Correct wire needed for wiring a caravan away from house

[JohnW2]

Correct me if i'm wrong here but one significant difference I see between DNO and customer systems is volt drop. DNOs are allowed to supply power to customers at anywhere between 216.2V and 253V. If the DNO sets the output of their transformer near the top of the acceptable range then they have over 30V of volt drop to play with while still delivering an in-spec voltage to customers.

Indeed so.

AIUI a supply to a lighting circuit (and most submains will be supplying lighting circuits is only allowed to drop 5% and that includes both drop in the submain(s) and drop in the final circuit. So in practice you probablly don't want to be dropping more than 10V or so in your submains.

That would be 3% for lighting (5% for everything else), so only 6.9V - so "you wouldn't want to be dropping more than 6V or so in your submains".

So the DNO has effectively three times the volt drop budget that the end user would have.

As above, "effectively five times the volt drop budget".

... so your point is actually even stronger than you thought (assuming there is at least some lighting involved)!

Kind Regards, John

 

[ericmark]

Living in a narrow boat the marina supply was from a 4A MCB but today you can't buy hot fill washing machines so in order to run the washing machine my son used a 3 stage 25A charger feeding three 160AH batteries and a 3kW inverter with 6kW peak to supply the washing machine.

I am not saying the best method but there are ways with low current supplies to still run near everything you want where the problem lies is deciding which method to use.

The problem with my sons method was using a cheap simulated sin wave inverter rather than a good quality true sin wave and the inverter did not do as it said on the box and failed. This was not because of wrong system it was due to cheap china made inverter.

Although the regulations say 3 and 5% volt drop careful selection of items can allow a far greater volt drop without a problem. This quote from Philips is an example.

Input Voltage (dedicated vs. multi).
Most ballasts are designed to
operate at specific voltages. Newer electronic ballasts, including Philips
Advance models that use IntelliVolt technology, offer much greater
flexibility and other advantages such as inventory reduction. Today’s
increasing demands on electrical utilities can cause wide voltage
variations during load demand changes which in turn cause light
output from lamps operated on dedicated electronic and
electromagnetic ballasts to vary with the input voltage changes.
With IntelliVolt technology, many Philips Advance ballasts maintain
constant light output through nominal input voltage ranges of 120
to 277 volts, thereby compensating for any change in input voltage.
Some ballasts operate from 277 to 480 volts or 347 to 480 volts

Clearly if the lights will still work at 120 volt then volt drop for lights is not a problem. Look at most Lap tops and 100 - 250v is common. It does mean one has to be selective to what is used but the switch mode power supply has transformed what volt drop can be accepted. OK breaks the rules but it works and it has to be considered.

This guy has a problem and thinking outside of the BS7671 box there are ways around the problem. However one has to realise not all items will work once you leave the cotton wool of the BS7671 regulations.

 

[JohnW2]

Although the regulations say 3 and 5% volt drop careful selection of items can allow a far greater volt drop without a problem. ... This guy has a problem and thinking outside of the BS7671 box there are ways around the problem.

In fact, one doesn't really have to think outside of the BS7671 box. The 3% and 5% figures are merely those which are 'deemed to satisfy' the VD requirements (i.e. for those who don't want to think). However, the regs are also satisfied if the voltage delivered to the equipment is at least as high as the minimum voltage specified by a relevant product standard or, in the absence of such a product standard (which will most often be the case), a voltage adequate 'to not impair the safe functioning of the equipment'. It doesn't even say 'satisfactory functioning', merely 'safe functioning'. Hence, as you suggest, VDs considerably in excess of the default 3% are likely to be acceptable (and compliant) for many lighting loads. The default 5% (for everything else, including sockets circuits) is perhaps more difficult to get around- whilst one could confirm that greater VDs did not impair the 'safe functioning' of fixed-wired loads, one could not really do so for sockets circuits, since one would not know what might be plugged into them.

In any event, the BS7671 default figures are perhaps a bit naive in talking about VD rather than delivered voltage. A VD of 3% or 5% from a supply voltage of 216.2V is (in terms of voltage delivered to the load) clearly a very different kettle of fish from a VD of 3% or 5% from a supply voltage of 253V. I suppose the implication has to be that they believe that all lighting loads will work satisfactorily with voltages as low as 209.7V and all other loads will work satisfactorily with voltages as low as 194.6V - which I'm far from convinced would always be the case. The BS7671 defaults may therefore, on occasions, 'fail' in both directions.

In reality (although I realise that this could theoretically change 'overnight') many of us, like me, are in the position that are supply voltage is never even as low as 240V. So long as that remains the case, I could have VD of at least 13% before the voltage delivered to the lights fell below that in the 'worst case' of BS7671's default 3%.

I have to say that, in 'common sense' (rather than regulatory) terms, VD is usually the aspect of circuit design which least concerns me (but don't tell anyone that I said that!).

Kind Regards, John