EV charge point approved with remedial works

[Ken]

Thanks for that very good explanation.

A modern electric house could easily have the instant shower,HP and charger going at once = 90A say

Also the 100A fuse i understand does not blow until considerably more so one ends up wondering what the real limit is.

I know of someone who regularly pulls close to 100A on TOU tariff and it gets hot! Batts,EV, Immersion, HP, storage heaters. The mind boggles to where this ends up. Perhaps what is needed is a shut off valve/switch at 65A longtime and say 100A for 15mins and people learn to manage within that criteria.

[Oldgreybeard]

Thanks for that very good explanation.

A modern electric house could easily have the instant shower,HP and charger going at once = 90A say

Also the 100A fuse i understand does not blow until considerably more so one ends up wondering what the real limit is.

I know of someone who regularly pulls close to 100A on TOU tariff and it gets hot! Batts,EV, Immersion, HP, storage heaters. The mind boggles to where this ends up. Perhaps what is needed is a shut off valve/switch at 65A longtime and say 100A for 15mins and people learn to manage within that criteria.

There will undoubtedly be a fairly big safety factor built-in to all the component parts of the system. For example, my supply comes more or less directly from a 100kVA pole mounted transformer, via a mix of an overhead run (about 100m or so) of four conductor 95mm² aerial bundled cable (ABC) and a shorter run of underground 95mm² Wavecon ( a three core conductor cable with a wave wound outer combined protective earth and neutral around the outside). The underground cable has a big pot joint, right under our meter kiosk, that converts it to a short bit of 35mm² concentric cable (single core plus a wave wound outer combined protective earth and neutral). That 35mm² concentric cable connects to the main fuse. This is pretty much the standard sort of arrangement for most installations over the past few decades, I believe, although many will have the 35mm² concentric cable coming in overhead from a joint on a pole.

The ratings of these cables are conservative, I'm sure, but the manufacturers seem to rate the four conductor 95mm² ABC overhead cable at 240A, the 95mm² three core plus outer PEN underground cable at 244A when buried directly in the ground, 227A when it's inside a duct and 232A when it's in air (say clipped down the side of a pole, as a part of our supply is).

Looking at our supply cable runs, One of the 95mm² overhead ABC cables seems to supply 9 homes, from what I can see (just looking at where it goes along the poles from the transformer), the other similar cable from the transformer supplies at least 4 homes as far as I can tell, but it does go underground at one point so may feed one or two other homes as well.

Taking the one cable that I can see and trace, and assuming that the 9 homes it supplies are split evenly across all phases, then that cable can provide each home with a maximum of 80A at the manufacturer's rating, assuming an even split (which most probably never happens in reality).

The 35mm² single phase concentric cables that supply each house from the 95mm² distribution cables are rated at 158A when in free air (i.e. as an overhead into a house), 147A when clipped to a wall, and 128A when enclosed in a duct or wall. Seems to be a pretty good safety factor built in to those cables.

The 25mm double insulated single core cables used for the tails from the main fuse to the meter and from the meter to the consumer unit, are rated at 143A when clipped to a wall, or 106A when enclosed in a wall or conduit, so again there seems to be a useful safety factor.

The two areas where overloads seem likely are in the distribution cables to the transformer (seems reasonably easy to moderately overload them), but mostly in the transformer itself. The largest pole mounted transformers made are only rated at 100kVA, so if feeding 12 homes, all equally balanced across all three phases, then the maximum continuous current per home will only be 36A. It seems that the DNOs rely very heavily on their own diversity rules to be reasonably confident that not every home will be drawing a lot of current at the same time.

This is probably reasonable. Looking at our demand over the time since I got Home Assistant up and running earlier this year, the peak has been around 14.6kW. That was a night when the battery was charging at 3kW, the car was charging at 7.4 kW, the hot water was charging at 3kW, the heat pump was running at a bit under 1kW and the house background (ventilation system, treatment plant air pump, UV disinfection unit) was about 200W. This 14.6kW demand was only for around an hour and a half, it then dropped back to about 11.6kW once the hot water was charged.

Clearly if all the houses connected to our pole mounted transformer were drawing even the sustained 11.6kW we were drawing after the hot water was charged, then that transformer would be trying to supply over 150kW, so about a 50% overload.

[AGT]

I think the dno allow 2.5kw per house in a new estate that has gas.

Today might be challenging for them.

[Oldgreybeard]

I think the dno allow 2.5kw per house in a new estate that has gas.

Today might be challenging for them.

I've spent ages searching around the web to try and find EXACTLY what each DNO offers, in terms of continuous power, but this data seems very hard to pin down. Similarly, I've not been able to find a source for the diversity allowance they use, it must be substantially more generous than the allowances the IET publish for the electrical installation inside the house.

