"Consecutive" Voltage Drops

[OnOff]

Felt best to post this here. I'm not sure, ran it by 3 sparks at works who all had a stab but weren't sure so here it is:

Declared single phase supply voltage can be 230V -6/+_10% so 216.6V at the lower end. Appendix F of the OSG says permissable drop is 3% for lighting & 5% for other uses. But do you assume 3 or 5% below 230V or the actual measured voltage on site at the time of test, the issue then being that that figure can vary?

In the case of say a house supplying an outbuilding you would obviously take the origin voltage as being at the house.

Assuming then that you've calculated acceptable volt drop at the outbuilding, if at a later point in time you supply a further, remote building from this outbuilding (maybe with a smaller cable) do you take the origin voltage as being at the outbuilding now? I think so but am not sure.

Hope this makes sense. Apologies if a numpy question!

 

[Andy™]

your 3&5% is within your installation regardless of supply. supply should be 230 + or - is in reality you could have an installation with a 2% drop with a supply of 216 so actual voltage 212ish and comply but an install with a 253v supply be 6% volt drop that doesnt comply

volt drop would be calculated from origin of supply to the installation. however there is nothing in BS7671 to say you can have more than 1 instllation so you could claim the origin is a remote building if you then extend from there. as usual, clear as ever

 

[Risteard]

your 3&5% is within your installation regardless of supply. supply should be 230 + or - is in reality you could have an installation with a 2% drop with a supply of 216 so actual voltage 212ish and comply but an install with a 253v supply be 6% volt drop that doesnt comply

volt drop would be calculated from origin of supply to the installation. however there is nothing in BS7671 to say you can have more than 1 instllation so you could claim the origin is a remote building if you then extend from there. as usual, clear as ever

Be careful using terms such as "comply" and "doesn't comply". The 3%/5% figures quoted from BS7671 appear in an Informative Appendix and are not actually Regulations. The actual Regulation merely states that the voltage drop should not be excessive so as to ensure proper/safe functioning of equipment. (I haven't gone to BS7671 for the actual wording, but essentially that is the gist of it.)

3% and 5% are guidance - nothing more.

 

[Ardet R]

The volt drop is measured from the origin of the installation to the end of the circuit (or the worst case point in the circuit if it branches or is a ring).

Volt drop is generally only considered during the design stage so the voltage would be taken to be the nominal supply voltage of 230V.

Andy correctly identifies that an outbuilding with a different earthing system may be a separate installation and Risteard points out that a piece of equipment must function correctly and safely; the regulation also states it is OK if the voltage drop permissible be identified within a product standard.  The compliance with these statements is deemed to be in place if the voltage drops in the table are complied with.

In general the origin of the installation will be the point at which the incoming supply becomes the property of the owner of the installation, i.e. at the meter.

 

[OnOff]

The volt drop is measured from the origin of the installation to the end of the circuit

Yep but what I can't get my head round is the calc if you have say two consecutive runs of cable each with a different mV/A/m value. Do you consider two origins?

 

[Ardet R]

You add the volt drop for each section together to find the total volt drop from origin to end of circuit.

I did this diagram earlier to show calculation of volt drop for varying cables in a circuit.

Obviously it would usually be larger csa cables first and smaller csa toward the end, and the earliest cables could be distribution circuits from other DBs.

 

[Tony S]

It’s called cumulative volt drop.

PM your e-mail address and I’ll send a spreadsheet.

BTW, it’s why we adopted the ring main.

 

[OnOff]

You add the volt drop for each section together to find the total volt drop from origin to end of circuit.

I did this diagram earlier to show calculation of volt drop for varying cables in a circuit.

Obviously it would usually be larger csa cables first and smaller csa toward the end, and the earliest cables could be distribution circuits from other DBs.

View attachment 10263

That's great thanks but the source voltage for each section varies surely because of volt drop to that point?

 

[Doc Hudson]

That's great thanks but the source voltage for each section varies surely because of volt drop to that point?

No different in principle to doing a volt drop calculation on a radial circuit with loads distributed along its length -vs- single load a far and of cable. As the calculation is mV/A/m the individual sections will have different volt drops than if it was just a single load at the furthest point.  The "source" is the same, but the voltage gradually drops along the circuit, and the load reduces the further along the circuit you go.  

Consider a 100meter 1.0mm T&E radial, with 5x 100watt loads at 20m intervals. first 20m will carry 500w, second 20m 400w, third 20m 300w, fourth 20m 200w, last 20 100w.  Whereas a single 500w load would be 100m carrying the full 500w.  If you work it out I think you find the distributed load is below 3% but the single load is above 5%.

If one of these hypothetical loads was actual a shed CU splitting off to some other accessories, the basic concept of the calculation hasn't changed. You still have to take into account how much load is being consumed along each section of the circuit.

Doc H.

 

[Ardet R]

That's great thanks but the source voltage for each section varies surely because of volt drop to that point?

Going back to basic electronics the voltage dropped over a resistor is only dependent on the current and resistance i.e. V=IR. The same principle applies when the "resistor" is a cable. Therefore the value of volts dropped is correct for each section and the total is then compared to 230V to get your percentage VD.