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Answer:
The calculation for volt drop in a cable is carried out using the mV/A/m values from the voltage drop tables in Appendix 4 of BS 7671 for the type of cable in question.
The mV/A/m value is multiplied by the current carried and the length of the cable.
i.e. VD = mV/A/m x Ib x L where mV/A/m is taken from the
1000 appropriate table in Appendix 4
Ib = design current
L = length of cable in metres
Example:
Calculate the volt drop in a PVC sheathed 10mm2 cable supplying an 9kW single phase electric shower. The cable length is 20 metres.
VD = mV/A/m x Ib x L where mV/A/m = 4.4 from table 4D2B
1000 Ib = kW = 9000
Volts 230
VD = 4.4 x 39.13 x 20 Ib = 39.13A
1000
L = 20 metres
VD = 3.44Volts
Note: The maximum VD allowed is normally 4% of 230V = 9.2V
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