Chewing The Fat..... "Volt-Drop"...

[SPECIAL LOCATION]

As a sole trader its great being able to manage jobs and plan your diary to suit your own personal work and family commitments with no external pressure from your boss.. [side note: Me- Self-employed since Feb 1999]

But as a sole trader you also don’t have any work colleagues to debate, discuss, chew-the-fat, over any work-related bits that you may want a second opinion on!

So just a bit of “Chewing The Fat” on a Friday night.. theme: “Volt Drop”

I have no problems ensuring that all circuits I design and install comply with BS7671 volt drop requirements…

But sometimes trying to explain to a customer why you need to use a larger CSA cable for a shed 50m down the garden, than the cable that their mate down the pub, who knows electrics, said they needed..

It can sometimes be a tad difficult telling the customer that their mate is an idjut!!

And/Or, I also never really understood why did the permitted volt drop become more critical for lighting circuits, but less critical for other circuits in 2008?? (It’s obviously nothing to do with nominal voltage as that changed in 1995).

So are there any other fat-chewers out there with much greater wisdom and/or easy explanations that can be passed on to myself and/or other lurkers on the forum who may be sh1t-scared to ask, for fear of being called an idjut!!!


Side Notes:- A bit of historic context for any of the younger members;

Nominal Voltage
pre 1995 nominal voltage 240volts -6% (225.6 volt), +6% (254.4 volt)
Since 1 Jan 1995 nominal voltage 230volts -6% (216.2 volt), +10% (253.0 volt)

Permitted Volt Drop
Pre 2008 permitted volt-drop on all circuits was 4% (9.2volts), ref 525-01-02.
Since 2008 to current time, permitted volt drop on lights is 5% (11.5volts), other circuits 3% (6.9volts)
ref 525.202 & 643.11 & Appendix 4 (previously Appendix 12).


I will now open another Friday night beer and await some constructive debate and discussion!!!

 

[binky]

You forgot one, solar 1%, volt drop.

I never understood why lights should be different to power, but I would assume all this is about energy efficiency reducing demand for power generation or prevention of cables overheating. I did once calculate the cable heating effect in a long sub main to a stable block the customer wanted solar panels on. To replace the cable with something larger would have involved digging a 100m trench and £3k in cable to meet the 1%. Customer wasn't happy about that, so I calculated that the energy loss would be around 80watts max. In other words quite neglible.

 

[kerching]

The Copper Development Association used to have an online calculator for cable losses. I used it a few times to calculate supply cable sizes for data centres. Not only did it calculate sizes it also illustrated pay back time due to reduced loses if a larger size was used

 

[Phoenix]

I never understood why lights should be different to power,

I always presumed that it would be because equipment itself should be fine with a 5% voltage drop, but when it comes to lighting, fluctuations in voltage can cause noticeable changes in lighting level that are distracting and annoying. I should imagine that these days with LED fittings and drivers that incorporate switch mode power supplies, in theory it shouldn't be an issue, how it pans out with some bargain basment fittings and lamps though is anyones guess. Used to be 4% for everything didn't it, under 16th, there is also likely a large factor of change for change sake in there too...

I did once calculate the cable heating effect in a long sub main to a stable block the customer wanted solar panels on. To replace the cable with something larger would have involved digging a 100m trench and £3k in cable to meet the 1%. Customer wasn't happy about that, so I calculated that the energy loss would be around 80watts max. In other words quite neglible.

3k/100m... £30/m That would have been a 4c 50mm you were pricing up? Cable is very dear these days. I got roped in as part of a gang in the week to help with a cable pull. Turned out to be a 130m run between buildings with two parallel 4C 240mm. Did a quick calculation on what it would have cost - and what it weighed - the result was pretty much equal to one of the vans on both counts I suppose the key thing, is that you did the calculation, and found that upsizing wasn't worth it - thats the way it should work IMHO, Its possible that if the feed to the barn wasn't existing and it had to be pulled in regardless with the trench and labour required anyway, that it may have made sense to go up one size on the cable, but not if it already exists, its a worthwhile excercise to consider such things, not least because the customer has been given all information to allow him to make an informed decision

 

[binky]

I suppose the key thing, is that you did the calculation, and found that upsizing wasn't worth it - thats the way it should work IMHO, Its possible that if the feed to the barn wasn't existing and it had to be pulled in regardless with the trench and labour required anyway, that it may have made sense to go up one size on the cable, but not if it already exists, its a worthwhile excercise to consider such things, not least because the customer has been given all information to allow him to make an informed decision

Customer was very happy not to spend the money. Half way along the run he had his own water supply and pump house, so had filter pumps running a lot of the time which would absorb a fair portion of the energy generated. Plus it was a split array, so whilst I calculated for max 4kw, it would not run at that level very often. Anyway, it's been in place for about 8 years now and is running fine.

 

[Bruspark]

As others have said the issue with old fashioned incandescent lamps inthe old days was that a small voltage change produced a large change in light output. So room lights dimmed when the kettle went on. Separate circuits helped a lot and the tighter volt drop requirement helped some more.

It may be that the next version of the regs could review how many incandescent lights still exist and if anyone cares about them ...

Until then my answer is that the regulations are set considering not only normal usage but many other conditions. We have to work to them and do so gladly as it lets us be sure all reasonable eventualities are covered too. Is the bloke at the pub well insured ?

 

[HappyHippyDad]

I do think that in a domestic situation VD calculations do seem a tad too risk averse.

I have come across many sheds at the bottom of the garden, wired in 2.5mm from the house. Sometimes the calcs show that VD is above the permitted amount, yet I have never come across any item of equipment that has not functioned correctly due to VD.

This may be the only time when 'John down the pub' is actually making a bit of sense!