Understanding Voltage Drop

To ensure satisfactory operation of electrical equipment, it is necessary to maintain the voltage at which it is supplied within certain limits and close to the nameplate rating.
We often meet people who do not know what “Voltage Drop” means. An attempt at an easy explanation is:
“With a conductor carrying current, the resistance in that conductor causes a drop in the voltage every metre and so the final voltage at the end of the length of conductor is less than at the start of the conductor. If there is more current, there is more voltage drop, if the conductors get hotter there is more resistance and so there is more voltage drop.
This means that voltage drop gets worse at full current in a circuit and gets worse as the conductor heats up, and it could be because of the current or because of the ambient getting hotter.”

Use of Tabulated mV/A.m Figures
The voltage drop (mV/A.m) figures for all sizes of conductors in a variety of cables are given in the AS/NZS 3008.1.1. They are calculated assuming the cable is carrying the full current and is at maximum rated
temperature. This gives a kind of “safety factor”, but it also assumes the condition of a perfectly balanced three phase system and a specific power factor. So there are special conditions in real circuits that differ from these assumptions and many such special cases are addressed specifically in the AS/NZS 3008.1.1.

Voltage Drop Limitations
In Australia, the nominal supply system is 230/400 volts.
The maximum voltage drop from the point of supply to any point in the installation is required to be no more than 5% of the nominal supply voltage, ie, 11.5 V for 230 V phase to earth or 20 V for 400 V phase to phase.
The voltage drop limitation applies to the full circuit right to the end. For example, other voltage drop limits may apply in ELV circuits or may be dictated by motor starting considerations. Another example: the voltage drop in mains and submains circuits should take account of the voltage drop in final sub-circuits (and vice versa) to ensure the total voltage drop in the installation is within the required limits.