It is part of the inspection process to ensure that installed conductors have been correctly selected for current carrying capacity and voltage drop. To check the suitability of the current carrying capacity it is simply a matter of looking at the installation method, and then checking on the current carrying capacity tables for the cable in Appendix 4 of BS 7671.
To ensure that the cable meets the voltage drop requirements is slightly more complex. A simple method is to measure the voltage at the origin of the circuit, and then measure the voltage at the end of the circuit with the load connected and switched on. The difference between the two measurements will be the volt drop.
If the first method is impractical, then a resistance test should be carried out between the phase and neutral of the circuit. This test is carried out using the same method as the R1 + R2 test although, instead of the test being between phase and circuit protective conductor (CPC), it is between the phase and neutral for the circuit.
Once the resistance R1 + Rn of the circuit has been measured it should be multiplied by the current that will flow in the circuit.
This will give you the volt drop for the circuit.
Example of voltage drop calculation
A circuit is wired in 2.5 mm2 and is 25 metres in length. The current in the circuit is 18 amps.
The measured value of resistance is 0.37 Ω.
Voltage drop V = I x R
Voltage drop V = 18 x 0.37 = 6.66 volts
This is the voltage drop for the circuit.
Voltage drop in conductor can be calculated by following software and MS Excel Spreadsheets:
Resource: Practical guide to inspection, testing and certification of electrical installations by Christopher Kitcher