When circuit conductors carry load current, a voltage drop is produced in them due to their impedance. This means that the voltage at the load end of the circuit should be expected to be less than that at the supply end, as illustrated in Fig 1.

Voltage drop in conductors carrying load current

Fig 1

Clearly, it is necessary to limit the voltage drop in a circuit so that the safe and satisfactory operation of current-using equipment supplied through the circuit is not impaired. In order to limit the voltage drop, circuit conductors larger than would otherwise be required may have to be used, depending on the load and the circuit length.

Requirements relating to maximum permitted voltage drop
The general requirement of BS 7671 relating to maximum permitted voltage drop is given in Section 525 of BS 7671. Under normal operating conditions, the voltage at the terminals of the fixed current-using equipment is required to be greater than the lower limit given in the product standard for the equipment. Where the fixed current-using equipment is not subject to a product standard, the voltage at the terminals has to be such as not to impair the safe functioning of the equipment.

Unfortunately, product standards or other sources of information about the lowest acceptable voltage at the terminals of fixed current-using equipment may not be readily available. However, a way of proceeding without such information is given in Regulation 525.3. Where the supply to the installation is given in accordance with the Electricity Safety, Quality and Continuity Regulations 2002 (ESQCR), the requirement of Regulation 525.3 is deemed to be met if the voltage drop between the origin of the installation (usually the supply terminals) and a socket-outlet or the terminals of fixed current-using equipment does not exceed the values given in Appendix 12 of BS 7671, that is, 3% for lighting installations and 5% for other uses.

A maximum voltage drop in accordance with the values given in Appendix 12 of BS 7671 is accepted by Regulation 525.3 because the ESQCR generally allows an electricity distributer to reduce the voltage of a low voltage supply by no more than 6% of the nominal value. Thus, for example, the voltage at the terminals of an item of fixed current-using equipment (other than a luminaire) or a socket-outlet in an installation should never be less than 89% of the nominal supply voltage (that is 100% - 6% - 5%). This voltage is usually adequate for the safe and satisfactory operation of current-using equipment having a rated voltage corresponding to the nominal voltage of the supply.
A maximum voltage drop of 6% for lighting and 8% for other circuits is normally acceptable in a large installation where the low voltage supply is derived from a privately-owned distribution transformer supplied at high voltage.

A voltage drop greater than the values given in Appendix 12 of BS 7671 is also normally acceptable in any installation during motor starting periods and for other equipment with high inrush currents, such as capacitors, transformers and certain types of lighting equipment. However, it has to be verified that the voltage variations during such periods are within the limits specified in the relevant product standards for the equipment. In the absence of a product standard, the voltage variations have to be in accordance with the manufacturer’s recommendations.