UK Supply Voltage

[MidhurstNigel]

I've just had a Zappi Mk II fitted and when charging my M3 at a full 32A it typically shows a voltage (presumably at the Zappi unit itself) of about 227 volts whilst the car screen shows about 225 volts. Given the 6.5m of cable from the Zappi to the car, and metering error these are pretty consistent. I was a bit surprised by the figures given the Zappi is connected back to the consumer unit with 10mm cable and there is only 15m of it. My no-load supply voltage is normally almost spot on 240 volts (say plus or minus 1 volt). When I started chasing the voltage around I found that when the Zappi was running at 32A the voltage on my side of the electricity meter was only 230 volts. In other words the voltage drop from meter to Zappi was only 3 volts, but putting on a 32A load dropped the voltage at the meter from 240 to 230. Is this to be expected? The house is about 30 years old with an underground supply and 100amp main fuse. We are only about 300m from a big substation.

As a separate (but obviously related matter) do meters (ours is now a SMETS2 smart meter) record true watts - i.e. do they take account of voltage as well as current when calculating power used?

 

[booyaka76]

Does the car charge at 32A? Does it charge ok?

If so what's the problem?

The voltage used throughout Europe (including the UK) has been harmonised since January 2003 at a nominal 230v 50 Hz (formerly 240V in UK, 220V in the rest of Europe) but this does not mean there has been a real change in the supply.

 

[pdk42]

My charging voltage is usually around 225V when pulling 32A. With lighter load, it's around 240V, so your results are not too different to mine. OTOH, I've got an older property which is at the end of a long run of overhead cables, so I sort of expected it.

 

[Glan gluaisne]

The UK mains voltage spec is 230 VAC +10%, -6%, so the voltage at the cut out can be anything between 216.2 VAC and 253 VAC and still be within the normal acceptable range.

Much of the time the UK LV supply will be above the nominal 230 VAC, because as a fudge to harmonise our previous 240 VAC nominal supply standard we didn't actually change anything, we just adopted an asymmetric tolerance.

 

[MidhurstNigel]

The voltage used throughout Europe (including the UK) has been harmonised since January 2003 at a nominal 230v 50 Hz (formerly 240V in UK, 220V in the rest of Europe) but this does not mean there has been a real change in the supply.

I know this, and it's not reaaly relevant to my point which is - is it usual for the supply voltage to DROP by 10 volts when you draw about 7.4kW of power?

 

[Glan gluaisne]

I know this, and it's not reaaly relevant to my point which is - is it usual for the supply voltage to DROP by 10 volts when you draw about 7.4kW of power?

The normal allowable voltage drop on a cable depends on what it's supplying, but in general it's 5% for power circuits, so, for a nominal 230 VAC supply a voltage drop of up to 11.5 VAC is acceptable.

 

[MidhurstNigel]

The normal allowable voltage drop on a cable depends on what it's supplying, but in general it's 5% for power circuits, so, for a nominal 230 VAC supply a voltage drop of up to 11.5 VAC is acceptable.

It's not the power circuit downstream of the meter that surprises me - a total of about 5 volts from meter to car seems fine. It's the 10 volt drop in the supply that I didn't expect. I don't know what the incoming cable size is but I would have expected it to supply 7.4kW without dropping the voltage coming into the house by 10 volts

 

[Glan gluaisne]

It's not the power circuit downstream of the meter that surprises me - a total of about 5 volts from meter to car seems fine. It's the 10 volt drop in the supply that I didn't expect. I don't know what the incoming cable size is but I would have expected it to supply 7.4kW without dropping the voltage coming into the house by 10 volts

The rules for the DNO side are different to the rules for the consumer side, so all the DNO have to guarantee is that the supply remains within the allowable range. Unless you were getting a drop down below 216.2 VAC then the supply's in spec.

The size of the incoming cable depends a bit on the age of the installation, but most commonly it will be 35mm² concentric coming into the house. The drop on this size cable is about 1.25 mV/A/m, so if you know the length of the run of cable coming in you can take a stab at estimating the cable drop. The chances are that the cable coming into the house will connect to a thicker cable that runs back to the transformer, often 95mm² ABC or maybe 95mm² Wavecon if it's an underground feed. The snag is trying to estimate the respective lengths of the incoming cables as well as assess their size/type.

In addition, the source impedance at the transformer has to be taken into account. It's possible to measure the supply loop impedance, and use this to work back to the sort of voltage drop on the LV side, not allowing for any drop on the HV side.

There's a healthy diversity allowance on the DNO side of the supply, it's not uncommon to find a 400 A capacity cable running maybe half a dozen 100 A supplies. The system allows for the fact that it's unlikely that every consumer will be drawing the maximum they can at the same time. It's one reason why EV charge points can cause a bit of concern for DNOs, as when they become popular there is a higher chance that several EVs could be charging at the same time from the same LV cable.

