Volt drop for entire installations

[Monkeyblaine]

It states that you are allowed a permissible volt drop of 3% for lighting circuits and 5% for power circuits in domestic use. Now is the limits for the entire electrical installation? I was under the impression that it was per circuit. For example 3% of every lighting circuit and 5% for every other circuit. I have been reading a bit about it and now I'm seeing you work out VD in all circuits add them together and then you can only have up to 5% for everything. I'm confused because if you have a CU containing lighting and power does that mean you can only have 3% for the entire installation from the origin???

 

[johnboy6083]

This depends on the installation. If it is a very large installation then use can be made of the fact that the transformer terminal voltage at 253v to earth.

as long as the voltage remains within operating limits of the equipment, then the volt drop considerations are met.

the 3% and 5% limits are designed more for smaller type installations fed from a shared LV system.

as the network in a large factory is under the management of one person, then it's easier to design the distribution system to more exacting standards than the DNO who often have no idea of exact loads.

Buy a book by the IET called "Electricians Design Guide"

 

[Sintra]

Yes 3% from origin for lighting and 5% for power. Volt drop has to include all submains or it would be pointless.

 

[johnboy6083]

The answer to the situation you're describing is yes, you design the system so that VD from origin to the end of the lighting circuit is 3%.

on a commercial premises for example with lots of submains. Design the submains to have a max volt drop, so that when the 3% or 5% is applied from the final DB, the terminal voltage is above 230.

 

[Monkeyblaine]

So is it per circuit? Each gets its own 3 or 5% allowance.

 

[kingeri]

So is it per circuit? Each gets its own 3 or 5% allowance.

If you meet the 3% and 5% limits per circuit, then the entire installation will also meet those requirements.....we are talking percentages here!

 

[Monkeyblaine]

So with submains containing lighting circuits. The entire installation cannot fall below 3% even if there are othe circuits with 5% allowances and DNO supplies are already calculated in assumption that they are up to loads of 100a for example (in domestic) so we just have to make sure again the entire installation doesn't fall below 3% as all homes have lighting circuits. An I getting it or missing something obvious.

- - - Updated - - -

So with submains containing lighting circuits. The entire installation cannot fall below 3% even if there are othe circuits with 5% allowances and DNO supplies are already calculated in assumption that they are up to loads of 100a for example (in domestic) so we just have to make sure again the entire installation doesn't fall below 3% as all homes have lighting circuits. An I getting it or missing something obvious.

 

[Monkeyblaine]

Ok another way of saying what I'm trying to understand. If by regulation standard you are permitted to lose 11.5v on a power circuit in LV systems. Then hyperthetically if you do lose that on one circuit does that mean you can't lose any more on any other circuits? And what percentage can be lost from entire installation can be lost 3% or 5% if the installation contains lighting surely it's only 3???? I'm struggling here.

 

[Richard Burns]

The voltage drop for any circuit is measured from the origin of the supply to the furthest end of that circuit. If that circuit contains lighting then the maximum volt drop is 3% if it does not then it is 5%.

You cannot measure the volt drop of the installation since there is no reference point to measure to.
You can state that the longest lighting circuit will have the most onerous volt drop e.g if it is four dbs downstream and then a 100m circuit run you might need some big cables!
However if there was a power circuit on that DB as well then it would likely comply because you might have only say 1% volt drop up to the DB and 2% out to the lighting, totaling 3%. So the power circuit from the DB would have 1% up to the DB and would have the capacity for another 4% VD for the circuit.
Pretty diagram for you!

 

[Monkeyblaine]

Or am I looking at it wrong. At the origin will there always be 230v (ish) due to how the DNO have designed it and then every power circuit added to it can have up to 11.5v drop each and every lighting circuit 6.9v drop each. The original 230 at origin doesn't change??

 

[Monkeyblaine]

Thanks you for your help. So I'm right in thinking the origin stays at 230v then anything south from that can have up to 5% for every power circuit and 3% at lighting. So regardless how much volt drop combined on all circuits it doesn't affect the origins 230v(ish)

 

[Monkeyblaine]

In other words you always start at 230 from busbar and then lose voltage on each individual circuit as they get longer/add more load.

 

[Richard Burns]

Yes the 230V is the reference "assumed" supply voltage that you use for your calculations irrespective of what voltage you actually have at the origin.
It is a calculation you are doing.

It may be that operationally for voltage sensitive devices you have to use the measured voltage but in general for designing a circuit / installation you are using the reference values.

 

[Monkeyblaine]

Thank you very much for clearing it up.

 

[Knobhead]

This shows the cumulative effect of volt drop