Chris Pearson said:

Just a thought here. Is there any provision which specifies voltage drop at HV. Surely, if HV is low, Lv must also be low?

No specific provision in BS 7671 ... and agreed, but transformers with tap-changers and AVC can be used to overcome that (or, in the LV system, autotransformers with tap-changers)

The key thing about a private transformer - one where only you have the secondary side - usually the HV is shared with others (!) , is that you are free to set the taps (or call in the DNO to set the taps to your needs, depending on contract agreement and who has suitably qualified staff )

This means that the voltage drop allowance normally reserved for the street cables is yours to play with - which if you have a large campus you will probably need to.

Mike

PS A private supply is defined as a supply that comes from a non-public source of energy, The HV supply is still a public source of energy, even if it is not considered.

I think the authors of BS7671, and a number of designers of large sites do not agree with that interpretation.  :-)

fully agree with you. l

I think we need to distinguish between supply/ origin and source. 525.202 clearly states the volt drop measurement should origin of the installation (usually the supply terminals) and the socket outlet....

BS7671 only applies to installations 1000v AC / 1500 DC and below. Thus it would deem the supply terminals on the LV side of the transformer as the oringin of the installation. If those terminals are privately owned, it would be considered a private supply.

Annex 4 - 6.4, Table 4Ab - Voltage drop (ii) Low Voltage installation supplied from private LV supply.

The HV supply or the source of energy is not considered for the purposes of Volt drop, but Origin of the LV supply is.

If those terminals are privately owned, it would be considered a private supply.

But the DNOs themselves were privatised after they took over from the old Electricity Boards - they may be regulated, provide a supply to the public, but their equipment isn't publicly owned.

   - Andy.

Much like the railways!

Peter Hartley said:

Thus it would deem the supply terminals on the LV side of the transformer as the oringin of the installation. If those terminals are privately owned, it would be considered a private supply.

Agreed, although ownership is not the issue because the transformer could be leased, but still be private.

It seems reasonable to regard the LV terminals of a private transformer as the origin, but otherwise it is the service head.

The real point is whether the transformer supplies any other customer or not. So if it is just the one, it is possible to compensate for voltage drop within the installation. However if there is more than one customer, if the wick is turned up because of voltage drop in one installation, the voltage may be too high for another so there is less scope for adjustment.

And whether or not the cable between the transformer secondary and your first load  or disboard is included in the sums or not.

For a house in the street the volt drop sums start at the meter board, not at the transformer terminals, the voltage drop in the street mains is very real but it  "belongs" to the DNO - they only guarantee to hit somewhere between 208 to 253 at your cut out (and are usually quite well in, but not always)

When you have a bit more control, you can usually do quite a bit better than that.

Normally drop at HV can be neglected because the voltage  loss is scaled by the transformation ratio. (11kV/400 or whatever)

Mike

Peter Hartley said:

It also applies to Extra Low Voltage (ELV) ranges and Low Voltage (LV) ranges, across a variety of frequencies.

yes, so in those cases, the v.d. is from the transformer (or converter) for those sources.

Regs 525.1 and 525.201 also still apply ... but in this case, the designer can take into account vd on both sides of the source to achieve those requirements, or use a "regulated" source (SMPSU for DC, for example). In any event, the source is not a public LV supply according to ESQCR, but in the case of ELV, you might ask whether the other "deemed to comply" models work too - I don't think they would always be valid TBH ... what's perhaps more pertinent here, is where a constant current driver is used as a source.

Energy efficiency measures may also take precedence !

In what situations would voltage drop be less relevant than energy efficiency? 

In what situations would voltage drop be less relevant than energy efficiency? 

AMK said:

In what situations would voltage drop be less relevant than energy efficiency? 

Well ... since you ask ... Power over Ethernet is a good example.

But in general:

(a) voltage drop can be controlled with power converters or transformers;

(b) where constant current (or constant power) is used, voltage drop to an extent can be ignored, and energy efficiency measures (perhaps whole lifecycle cost) could be considered.

Just to add, power converters and transformers are not without disadvatages. For instance, they can cause power losses. Also will make the system more expensive, bulky, and complicated. Therefore, power converters and transformers may not be the optimal choice for voltage drop.

True - and in many cases, a fatter cable is the way to go, but if you have to get 1kW over more than say 1km, then it is less lossy to install step up step down transformers at both ends - there is a reason there is an EHV national grid ;-)

As a very rough rule of thumb, when the distance in metres is comparable to the voltage between the lines, it becomes worth considering, and when it is several times further, a change of voltage  is pretty much the only practical solution..

Mike