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Maximum voltage drop on SWA runs to outbuildings, am I over engineering the solution

Alan B

I do quite a few EV charger installs, more and more seem to end involving long SWA cable runs to garages etc with voltage drop driving cable size.

In many cases I am running an EV charger, other bits in the garage and garage lighting circuits from the same cable run.

According to my understanding of the regulations lighting circuits are only allowed to have a 3% voltage drop between the incoming supply and the accessory compared to 5% for power. Therefore voltage drop for the garage lighting ends up being the driver for cable size on the garage supply cable.

I get the impression that others bend the rules and have certainly found some installs that do not comply even with the 5% guidance, I appreciate that most of the time we can get away with it but I don't want to end up with upset customers and maybe having to replace an expensive cable.

But by calculating this way, am I being unnecessarily stringent with my calculations. I realise that this assumes worst case supply voltage from the grid which I feel I have to stick with, but for example are typical modern LED light fittings more tolerant to low voltage supply compared to filament lamps and therefore the guidance in 7671 is actually out of date? 

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    Flicker on LEDS is a very variable thing - unlike hot filaments that have a natural time constant of 10-50msec depending on the filament weight, or gas tubes that have some afterglow and do not completely die away between mains half cycles, the LEDs themselves are very fast on-off indeed, tens to hundreds of nanoseconds in fact and can be used for high speed signalling.. (e.g. LiFi  as used in places that do not want to radiate their data outside the room)


    So it falls to the power supply and fitting designers to determine how flicker sensitive or not.  There are those that contain complex switching supplies and can track up and down mains variations and maintain perfect output.

    Some designs are less complex, but clearly well smoothed and can ride over voltage fluctuations that would be objectionable on filament lamps bit slowly dim in response to a failing supply.  Others are much simpler, being little more than a series capacitor, a bridge rectifier and some LEDS as a load. This sort are cheap and popular, but very vulnerable to supply flicker - in some case more so than the old filament lamps.

    So there is no simple answer - apart perhaps from using 12V LED strip and a power supply you smooth yourself,

    Mike.

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  • 0

    Flicker on LEDS is a very variable thing - unlike hot filaments that have a natural time constant of 10-50msec depending on the filament weight, or gas tubes that have some afterglow and do not completely die away between mains half cycles, the LEDs themselves are very fast on-off indeed, tens to hundreds of nanoseconds in fact and can be used for high speed signalling.. (e.g. LiFi  as used in places that do not want to radiate their data outside the room)


    So it falls to the power supply and fitting designers to determine how flicker sensitive or not.  There are those that contain complex switching supplies and can track up and down mains variations and maintain perfect output.

    Some designs are less complex, but clearly well smoothed and can ride over voltage fluctuations that would be objectionable on filament lamps bit slowly dim in response to a failing supply.  Others are much simpler, being little more than a series capacitor, a bridge rectifier and some LEDS as a load. This sort are cheap and popular, but very vulnerable to supply flicker - in some case more so than the old filament lamps.

    So there is no simple answer - apart perhaps from using 12V LED strip and a power supply you smooth yourself,

    Mike.

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