EricMark
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- Reaction score
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The formula is in BS7671:2008 for working out the correction factor, I seem to remember involves square routes, as far as the 26 amp for ring final never found it written down, I was attending IET lectures when BS7671:2008 came in and in the lecture it was stated moving from the 4% for all to 3% lighting and 5% power resulted in the length of a ring final going from 80 something meters to 106 meters, however try as I could I could not get this 106 meters, so I asked and was told we consider 20 amp draw at centre and 12 amp even spaced so use 26 amp as design current. I have never found this written down, but it does give the 106 meters of 2.5 mm cable being just within volt drop limit.
With lights of course there is nothing to say 6 amp is the design current, it could well be considered as 3 amp if all lamps even spread, so 3 amp at 3% volt drop would allow 91 meters of 1.5 mm cable. If the DNO loop impedance is 0.35 ohms the end of run loop impedance should be 3.25 ohms.
However in real terms since only line goes to switch, not that easy to work out the limit.
My worry was if I do an EICR and say all is OK and there is excessive volt drop, could some one at a latter date say I got it wrong and try to claim I had not done the EICR correctly, or PIR as it was called then, however one there are laid out forms to show an EICR results and also set criteria as to issuing C1, C2, C3 etc. And unless volt drop was massively over limits it is unlikely to be picked up on.
My original idea was to include the calculations in the test results to auto flag up if non compliant, but a loop impedance meter is not that accurate, test three times and you will normally find 0.02 ohm variation, this is doubled up when also measuring the DNO at head loop impedance, 0.04 ohms makes a big difference to volt drop calculations, and if for example it was found you had 130 meters instead of 106 meters, all you need to say is worked at at 13 amp at centre and 19 even spread so using 22.5 amp as design current, and no one can say that's wrong.
With lights of course there is nothing to say 6 amp is the design current, it could well be considered as 3 amp if all lamps even spread, so 3 amp at 3% volt drop would allow 91 meters of 1.5 mm cable. If the DNO loop impedance is 0.35 ohms the end of run loop impedance should be 3.25 ohms.
However in real terms since only line goes to switch, not that easy to work out the limit.
My worry was if I do an EICR and say all is OK and there is excessive volt drop, could some one at a latter date say I got it wrong and try to claim I had not done the EICR correctly, or PIR as it was called then, however one there are laid out forms to show an EICR results and also set criteria as to issuing C1, C2, C3 etc. And unless volt drop was massively over limits it is unlikely to be picked up on.
My original idea was to include the calculations in the test results to auto flag up if non compliant, but a loop impedance meter is not that accurate, test three times and you will normally find 0.02 ohm variation, this is doubled up when also measuring the DNO at head loop impedance, 0.04 ohms makes a big difference to volt drop calculations, and if for example it was found you had 130 meters instead of 106 meters, all you need to say is worked at at 13 amp at centre and 19 even spread so using 22.5 amp as design current, and no one can say that's wrong.