Mind you, some of the IET design figures seem well out of date too. They are still stating that lighting circuit loads in domestic premises should be calculated on the basis of a minimum of 100W per lamp holder, believe it or not. Our house isn't untypical as far as lighting goes, we have lots of it, but it's now all LED. With every single light in the house on (that's a total of 57 lamp holders, not including the lighting in the detached garage that has it's own supply) our lighting circuit could draw about 320W (in reality it's rarely more than about 50W). Under the IET design rules I should have designed the lighting circuits to handle 5.7kW . . .

[AGT]

https://www.spenergynetworks.co.uk/user ... 02-012.pdf

[Oldgreybeard]

https://www.spenergynetworks.co.uk/user ... 02-012.pdf

Many thanks for finding that, it looks like SPEN (what used to be SSEPD when I was dealing with them, I think) have followed suit from WPD and upped their maximum for new installations from 15kVA to 20kVA, which is good news for all those with recent installations. This is the bit from that useful document that I think applies:

SSEN domestic.jpg (120.54 KiB) Viewed 2914 times

The snag is that their demand estimator section in that document looks like it comes up with numbers that are a bit on the low side for our usage! Look at their worked example for 20 houses connected to a supply:

Demand estimator example.jpg (105.82 KiB) Viewed 2907 times

I bet some here are massively exceeding those numbers, me included, as we very often pull in well excess of 10kW for several hours at a time over night.

[AGT]

Those numbers only apply to the DNO.
I will pull the load I need to to, if I have 25mm tails and 100 amp fuse more fool them.

[Oldgreybeard]

Those numbers only apply to the DNO.
I will pull the load I need to to, if I have 25mm tails and 100 amp fuse more fool them.

The problem with that is, that if lots of people do the same (say as EVs and home batteries become more popular and there is a strong incentive from much cheaper short period off-peak tariffs) then the DNO's just cannot keep working on the basis of their transformers only being able to supply a few kW per house.

The most probable fix by the DNOs may well be to just adopt the system used by EDF in France, and mandate load limiters. Some friends live in France and if they exceed the allotted maximum current for their home an EDF trip goes in their basement. Their limit is low, something like 40A for the whole house. Apparently this is a standard fitment in France, if you want a higher current supply then you have to pay for a much more expensive tariff, and when you do that EDF come and fit a higher rated trip.

Personally I would not want to see that happen here, so perhaps it might be best if we all thought about the inadvertent impact our actions may have? I can't see the DNOs upgrading the supplies to deliver more current to every home at the same time, it would be a massive and very expensive undertaking. Even in a rural area like ours it would mean replacing thousands of miles of cables and hundreds of transformers. In an urban area the disruption would be a lot worse. Every customer will end up paying for it, too.

Seems better to stay under the radar and try and keep loads down as best we can, lest we end up with a system like France has, perhaps. I think we can be near 100% certain that the DNOs are not going to spend thousands per home on infrastructure upgrades if they can fit something like a limiter trip costing just a few pounds to every home in place of the fuse, as they do in France. Or, perhaps, some sort of current trip that uses the smart meter functionality instead.

[Countrypaul]

Perhaps they will look at those with smart meters and for those that draw more than a certain current regularly charge them a much higher standing charge, or only allow upto say 50A at low tariff rate and if you draw more if is at a much higher rate,

Given that to reduce ghg there will inevitably be more fully electric housing with HP, EV, and electric cooking, over time the DNO will have to ugrade the supply lines, or at the very least make it much easier to prevent high peak usage. I can see more options for TOU tariffs being based on very local loads and time shifting of loads.

[Oldgreybeard]

I've found a description of the way that load limiting with respect to the tariff charges works in France, here: https://www.frenchentree.com/living-in- ... l-systems/

In essence, the cheapest standing charge tariff allows you to pull up to 30A before the trip (they call it a disjoncteur différentiel) operates. The next tariff up allows you to pull up to 45A, the next more expensive tariff above that allows 60A and the most expensive standing charge tariff allows up to 90A.

It would pretty easy, and potentially cheap compared with upgrading transformers etc, for the same system to be adopted here. I don't have much faith that the private companies that operate our electricity distribution system would seek to take anything other than the cheapest solution to a potential local overload problem, I'm afraid.

Around here, for example, the DNO have absolutely no way of knowing which homes might be overloading the transformer, as smart meters seem to not work here. If there were a couple of people on the same transformer as us who regularly caused an overload at the transformer, and that resulted in all of us on that transformer supply having mandatory tariff-related load limiting, or be forced to pay a higher standing charge, for example, then I would not be a happy bunny at all.

This is why I feel that it makes sense to try and self-limit the peak current we pull, just to try and avoid such a scenario. It's also the least selfish thing to do, and the most sensible thing to do in terms of reducing our impact on the planet, which is, after all, one of the key tenets of this forum, isn't it?