Good morning all, quick one from me - Ive heard a lot of people within industry discussing this. A quick question from myself in terms of the overall installation process and the table released in the publication on line - wiring matters 96 July 23. In the table it references 230 volts, which is a fair starting point. However I very rarely see this constant voltage and almost all the issues I have seen with O-pen and problem tripping occur with an overvolatge. 

The question I have - is the figure of 3 % a generic guidance/illustration figure as I am now seeing this being used as the figure for Max VD in cable calculations? And people using terms such as we can only use 39 metres of cable  6mm? 

How is this viewed in terms of an install where the voltage sits at 246 volts for example - where the voltage above 230 is more than the 3% tolerance discussed? I appreciate volatges will always fluctuate - but restricting location and length of run based on this ????

I would love to know the thoughts of others on this - are we saying this is a guidance piece in terms of the 3% and open to change re the voltage at site? 

Thanks 

The supply voltage is rarely constant - Ohm's Law is at play in the DNO's cables as well as within installations. So 246V in the middle of the afternoon might well drop to something significantly lower at 1am when next door's storage heaters kick in on their E7 tariff, or when the factory down the road starts up in the morning, or (possibly more relevant these days) when a street full of EVs switch on when a cheap tariff starts. So 'acceptable' voltage drop has always been a bit arbitrary - it's been anything between 2.5% per final circuit to 5% for entire installation within my memory.

Operational voltage at the appliance isn't the only consideration though - 3% v.d. also means 3% wasted power - and with large loads - e.g. 7.2kW EV means over 200W lost in the cabling - that's quite a few pence every charging session, maybe tens of pounds a year, hundreds of pounds over the lifetime of the installation - so the cost of going up a cable size or two rapidly becomes easy to justify both economically and environmentally.

   - Andy.

Thanks as always Andy for the detailed answer.

I agree with every point you have made above. In regards to the new publication, how do you view that? Are you of the opinion that the 3% is a guidance or would you look to use that figure when designing a circuit?

Are you of the opinion that the 3% is a guidance or would you look to use that figure when designing a circuit?

I don't think it's in BS 7671, so would treat it as good practice rather than a requirement (probably similar to the 1% that's often recommended for PV AC circuits) - so try to stick with that in the vast majority of cases, but keep the option not to in reserve for any particularly awkward cases where especially where there may be other mitigating factors.

   - Andy.

Are you of the opinion that the 3% is a guidance or would you look to use that figure when designing a circuit?

I don't think it's in BS 7671, so would treat it as good practice rather than a requirement (probably similar to the 1% that's often recommended for PV AC circuits) - so try to stick with that in the vast majority of cases, but keep the option not to in reserve for any particularly awkward cases where especially where there may be other mitigating factors.

   - Andy.

Yes my thinking as well.

Thanks Andy,

Dom