Low Mains Voltage (~190-220v) - How to resolve?

[toontoonizer]

Hello,

We have been grappling with low voltage being supplied to our home for many months now. I was able to first identify the issue in June of this year when I got an electric car. The car displays the voltage it is getting from the mains while charging via a 3pin socket (10amp max). This reading showed voltage in the region of 220v (which I am aware is in spec).

However, i soon began getting notifications that the charging was interrupted (multiple times) and that I should check the power supply. When unplugging and reinserting the charger the car resumed charging. Since then I kept an eye on the charge voltage and have seen it dip down to 180v. 

I also invested in Uninterruptible Power Supplies (UPS) for my pc and av equipment (we live in the sticks so power cuts are not unusual). The UPS also display the input voltage from the mains. From this I was able to see that the voltage would very rarely go above 210v. Often, the voltage would drop so low (~175v) that the UPS would switch to battery power. 

I raised this with our network operator (UK Power Networks) and I must have got a competent engineer as he agreed there was an issue after some testing. There was significant digging following this and the fault was eventually traced back to a fault on the main line in the road. Once resolved, this seemed to have fixed the issue. We had a voltage in the region of ~235v. However, we still experienced significant voltage drops - I.e. turning on the microwave and the voltage would drop by 10 volts. I noticed it was 1v per 1a that was being lost.

Recently however there was a significant area wide power cut. The issue was quite severe as they had generators plugged directly in to the network while they resolved the issue - which in total took 7 days. During this time we had low voltage again, and I notified the operator. As generators were being this was likely the culprit. Once the issue was resolved, our supply has reverted to an extremely low voltage once again - similar to before. 

I have raised the issue of low voltage and excessive voltage drops to the operator now at least 30 times. The most recent time, they sent an engineer who switched us to another phase (we have a 3 phase supply incoming). This was better for a few hours but then reverted back to the low voltage. 

We even had a voltage monitor/logger placed on our supply for 2 months. The guy who did the voltage logging looked at the data and saw we were pulling over 100amps on the phase and concluded that this is why the voltage drops to 190v and that there was nothing to be done. 

I'm extremely skeptical of this. Even if the load was over 100amps I don't think the mains supply voltage should drop to 190v. The main lines carry for more amperage and maintain their voltage. 

My question is how do I get the operator to look into the issue seriously. I feel like I am being fobbed off even though their supply is falling out of the legally acceptable specification. I have raised a complaint via their complaints process in the hopes that this will get me further. 

Does anyone have any advice? Our property is electric only (no gas) so all our heating is via electric and therefore is crucial that we have a stable supply. 

 

[ProDave]

What on earth would you be using in your house to draw 100A?

I would get an electrician in to do some tests on your installation.  He would be looking for poor joints or faulty equipment in your consumer unit(s) and a vital test he needs to perform is the supply loop impedance or loop test.

If all your own equipment is okay and not responsible for the voltage drop then you need to pester your supplier again.  It does sound like they are having network problems.

It would probably be worth you buying 2 bits of test equipment, a clamp on ammeter that you can leave on one of the meter tails that shows how much current your house is drawing, and a multimeter to measure the mains voltage.

 

[toontoonizer]

We have a large residential property (27 acres) with 2 powered annexes, gate motors plus lighting. One annex is fully featured with kitchen + boiler + underfloor heating hence the draw could exceed 100amps in certain scenarios. 

The internal wiring was one of the things I considered, but during the time they were repairing the first fault we were put on a generator and we experienced no voltage drop issues on the generator. If the internal wiring was an issue this would have persisted even while running on a generator. 

I believe the supply loop impedance is what indicated a fault to them the first time as it was around 1.2 (and I believe the maximum should be 0.8ish).

Thanks for the reply

 

[Sidney]

So not a supply issue as such more a demand issue.

You say you have a new three phase supply being installed in the future, this should solve the problem. In the mean time I would consider reducing usage where possible such as not using under floor heating and not having large loads on at the same time.

 

[ProDave]

Yes this is a bit more than one normal dwelling.  Definitely get that upgrade to 3 phase so you can share the house load between the 3 phases.

 

[toontoonizer]

We actually already have a 3 phase supply coming into to the property. For some, reason the previous owner was only using a single phase. We have a 3 phase meter Exchange scheduled this month and then we can distribute load between the annexes, gate etc. 

