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

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!!!

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.

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

binky said:

I never understood why lights should be different to power,

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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...

binky said:

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.

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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

Phoenix said:

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

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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.

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 ?

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!

So... lets just chuck a bit more fat into the pan from a real-world domestic installation perspective..

A customer I have, has an existing weatherproof out door socket on the rear of property...
Wired in 2.5mm PVC T&E radial from a 20A type B MCB on the 30ma RCD side of an old split-load CU.
From which they want to supply a new outbuilding including lights & sockets further down the garden.

From my R1+R2 calculations and looking at the physical property structure, I have estimated that this cable run is approx max 20m long from CU to existing socket..

So if the existing cable was supplying the full 20A available from the MCB.....
20A over 20m of 2.5mm T&E, volt drop is 7.2v.. which is within the 11.5v required for sockets...
But it exceeds the 6.9v permitted for lighting...

And the total length from socket to outbuilding and the wiring around the outbuilding itself is approx 50m max..
20m existing + 30m new.

There are zero options with getting a new larger CSA cable directly back to the CU without serious major physical damage to the property structure that the customer is not wiling to consider.

So to extend it further with a few LED lights, (probably less than 40watts max extra load)..
Technically will NOT meet the volt drop calc's if the full 20Amps is being drawn via sockets & lights down the single cable!

Customer wants 5x double sockets in this outbuilding which is more of a summerhouse/office rather than a workshop/shed..
plus 6x internal lights and 2x external lights.... Lets assume the lights are all 5watt LED's 8x5watts=40watt.
But they will probably not all be on together as the 2x external lights are via a PIR sensor..

5x Double sockets with unknown max loads... But probably at a guess some form of plug-in heater may be used as Mr or Mrs customer may want to be down in the garden office when the weather gets a bit cooler... ?

So now lets bring the "Elephant into the room"..
If the new work was only supplying sockets we could design and install it for a 11.5volt-drop..
and ignore the 6.9volts if we were installing fixed lights...

But who is to say that the customer isn't going to just connect the same original lights via a 13a plug-top fused at 3a and still draw the same max load???? e.g. Sockets only, No fixed lights, But customer plus in some lights?????

So how do you explain to them that if they want the lights hard-wired it's more expensive, (bigger cable), than if they choose to have sockets only and may or may not plug in table lamps etc..???

Confused.com!!!!!


While I have no personal experience.. I have heard stories of lighting ballasts and ignitions not working correctly on some long cable runs in car-parks due to volt-drop problems.... But I can't get my head around how modern LED lighting has any similar issues??

p.s.
I have already come up with a solution for my customer..
lets say it involves a 16A RCBO, type B overload, type A RCD, fitted to the non-RCD side of the CU!!


SIDE NOTE:-
I have never previously attempted to measure any actual volt-drop...

But a few days ago I did a totally un-scientific test in my own garden shed with a 1200watt Aldi angle grinder...
Grinder data label.... 1200watts



Plus two meters that can display voltage plus one clamp meter that can display current

View attachment IMG_6934.MOV

Looks like around 2volts dropped with just under 2.3amps load....??

50m of 2.5mm pulling 20A gives 18V VD approx. That is quite a bit outside of the limits.

Could you not use 4mm, 6mm or perhaps even 10mm from socket to outbuilding.
4mm = 13.8V (total VD for whole run)
6mm = 11.58V
10mm = 9.84V

Plus, change the MCB to 16A which would give a total VD of 7.88V from CU to outbuilding if you use 10mm from socket to shed.

Back to your original point, 2.5mm would probably be fine and never cause problems

Plus, voltage still always seems to be closer to 240V rather than 230V!

SPECIAL LOCATION said:

So now lets bring the "Elephant into the room"..

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To be honest, it’s just a light in a shed I’ve been known to overlook the lighting volt drop requirement as it simply doesn’t seem sensible since LED lighting has pretty much taken over. As you have said, the fact a customer could just plug in a light adds further fuel to the argument.

