Electrical Vehicle Chargers - No Diversity?

Essentially you have got to the root problem of EV charging. There is simply not enough electricity to go round! Let's look one stage higher, a street. This has a high level of diversity applied already by the DNO installation, ie 5-10A per property. How would load management help at all when the street full of cars are all charged? The answer is it won't and so a lot of people are going to loose their supply, either by fuses blowing or smart meter switch off. In some places it is probably happening already, but the regulator and Government are remarkably quiet about it, I wonder why?

pww235 said:

Yes, but in the absence of a load management system it's still zero diversity.

BS 7671 does not prohibit diversity for EV charging points.

The application of diversity will depend on the individual installation.

Diversity in general is applied:

(a) to circuits, and

(b) to parts of the installation, and the installation itself (effectively, switchgear, submains, etc.)

NOTE: Each EV charging point must be on its own final circuit ... BUT that final circuit may be within the charging equipment itself. This analogy should not really be applied for domestic charging points in general, but in terms of commercial/industrial/public charging equipment, there may be more than one charging point per item of charging equipment. So, there are a number of cases where one submain or feeder pillar etc. feeds many charging points.

In installations where EV is a significant proportion of the load of the installation, or swtichboard/distribution board/consumer unit, then it's unlikely to be able to apply diversity, especially if it's impossible to make "time of use" type assessments.

In larger installations, if a designer's modelling shows that it is highly improbable that overloads will occur, BS 7671 does not prevent that -- but that is at the designer's own liability.

In practice, many charging points in domestic installations are being installed with load control these days, to spread more widely the available power, and also larger installations are beginning to take advantage of that (for example, may be much better for a business whose employees work 8 hours per day to offer on average curtailed, or even fully-curtailed, i.e. 3 kW only, charging for employees' vehicles during the day)

smart meter switch off

Smart meters are capable for a lot more than just switching the entire installation on or off. They already have the capability of signalling current tariff rates to equipment within the installation, and for different customers on the same street to be on different tariffs at the same moment. If problems were to arise (and given that we coped with lots of people in some districts having off peak storage heating I'm not sure that's a given) it doesn't seem to be beyond the wit of man to stagger off peak tariffs across homes in the same street in some form or another.

Average car mileage is apparently 7,400 miles/year (pre-pandemic) - at around 3 miles per kWh for a typical EV - so would need around 2,500kWh/year - or about 6.75kWh/day on average - so less and an hour for a typical 7kW charger.

If you want an example of how thin looking infrastructure can actually supply a lot more power - just look at France. A quick glance at their small transformers feeding mile-long set of ABCs through a village and a 30A or 45A single phase limit per house might convince you that they must have very modest demands - yet looking at the figures they consumer about twice as much electrical energy per head as we do.  The trick they use is to spread the consumption far more evenly though the day and night, rather than bunching it up like we do (morning and evening for domestics, during the day for commercial/industrial). The driver for them was their fleet of nuclear power stations which are much more efficient when feeding a constant load, but the same techniques could work for us.

   - Andy.

Yes Andy, but somehow you need to tell the customer how much they will pay for goods before sale, and just changing the tariff for load management is probably illegal. The obvious answer is that the tariff for electric vehicles should be high enough to discourage use, or make people not charge at home because the infrastructure costs are not considered for electric vehicles for political reasons. Boris wants to build 8 more nuclear stations at a probable cost of £250 Billion, presumably to be added to our electricity bills over the next few years. That is roughly £10,000 per household, so another £2,000 per year whatever the consumption, and the building is so ungreen that the outcome is no gain in anything! The point is that none of these ideas actually work, and are hopelessly expensive. Nuclear is useless for turbine backup too, so what is the point, we still need gas!

Would you accept an electricity tariff that depended on your neighbours useage, and you didn't know the price up front? Do you never look at goods prices in the shops? If so you must be very rich!

That seems a terrible response to having rubbish infrastructure.

If we discourage people from charging at homes, we'll need even more rapid chargers (at least 50kW each) than we do already.  Because everybody would have to be using them to charge their cars.  And all those rapid chargers are going to need new infrastructure installed.

Rather than bodging it with an inferior solution, why not have a rolling program of upgrading residential substations to allow people to slow-charge at home?

but somehow you need to tell the customer how much they will pay for goods before sale, and just changing the tariff for load management is probably illegal.

I don't understand - prices could either be fixed (e.g. super-cheap for say 1 hour of the overnight off-peak, different houses on the street being allocated different hours) or could be dynamic with prices pushed to the customer up to 24 hours in advance - like the Octopus Agile tariff already does. Neither sounds even slightly illegal to me - it's the same basic principle as the old E12/E10/E7 tariffs - helping to match demand with available supply - if done a little more smartly.

   - Andy.

Most homes could accept charging an EV at 7 kw overnight when other use is limited. Charging during the evening peak could be discouraged by tariffs.

Whilst existing street mains and substations could not supply a 7kw EV charger in every home, does anyone really expect an EV charger in EVERY home, and all used every night ?

Agreed, 'cos that's where diversity does come into play.

Over the years, my commuting has varied enormously, from an 800-odd mile weekly round trip, but walking to work during the week; to a regular 8 miles per day, 5 days per week.

In the former case, ignoring the need to charge up en route (I could just do it without re-fuelling, but not without replenishing the oil!) I'd have 5 days to recharge; and in the latter case, it would hardly have been worth plugging in each evening.

As for the short commute, there would never have been any recharging at work because of Government policy although oddly, had I chosen to ride to work, I could still have parked my horse in the paddock like everybody else.

Yes, the installation as a whole may have diversity applied, and if a load management system is employed then that can be used to limit the loading by setting a maximum power consumption for the EV chargers that is lower than their rated delivery. But in the instance of the original post, a TPN distribution board supplying 5no. 32A TPN chargers, no diversity should be applied to the EV circuits when calculating the total DB load and it's submains cable....unless there's load management.

pww235 said:

But in the instance of the original post, a TPN distribution board supplying 5no. 32A TPN chargers, no diversity should be applied to the EV circuits when calculating the total DB load and it's submains cable....unless there's load management.

Well, I'm almost with you and tend to agree in most cases with such a small number it's probably best not to apply diversity just in case all chargers were to start the cycle and request full 7 kW at the same time ... but BS 7671 doesn't specifically say you can't

It remains the case that 7 kW chargers in use don't use 7 kW all the time, and even with < 10 no. chargers, we might think about models to consider the probability of all charging points demanding full load at the same time.

The situation is complicated by the fact that in some situations, the concurrent max load draw being requested is more likely than others.