However, regardless, a 120amp load should not cause the supply to drop below 200v. I don't see how that would change even if we were on a 3 phase supply - clearly there is excessive voltage drop. For examples when I turn on an oven the supply drops by 10volts. I'm pretty confident that is not normal. 

 

[ProDave]

A typical acceptable loop impedance might be 0.3 ohms, and at 120A that could drop 36 volts, so if you only have 230 to start with that would be down under 200V

But a 10A oven dropping 10 volts does not sound right which is why I suggest get an electrician to do a loop test.

With that sort of load through a domestic single phase consumer unit it would not be unheard of to find some issues.

You need to be sure all your stuff is squeaky clean before calling the DNO again.

 

[toontoonizer]

Well the main house and annex have separate consumer units so the load would never be over 100amps at any consumer unit - either way, I appreciate your advice and indeed i think a loop test needs to be done at the main and each consumer unit.

Plugging my car to charge will drop 10volts, turning the hoover on will drop 3 volts, toaster is 5v, coffee machine is 5w, our largest underfloor heating loop is 6v. The stove is an induction hob with a max 7.2kw draw so I'm sure that would annihilate my supply if it was being used heavily. It's predictably always the same amount of drop as you can see, it's been happening for long enough that I know how much drop each appliance causes.

 

[Murdoch]

How far is the house from the road?

 

[toontoonizer]

The cable runs for more than 50m before it joins the main on the road. I know this because they practically dug up the whole cable until the join when investigating a previous fault. 

 

[SPECIAL LOCATION]

Just banding around random figures of voltages and items you plug in is of little use..

(Like me saying my shopping is expensive...  But not listed what items we bought! )

Unless you are also able to quote a circuit characteristics as well...

i.e. cables lengths..  conductor sizes..

As a 10A oven WILL drop 10v from the origin of your installation if it happens to be run via a 135m 6.0mm T&E cable..

(and/or via some combination of other sub-main sharing other significant loads..) 

AND  50m of 25mm carrying 100A will drop nearly 8.75V

Basically you have an overloaded single phase IMHO.

I am guessing 27 acres and two annexes are not going to be short cable runs?

How long and what size cables re on the runs to the annex's? 

 

[toontoonizer]

Just banding around random figures of voltages and items you plug in is of little use..

(Like me saying my shopping is expensive...  But not listed what items we bought! )

Unless you are also able to quote a circuit characteristics as well...

i.e. cables lengths..  conductor sizes..

As a 10A oven WILL drop 10v from the origin of your installation if it happens to be run via a 33m 6.0mm T&E cable!

AND  50m of 25mm carrying 100A will drop nearly 18.5V

Basically you have an overloaded single phase IMHO.

I am guessing 27 acres and two annexes are not going to be short cable runs?

Yup that's fair enough. 

We have a 3 phase supply coming in from the main line on the road >50m away. I don't know what cables they use for this delivery. 

The voltage at the 3phase cutout is 248v on 2 phases - it's ~235v on the other. This goes directly into the main house consumer unit which is right next to the meter. 

Thereis a 25mm twincore+neutral armored cable running from an isolator switch direct to the annex (annex plus kitchen, boiler etc) consumer unit which is probably a 40-50m run. The other annex is further out but not in use at the moment. I'm also not sure but I think only one core of the twin core SWA is being used by the annex with kitchen etc. The other core is going to the other annex.

What makes thing unclear for me is that the voltage in the main house drops the same way, and when the DNO measured the voltage of the loaded phase it is also in line with what is being delivered at the annex (the farthest away measurable point). I've seen the incoming voltage at the annex reach 245v at it's peak. Would this still happen on an overloaded single phase?

It seems like that is indeed partly what the problem here is.

 

[ProDave]

Yes part of the problem might be very long sub mains to the annexes.  Hence why I say get an electrician to find out WHERE the voltage drop is occurring.

If you find the voltage drop is in your installation then it is up to you to upgrade cables or whatever is needed to correct it.  Only if the electrician finds that it is excessive loop impedance at the supply will the DNO be able to fix anything,.

 

[SPECIAL LOCATION]

Yup that's fair enough. 

We have a 3 phase supply coming in from the main line on the road >50m away. I don't know what cables they use for this delivery. 

The voltage at the 3phase cutout is 248v on 2 phases - it's ~235v on the other. This goes directly into the main house consumer unit which is right next to the meter. 