 

[WannabeOwner]

Dunno if of any relevance but:

I had my meter moved (it was in the wrong place for some building extension work).

Turned out that the underground cable to my house was 4 wires instead of 2 - so 2xL and 2xN. I presume that way-back-then they had some skinny cable lying around and doubled up or, back then, the house was relatively large and builder wanted underground instead of overground, and that was the "solution".

When the Power People spliced the new cable for my new meter (underground) they said they weren't; allowed to splice cables so they just connected up one of each L and N ... so I now have only 50% of the cable that I had before

My charging is often on the low Voltage side ... so far it hasn't been a problem, if we get a good frost or some snow I'll look out for a melt-line to the telegraph pole

 

[MidhurstNigel]

The rules for the DNO side are different to the rules for the consumer side, so all the DNO have to guarantee is that the supply remains within the allowable range. Unless you were getting a drop down below 216.2 VAC then the supply's in spec.

The size of the incoming cable depends a bit on the age of the installation, but most commonly it will be 35mm² concentric coming into the house. The drop on this size cable is about 1.25 mV/A/m, so if you know the length of the run of cable coming in you can take a stab at estimating the cable drop. The chances are that the cable coming into the house will connect to a thicker cable that runs back to the transformer, often 95mm² ABC or maybe 95mm² Wavecon if it's an underground feed. The snag is trying to estimate the respective lengths of the incoming cables as well as assess their size/type.

In addition, the source impedance at the transformer has to be taken into account. It's possible to measure the supply loop impedance, and use this to work back to the sort of voltage drop on the LV side, not allowing for any drop on the HV side.

There's a healthy diversity allowance on the DNO side of the supply, it's not uncommon to find a 400 A capacity cable running maybe half a dozen 100 A supplies. The system allows for the fact that it's unlikely that every consumer will be drawing the maximum they can at the same time. It's one reason why EV charge points can cause a bit of concern for DNOs, as when they become popular there is a higher chance that several EVs could be charging at the same time from the same LV cable.

Jeremy, thanks for your knowlegable input. I appreciate that the supply is well within the 230 -10% figure the DNO has to provide, but as I said i'm 'surprised' by the 10 volt dropped and wondered if anyone else had had a similar experience? FWIW the last time we had some work done the Ze of our supply was measured at 0.26 ohms

 

[Adopado]

I know this, and it's not reaaly relevant to my point which is - is it usual for the supply voltage to DROP by 10 volts when you draw about 7.4kW of power?

Yes, well it's certainly normal for me! Admittedly we are in the country and with a little transformer up a pole supplying the house but we get a good old drop when charging... still well within spec.

 

[Glan gluaisne]

Jeremy, thanks for your knowlegable input. I appreciate that the supply is well within the 230 -10% figure the DNO has to provide, but as I said i'm 'surprised' by the 10 volt dropped and wondered if anyone else had had a similar experience? FWIW the last time we had some work done the Ze of our supply was measured at 0.26 ohms

Voltage drop on the supplier side would be 8.32 V at 32 A, so the 10 V measured at 32 A would indicate that Ze may now be around 0.31Ω, a bit higher than the 0.26Ω previously recorded, but within the normal limit for a TN-C-S supply of 0.35Ω.

 

[MidhurstNigel]

Voltage drop on the supplier side would be 8.32 V at 32 A, so the 10 V measured at 32 A would indicate that Ze may now be around 0.31Ω, a bit higher than the 0.26Ω previously recorded, but within the normal limit for a TN-C-S supply of 0.35Ω.

Never really thought of Ze in that way, but if the mains works as a resistive supply, then I can see how the voltage drop arises (I'm only an amateur in these matters and have only ever thought of Ze in the context of fault currents). In honest trueth there may have been addition load on my side of the meter when I checked out the voltage (maybe washing machine or something), so the total load could have been over 32A, and the 0.26Ω could still be correct. Presumably if I drew the full 100 amps the main fuse would allow I could see a voltage drop of 26 V which would be below the 240 -10% limit. Don't think I'll raise this with the DNO - they'd probably want to drop the main fuse rating! Thanks again for your input.

 

[WannabeOwner]

Don't think I'll raise this with the DNO

Very wise!

I raised it with mine : "Oh, you have an electric car, you need 3-Phase for that, price for upgrade will be £X0,000" ... I could even hear the rubbing-of-hands ...

 

[Glan gluaisne]

You can use Ze to calculate the anticipated voltage drop reasonably accurately, as it's the total impedance of the loop that includes the line conductor, the transformer secondary and the PEN conductor, as measured at the incoming isolator switch/cut out.