Voltage Drop Requirements
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:
- Ecodial (Schneider Electric)
- Simaris Design (Siemens)
- Etap
- Calculator of Bus Bar Size and Voltage Drop
- Voltage Drop Calculator (Albany Technical College)
Further Study – Voltage drop calculation methods with examples explained in details
Voltage drop calculation methods with examples explained in details
Resource: Practical guide to inspection, testing and certification of electrical installations by Christopher Kitcher
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Edvard Csanyi
Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.
Profile: Edvard Csanyi
Hi sir, i am very much found of ur topics, i am interested to work with you, give me an opportunity sir.
Hello.
I computed my system
From 3.3kV/400V IT System, 50Hz,
Length is 100 mts from Transformer to Main CB of LV Switchgear
Length is 50 mts from Protection Devices to Motor Loads
It calculates the voltage drop but is too high around 17%
Requirement Voltage drop should be atleast 10% only.
I used EPR type of Cable for Mining
Please help me reduce voltage drop
Thank you
Check if you selected the right cable and its laying method from appropriate table.
I am interested in such design works.