When it comes to explaining volt drop to customers, it’s always easiest if there’s a significant load like a hot tub the other end. Then I can just say that they want their electricity to be heating the water not the cable under the ground. This seems to be accepted.

Finally a funny situation - there was a house with a garden and orchard that had about 150m of 3 core 0.75mm flex in a hosepipe running to a little tool shed at the end of the orchard. The customer wanted to re-use this to supply occasional garden lights and I had to say no. He then said he urgently needs a light in this little shack half way along. We looked and I noticed a very rust carbide miners lamp on a shelf. Half joking I said you should get that working. He had no idea how it worked and became more and more fascinated when I explained that water drips onto carbide, makes gas, you light it and it gives off light. He then asked if it could be made to work again and I said there’s not much to them, so probably. Turns out it was his grandfather’s.
Next time I went there he proudly showed it working to me - he’d gone on YouTube and found some videos and cleaned it all up. That was the end of the outdoor power project, he said he loves that he can use his grandads old light and it’s really cheap to run!

I suppose those writing the regs assume worst case scenario that grid voltage is only 230V, as if! I don't think I've seen lower than 245V in 20 years as the DNOs are keeping voltages up to cope with higher demands on out of date cable infrastructure. Ergo, even if the VD exceeds what it should be, everything still works fine.

As for sheds, if the sub main is correct, and feeding a mini board, I don't really take much notice of VD for the lights. I don't see how you can apply VD for lights along the entire supply.

plenty places here where voltage is 230v

Andy™ said:

plenty places here where voltage is 230v

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Not down here there isn't. Solar inverters show grid voltages, and Ive fitted hundreds of those, in between regular electrical works. My own house today is in the photo below. Might be different on new build estates, but I don't get involved with those.

varies a lot round here. some estates low 230's. adjacent can be high 240's. completely random

Andy™ said:

varies a lot round here. some estates low 230's. adjacent can be high 240's. completely random

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Ive seen it reach 250V and as little as 229V both still within the acceptable range.

7029 Dave said:

Ive seen it reach 250V and as little as 229V both still within the acceptable range.

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Plymouth city centre is up around 250V, although that might have changed since they built some large blocks of flats and replaced a lot of cabling

About 10 years ago one job I was on the voltage dropped as low as 211v and rarely got above 220v this was following a recent network / tranformer upgrade and just recently I left the power analyser on a domestic supply because the PV inverter was ocassionally dropping out and the voltage was peaking at 256 - 257v and the DNO won't take any action unless the voltage goes over 258v

Tim Howard said:

To be honest, it’s just a light in a shed I’ve been known to overlook the lighting volt drop requirement as it simply doesn’t seem sensible since LED lighting has pretty much taken over. As you have said, the fact a customer could just plug in a light adds further fuel to the argument.

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This is pretty much the reason for me posting this thread in the first place..

Working solo as a sole trader it is indeed interesting to see that other qualified, competent persons also can't seem to explain why one or more LED lights less than 50w max should need a more critical volt-drop limit than a handful of sockets in a domestic outbuilding!?

UNG said:

just recently I left the power analyser on a domestic supply because the PV inverter was ocassionally dropping out and the voltage was peaking at 256 - 257v

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Out of interest, was there a lot of PV in the immediate area?
I had similar, and my suspicion was that in this upper class residential area a lot of early PV adopters without battery storage and lack of daytime loads was pushing the supply voltage up. I couldn’t prove it though.

Tim Howard said:

Out of interest, was there a lot of PV in the immediate area?
I had similar, and my suspicion was that in this upper class residential area a lot of early PV adopters without battery storage and lack of daytime loads was pushing the supply voltage up. I couldn’t prove it though.

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That can happen where you have a social housing estate and every house has an array. But generally doesn't. I've never understood how inverters seem to differentiate between grid voltage and backfeed from other inverters.

I'm my own experience though, if the inverters are tripping out due to high voltages, it's a fault with the local transformer.