Martin asks this question, which is about Building Network Operators (BNO) supplies, volt drops and the establishment of the supply origin for the purposes of the apartment final circuit calculations. It has been answered by the NICEIC (Theme – Distribution Equipment including Cable Management):
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Question: We have worked on a wide range of apartments and similar buildings where there is group metering on the ground floor of the building. As such, the cables between the meter room and the dwellings, now seem to be referred to as ‘Building Network Operators (BNO) supplies’ - i.e., they are the responsibility of the landlord and are not owned by the apartment.
Sometimes, the dwellings concerned can be in excess of 100m away from the group metering room and the sub-main cables have, therefore, large cross-sectional areas (up to 70 or 95mm2) because of the volt drop (with the volt drop being based on the sub-main cable all the way through to the end of the final circuit of the dwelling).
Apart from cost, this causes issues regarding physically terminating this size cable at both ends. One thought that we have investigated, in the eyes of the Wiring Regulations, is whether the origin of the installation of the apartment - in particular for volt drop purposes - could be considered as the end of the BNO cable at the consumer unit.
As the voltage at the incoming supply is 230V -6% +10%, this gives a range of approximately 216 - 253V. Effectively, the maximum volt drop of the installation is typically based on, for lighting, 3% of 230V, which is approximately 223V - higher than the permissible lower limit of the incoming supply of 216V.
Effectively, as we understand it, a 3% volt drop limit could be applied to the 216V, if this was a measured incoming supply voltage.
Although we are not looking at the very lowest volt drop limit, do you think it is permissible to take the original of the apartment installation as the end of the BNO cable (which may have, for example, a voltage above 216V) and then take this as the supply origin for the purposes of the apartment final circuit calculations?
Answer: Your thoughts are correct.
If there is adequate voltage at the dwelling that would allow the correct operation of equipment, then a further volt drop (if required) would be calculated from that point, and not from all the way back at the supply intake.
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