs take into account that all of this is out of the installers / designers control and dont insist on a lower / tighter volt drop limit thats reliant on the dno voltage value , which could result is ridiculously oversized cables in trying to comply.
  6. davesparks

    davesparks Forum Mentor

    I think DNO transformers are still normally set up for 440/250V
  7. Guest111

    Guest111 Guest

    This is why the argument about standardising the supply at 230v is wrong,to drop the voltage at the transformers would lead to an increase in the current across the network,which would cause major problems.In fact I don't think I have ever measured 230v at a house,they are usually around the 240v mark,I,ve actually had it as high as 253v in my house.
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