Thereis a 25mm twincore+neutral armored cable running from an isolator switch direct to the annex (annex plus kitchen, boiler etc) consumer unit which is probably a 40-50m run. The other annex is further out but not in use at the moment. I'm also not sure but I think only one core of the twin core SWA is being used by the annex with kitchen etc. The other core is going to the other annex.

What makes thing unclear for me is that the voltage in the main house drops the same way, and when the DNO measured the voltage of the loaded phase it is also in line with what is being delivered at the annex (the farthest away measurable point). I've seen the incoming voltage at the annex reach 245v at it's peak. Would this still happen on an overloaded single phase?

It seems like that is indeed partly what the problem here is.

{ You answered whilst I was doing some corrections to my calculations.. (reading wrong lines off the volt-drop tables)..  }

Anyway..

I think you need full review of all of your circuits..

cable sizes..

expected loads etc..

As there are tables in the wiring regs where the expected volt-drops can be calculated..

Once you know what your expected volt drops for each circuit are..

You can then assess if the voltages you are actually reading are within spec..

Without that actual info and calculations you have very little ammunition to get the DNO to do anything..

It sounds to me like a classic problem of various bits being added or uprated over the years without an accurate re-assessment of loading and volt-drop effects on the overall installation.

e.g.  Two circuits run over long cables may be ok in themselves.. but if both are on together the combined load also increases the volt drop at the supply cable coming to your meter..

You almost need to treat this like a reverse design exercise..

taking loads and distances from source to see what cable sizes should be needed to comply with BS7671 volt-drop regulations.

 

[toontoonizer]

Yeah I get that and that would help explain why there's a drop under load. 

It doesn't explain why the incoming voltage is sometimes 205v at 11pm when there's definitely a minimal load (20amp)

 

[SPECIAL LOCATION]

Yeah I get that and that would help explain why there's a drop under load. 

It doesn't explain why the incoming voltage is sometimes 205v at 11pm when there's definitely a minimal load (20amp)

Appendix 2 of BS7671 states the supply voltage as 230v +10% -6%

which is 216.2V to 253.0V   This is the bit the supply company have to comply with..

And Appendix 4 give max permissible voltage drop.

This is the bit you, (your electricians), have to comply..

As I said earlier just banding around a voltage figure without any circuit characteristics is like me asking you to explain why our shopping is £45 more than the next door neighbours..

(insufficient information)

But lets take a hypothetical example..

If the supply at your meter is was 216v...  {bottom end of DNO's tolerance}

and you have a 20A load at the end of a 30m length of 2.5mm T&E cable...

the volt drop would be 10.8v giving 216-10.8= 205.2v

You need to do a thorough analysis of your whole installtion.

Without that you are just guessing!

 

[toontoonizer]

Sure, understand a thorough analysis needs to be done, but in your example, let's ignore the annex. I'm talking about voltage in the main house. For example, the voltage at the kitchen socket in the main house, which is the shortest run of cable ~5m (to th consumer unit which is 30cm from the cutout) is 205v. The mains supply would have to be less than 216 for that to be the case - and this is what we experience. 

 

[Sidney]

Load and voltage are directly related. The higher the current on the cable the greater the resistance and voltage drops accordingly.

 

[toontoonizer]

Load and voltage are directly related. The higher the current on the cable the greater the resistance and voltage drops accordingly.

Yup understand the relationship between current, voltage and resistance but there is no excessive load in the 205v example. No kitchen appliances - a maximum load of 20amps. 

I feel like I'm missing something obvious here

 

[SPECIAL LOCATION]

Yup understand the relationship between current, voltage and resistance but there is no excessive load in the 205v example. No kitchen appliances - a maximum load of 20amps. 

I feel like I'm missing something obvious here

Sure, understand a thorough analysis needs to be done, but in your example, let's ignore the annex. I'm talking about voltage in the main house. For example, the voltage at the kitchen socket in the main house, which is the shortest run of cable ~5m (to th consumer unit which is 30cm from the cutout) is 205v. The mains supply would have to be less than 216 for that to be the case - and this is what we experience. 

What is the composition of the socket circuit?

Cable size?

If radial, what's the R1+R2 value?

If ring, what are the r1, rn & r2 values?

What's the voltage at the CU terminals?