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Voltage drop in building networks

AJJewsbury
The recent posts about building networks has got me thinking - is there any recommendation for maximum permitted voltage drop within building networks?


Generally the DNO are allowed their +10/-6% variation and then BS 7671 generally allows 5% (or 3%) drop within the consumer's installation - and seemingly most ordinary appliances seemed to be based on a total of those - which is fine where the consumer's installation is directly connected to the DNO's system. But what about when there's maybe many tens of metres of extra cable involved courtesy of a BNO?


I guess that in the old days, when the distributor had responsibility all the way to the meter (so including the BNO as we'd now call it where the meter is at the consumer's end) the ESQCR limits would have applied at the meter terminals so the DNO would have had to designed to a tighter limit at the building's cutout - does anyone know if they still do that?


Otherwise there would seem to need to be some co-ordination between the design of the BNO and the consumer's installation - should the BNO formally 'declare' to the consumer's designer the v.d.? - perhaps as part of the tolerance on nominal voltage?


    - Andy.
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    I suspect that in most cases, that this is a non issue. Most building networks seem to feed relatively small premises such as small blocks of flats or a row of small lock up shops. In such circumstances it entails little extra cost, if indeed any cost, to keep the voltage drop in the building network to under 2%. In a small shop or flat, lighting circuits will often have only 1%voltage drop in any case, and 3% for power circuits.


    In the case of a larger or more complex building network, then IMO engineering judgement may be applied to justify a greater voltage drop. A large building network implies either an on site substation, or at least a dedicated cable from a nearby substation.

    In such circumstances it would be reasonable to design "as if" part of the installation belonged to the DNO, it would have done in the recent past. DNOs often allow for at least 5%voltage drop in street mains, and another 2% in service cables.

    So it might be reasonable to allow for 5% drop in the street mains or in that part of the building network that corresponds to the street mains. And another 2% voltage drop in the service cables, or that part of the building network that corresponds to the service cables.

    And then the usual voltage drop within the installation of the final consumer.
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    I suspect that in most cases, that this is a non issue. Most building networks seem to feed relatively small premises such as small blocks of flats or a row of small lock up shops. In such circumstances it entails little extra cost, if indeed any cost, to keep the voltage drop in the building network to under 2%. In a small shop or flat, lighting circuits will often have only 1%voltage drop in any case, and 3% for power circuits.


    In the case of a larger or more complex building network, then IMO engineering judgement may be applied to justify a greater voltage drop. A large building network implies either an on site substation, or at least a dedicated cable from a nearby substation.

    In such circumstances it would be reasonable to design "as if" part of the installation belonged to the DNO, it would have done in the recent past. DNOs often allow for at least 5%voltage drop in street mains, and another 2% in service cables.

    So it might be reasonable to allow for 5% drop in the street mains or in that part of the building network that corresponds to the street mains. And another 2% voltage drop in the service cables, or that part of the building network that corresponds to the service cables.

    And then the usual voltage drop within the installation of the final consumer.
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