How to Calculate Voltage Drop in an Electrical Installation

BS 7671 Regulations 612.14

Where required to verify compliance with Section 525 the voltage drop may be evaluated by measuring the circuit impedance
and using calculations using cable length and load current for different cable sizes.


As an electrician it is essential to be able to calculate the voltage drop in electrical installations.  As part of your electrical training you will normally be expected to calculate voltage drop ether in the classroom or as part of some written exams. 

There is now a specific requirement to verify the voltage drop does not exceed the limits specified in the products standards of installed equipment. Where there is not a product standard, the voltage drop should not impair the proper safe functioning of the equipment and this is deemed to have been met when the voltage drop between the origin of the installation and the equipment does not exceed the values stated in BS 7671 Appendix 4; Section 6.4 and Table 4Ab. Table 4Ab lists the maximum voltage drop allowed between the origin of the installation and any load point

For example; for low voltage installations supplied directly from a public low voltage distribution system lighting circuits are restricted to 3% of the distributor’s declared nominal supply voltage and other circuits are restricted to 5% of the distributor’s declared nominal supply voltage

Note: If the installation has been designed to meet the requirements of BS 7671 it is not normally a requirement to verify voltage drop during an initial verification. 

Acceptable methods of determining the voltage drop is to use one of the following formula. 

Vd = RLNx 1.2 x lb 


  • Rln is the resistance measurement (Ω) of Line and Neutral conductors from the origin of the supply to the connected load; 
  • 1.2 is a correction factor to allow for an increase in conductor resistance with circuit fully loaded; 
  • lb is the design current of the circuit.


An alternative method is to use the voltage drop Tables in BS 7671 Appendix 4, and calculate the voltage drop using the formula :

Vd= mVAm x lx lb ÷ 1000


  • mV/A/m is the voltage drop per metre per amp
  • I is the length of the circuit conductors
  • lb is the deign current of the circuit

There you have it. Calculating your voltage drop is not some science from the dark side, but actually fairly straight forward if you have the correct information.