Where a final circuit is supplied from a distribution board that is not at the origin of the installation, as shown in Fig 1, the total voltage drop between the origin of the installation and the load using equipment is equal to the sum of the voltage drops in the distribution circuit and the final circuit.Ĭare is needed when adding together the voltage drop of a three-phase distribution circuit and a single-phase final circuit, as the former voltage drop relates to the line-to-line voltage and the latter relates to the line-to-neutral voltage.Īs illustrated by the following worked example, expressing all the voltage drops as percentages (of the nominal line-to-line voltage or line-to-neutral voltage, as applicable) before adding them together helps to reduce the likelihood of errors. For AC circuit conductors having a csa of 25 mm 2 or larger the value of mV/A/m to be used in the above equation is the ‘impedance’ value (mV/A/m) z, from the appropriate column of the table.Ĭalculating the total voltage drop if a circuit is NOT directly supplied from the origin of the installation These are identified in the tables by the symbols r, x and z, respectively. L is the length of the run of the cables, in metresįor AC circuits using conductor sizes up to and including 16 mm2 and DC circuits with conductors of any csa, inductance may be ignored and the tabulated voltage drop values take account of resistance only.įor cables having a csa of greater than 16 mm 2, separate tabulated voltage drop values are given for the resistance, inductance and impedance of the conductors. MV/A/m is the tabulated mV/A/m value obtained from Appendix 4 of BS 7671 The voltage drop for a run of cable(s) is calculated using the following formula: The tabulated voltage drop values are for a current of one ampere for a one metre run along the route taken by the cables. Note: A greater voltage drop than stated may be accepted for a motor during starting periods and for other equipment with high inrush currents (Regulation 525.102).Īppendix 4 of BS 7671 includes tables of voltage drop in mV/A/m (millivolts per ampere per metre). Where the fixed current-using equipment is not subject to a product standard, Regulation 525.201 requires the voltage at the terminals to be such that the safe functioning of the equipment is not impaired.įor a low voltage installation supplied directly from the public network these requirements are deemed to be met if the voltage drop between the origin of the installation (usually the supply terminals) and the terminals of fixed current-using equipment or socket-outlets does not exceed the following limits stated in section 6.4 of Appendix 4 of BS 7671 (Regulation 525.202). Regulation 525.1 requires that, in the absence of any other considerations, under normal operating conditions the voltage at the terminals of fixed current‑using equipment shall be greater than the lower limit corresponding to the product standard for the equipment. Requirements relating to maximum permitted voltage drop
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