Volt Drop

In my view, the voltage drop should be calculated for the entire consumers installation, from the origin to the far point of the longest final subcircuit. And not just for the submain. The total voltage drop should not normally exceed 5% for power and 3% for lighting.

It is up to the designer as to how this is apportioned between the sub main and the final circuits. 2% in the sub main and then 1% in lighting final circuits and 3% in power final circuits might be a start.

These figures are not absolute requirements and in some circumstances engineering judgement may be applied to permit of a larger voltage drop. If the supply is from private generating plant, or from an HV DNO supply via a private transformer, then a larger figure may well be acceptable.

For an installation supplied at low voltage from public mains, I would normally keep within the accepted figures. The fact that the present supply voltage is generous is of little relevance for a public LV supply. What if the DNO reduce it to 220 volts which they might.


For a three phase submain feeding single phase loads, the intention should be to ensure that the single phase line to neutral voltage at the far end of the "worst" final circuit will be within the accepted figures of 3% or 5%. If the load is reasonably balanced then one might reasonably calculate the voltage drop in the sub main on a 3 phase basis. Alternatively calculate on a single basis, but allow only for voltage drop in the phase conductor, not the neutral. With a reasonably balanced load, the current in the neutral core and therefore the drop therein should be very small.

In my view, the voltage drop should be calculated for the entire consumers installation, from the origin to the far point of the longest final subcircuit. And not just for the submain. The total voltage drop should not normally exceed 5% for power and 3% for lighting.

It is up to the designer as to how this is apportioned between the sub main and the final circuits. 2% in the sub main and then 1% in lighting final circuits and 3% in power final circuits might be a start.

These figures are not absolute requirements and in some circumstances engineering judgement may be applied to permit of a larger voltage drop. If the supply is from private generating plant, or from an HV DNO supply via a private transformer, then a larger figure may well be acceptable.

For an installation supplied at low voltage from public mains, I would normally keep within the accepted figures. The fact that the present supply voltage is generous is of little relevance for a public LV supply. What if the DNO reduce it to 220 volts which they might.


For a three phase submain feeding single phase loads, the intention should be to ensure that the single phase line to neutral voltage at the far end of the "worst" final circuit will be within the accepted figures of 3% or 5%. If the load is reasonably balanced then one might reasonably calculate the voltage drop in the sub main on a 3 phase basis. Alternatively calculate on a single basis, but allow only for voltage drop in the phase conductor, not the neutral. With a reasonably balanced load, the current in the neutral core and therefore the drop therein should be very small.