Radial Voltage Drop Calculation?... By Load or MCB Rating?

You need to consider the total voltage drop from the origin of the installation up the point of use.

In a very simple installation, the consumer unit may be very close to the orign which is generally accepted to be the power companies electricity meter. Strictly speaking, one should calculate the voltage drop in the cables from the meter and into the consumer unit, but it is common to ignore this as being to small to worry about for short cables.

In such a simple installation, simply calculate the voltage drop in each sub circuit and ensure that it does not exceed 5% for power and 3% for lighting. If the voltage drop is excessive, then a larger cable is the obvious answer, but alternatives exist.


A slightly larger or more complex instalation may have the relevant consumer unit located in a workshop, outbuilding, or brewery or other place that is distant from the origin of the installation. A relatively large cable is then installed from the origin to this local consumer unit. Known as a sub main.

In such a situation the voltage drop in this sub main should be calculated.

AND then the voltage drop in each sub circuit calculated. 

The total of the voltage drop in the sub main and in the sub circuit should not exceed 5% for power and 3% for lighting.


Calculate the maximum load that can reasonably be expected on the submain, this may call for judgment and common sense rather than simply adding everything up. Calculate the voltage drop in this submain. Aim for no more than about  1% to 1.5%. Consider a larger cable if needed.

An apparently over sized sub main may be prudent to allow for expansion.


Then calculate voltage drop for each sub circuit. If say 1.6% has already been lost in the submain, then remember that only another 1.4% for lighting or another 3.4% voltage drop for power circuits is available.


If the submain is long, then I might allow as much as 2.5% voltage drop therein. That only leaves 0.5% available for voltage drop on lighting circuits, which is achievable for short and lightly loaded circuits.


Other arrangements may be applicable to VERY large installations, but are unlikely in this case.

You need to consider the total voltage drop from the origin of the installation up the point of use.

In a very simple installation, the consumer unit may be very close to the orign which is generally accepted to be the power companies electricity meter. Strictly speaking, one should calculate the voltage drop in the cables from the meter and into the consumer unit, but it is common to ignore this as being to small to worry about for short cables.

In such a simple installation, simply calculate the voltage drop in each sub circuit and ensure that it does not exceed 5% for power and 3% for lighting. If the voltage drop is excessive, then a larger cable is the obvious answer, but alternatives exist.


A slightly larger or more complex instalation may have the relevant consumer unit located in a workshop, outbuilding, or brewery or other place that is distant from the origin of the installation. A relatively large cable is then installed from the origin to this local consumer unit. Known as a sub main.

In such a situation the voltage drop in this sub main should be calculated.

AND then the voltage drop in each sub circuit calculated. 

The total of the voltage drop in the sub main and in the sub circuit should not exceed 5% for power and 3% for lighting.


Calculate the maximum load that can reasonably be expected on the submain, this may call for judgment and common sense rather than simply adding everything up. Calculate the voltage drop in this submain. Aim for no more than about  1% to 1.5%. Consider a larger cable if needed.

An apparently over sized sub main may be prudent to allow for expansion.


Then calculate voltage drop for each sub circuit. If say 1.6% has already been lost in the submain, then remember that only another 1.4% for lighting or another 3.4% voltage drop for power circuits is available.


If the submain is long, then I might allow as much as 2.5% voltage drop therein. That only leaves 0.5% available for voltage drop on lighting circuits, which is achievable for short and lightly loaded circuits.


Other arrangements may be applicable to VERY large installations, but are unlikely in this case.