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

Nano Brewery:

Thanks all.  Oh right!?... I was of the understanding that the allowance is for the whole installation?...

https://www.tlc-direct.co.uk/Book/4.3.11.htm
Qoute form Example 4.8 from the above link:-  "It is important to appreciate that the allowable volt drop of 4% of the supply voltage applies to the whole of an installation. If an installation has mains, sub-mains and final circuits, for instance, the volt drop in each must be calculated and added to give the total volt drop as indicated in {Fig 4.10}."


...is that incorrect?  Cheers!


Not exactly incorrect, but perhaps misunderstood.

"The whole installation" means all parts of the installation that are relevant to the circuit in question.

It does not mean including voltage drop in unrelated circuits that happen to be within the same building.

As an extreme case, what about a large office with 100 sub circuits ? Would 0.05 % voltage drop be realistic ? A very large building might have 1,000 sub circuits.


These days the generally accepted figures for voltage drop are 3% for lighting and 5% for power circuits.. These are not in fact absolute required figures but are a good guide for small and simple jobs.

I would normally design on the basis of about 1% drop in sub mains and 2% in final lighting circuits, or 4% in final power circuits.  The exact division would depend upon circumstances.

For a large or complex installation it MIGHT be acceptable to use engineering judgement to permit of significantly larger voltage drops.

Nano Brewery:

Thanks all.  Oh right!?... I was of the understanding that the allowance is for the whole installation?...

https://www.tlc-direct.co.uk/Book/4.3.11.htm
Qoute form Example 4.8 from the above link:-  "It is important to appreciate that the allowable volt drop of 4% of the supply voltage applies to the whole of an installation. If an installation has mains, sub-mains and final circuits, for instance, the volt drop in each must be calculated and added to give the total volt drop as indicated in {Fig 4.10}."


...is that incorrect?  Cheers!


Not exactly incorrect, but perhaps misunderstood.

"The whole installation" means all parts of the installation that are relevant to the circuit in question.

It does not mean including voltage drop in unrelated circuits that happen to be within the same building.

As an extreme case, what about a large office with 100 sub circuits ? Would 0.05 % voltage drop be realistic ? A very large building might have 1,000 sub circuits.


These days the generally accepted figures for voltage drop are 3% for lighting and 5% for power circuits.. These are not in fact absolute required figures but are a good guide for small and simple jobs.

I would normally design on the basis of about 1% drop in sub mains and 2% in final lighting circuits, or 4% in final power circuits.  The exact division would depend upon circumstances.

For a large or complex installation it MIGHT be acceptable to use engineering judgement to permit of significantly larger voltage drops.