Coming soon! The IET Electrician’s Guide to Domestic Electric Vehicle Charging Equipment Installation
By: Simon Ogborn FIET MSET LCGI
I have to admit that when I was asked to write the Electrician’s Guide to Domestic Electric Vehicle Charging Equipment Installation for the IET, I jumped at the chance.
This wasn’t driven by any urge to spend more hours tapping away on my computer keyboard, nor any particular desire to see my name in print. What made me so keen was the opportunity to play a part in ensuring the installation, of electric vehicle (EV) charging points, is as safe as possible.
Sadly, there is often the attitude that ‘if you can install, for example, an electric shower, then what is the difference?’
As it turns out, a lot.
Luckily for the readers of Wiring Matters, this isn’t an issue, in that you wouldn’t be reading Wiring Matters if you didn’t want to check and improve your knowledge. Hopefully, the guide will be a handy resource, both on-site and in the classroom.
Supporting installers and learners
With the latter in mind, it’s worth noting that the guide is written to complement and exceed the requirements of the City & Guilds 2921-34 and EAL 603/3929/9 qualifications. It is also written to complement both the IET Code of Practice for Electric Vehicle Charging Equipment Installation and the relevant requirements of BS 7671:2018+A4:2026.
Whereas the code of practice and BS 7671 are reference books, the guide is intended to be more instructional, and is limited to domestic and small-scale commercial EV charging point installations. There will typically be a maximum of three Mode 3 charging points in this type of installation.
Managing load demand and system capacity
One of the issues with EV charging is ensuring that the installation can adequately supply the additional load (s), which may involve an additional supply or phase, the introduction of a prosumer electrical installation (PEI), and/or a load curtailment system. Consideration must be given to ensuring that any switchgear assembly, typically a consumer unit, does not get inadvertently overloaded, especially where additional supplies are reverse fed from photovoltaic (PV) and/or electrical energy storage systems (EESSs).
Many electricians will, quite rightly, know that EV charging systems have a particular risk if the PEN conductor in a PME earthing arrangement were to open circuit, whilst an EV is connected to the supply.
What is less well known is that many TN-S systems also have PME sections in the supply network, and thus, this issue is wider than just for TN-C-S. Going back to the time before Amendment 1 of BS 7671, which was released in 2020, the solution to this was usually to use a TT earthing arrangement for EV charging points.
This, however, is not the easy option it appears, and from Amendment 1, open PEN detection devices (OPDDs) have been permitted and extensively employed. OPDDs do have limitations, however, and this is one of the many topics discussed in the guide.
Converting to a TT is still an option, but inserting an earth electrode into the ground is not practicable in many installations. It can be particularly problematic with regard to getting sufficient distances from metalwork connected to another earthing arrangement, be that underground or from a simultaneous contact point of view. This gets even more complicated where the ‘other’ metalwork is in a neighbour’s premises or, potentially, street furniture.
Why cable design matters
Some of the risks with EV charging points aren’t a result of the specific application. The risk exists in other types of circuit, for example, the electric shower mentioned earlier. It is possible to undersize, and thus overheat, the cable to a shower, and it does happen.
The reality is, you are much more likely to get away with an undersized cable for a shower than with an EV charging point, for two reasons. Firstly, the duration of the load. Cables take an hour or two to get to full temperature, even with a small overload. With a shower, that duration is much less likely to be achieved due to the nature of use.
EV charging, however, often lasts for many hours. The other issue is one that many electricians seem to miss: EV chargers draw constant power.
The significance is that, with a shower, the current drawn decreases as the voltage drop increases. It’s simple Ohm’s Law with a resistive load.
For an EV charger, though, because the power stays constant, an undersized cable increases the voltage drop, which, in turn, increases any overcurrent. It also has to be remembered that, with the exception of Mode 4 electric vehicle supply equipment (EVSE), the charger itself is in the EV. This is relevant as the charging cable is also included in the voltage drop issue, and that can also cause the installation cabling to overheat.
A small overload of long duration might take a number of years to degrade the cable, but it will do so eventually and a fire could be the result.
Thermal constraints and circuit calculations
It doesn’t take many factors to significantly lower a cable’s maximum current carrying capacity. The correct application of these factors is, therefore, very important.
The guide also covers the correct application of the formulas for steel wire armoured (SWA) cables, which take into account the use of the armouring as a circuit protective conductor (CPC). Another design aspect that often gets overlooked, but could be the difference between a court appearance or not, is thermal constraints, both for short-circuit and earth fault circumstances.
To this end, the guide has circuit design instructions and a set of examples to show this in action in various circumstances.
Choosing the right RCD protection
BS 7671 is an evolving publication. The reason for this is primarily due to a combination of experience, in that industry feeds back where there are issues, plus innovation. The latter is twofold - new equipment comes onto the market to solve previous issues, but some new equipment causes problems that need mitigating.
Residual current devices (RCDs) fall into both of these in that newer types of equipment, including EV chargers, are causing Type AC RCDs to be blinded by DC being introduced onto the AC supply, but also different types of RCD are now available which are designed to operate correctly with different waveforms present. The guide explains this in much more detail, giving guidance on both the requirement and the reasoning for this.
Addressing the challenges of cross-pavement charging
Another area that requires significant consideration is cross-pavement charging. This presents a range of potential issues, including charging cables requiring derating due to enclosure in pavement trunking, which can get to significant temperatures when the sun shines.
Other issues include the voltage drop in the charging cable, as mentioned earlier, as this can be an increased issue where an extra-long one is required. We mustn’t forget the simultaneous contact issue either. An EV being charged on a public highway could easily be within reach of another EV connected to the supply from a neighbouring property. There is also the likelihood of street furniture to consider in this regard as well.
Closing knowledge gaps
I am never going to say that the guide’s readers will be new to all aspects of all the topics, but what generally happens is that gaps in knowledge, and understanding, get filled along the way. As I have said many times when teaching, I have never known anybody have to apologise in court for double-checking.
Many people have had to for not doing so though. I, myself, study as much as I can, even after over 35 years in the industry. As such, I sincerely hope, if you are installing relevant EV charging, you get this guide and make sure you are undertaking the design correctly, and take the opportunity to improve if there are gaps in your knowledge and/or understanding.
Looking ahead to publication
Needless to say, I haven’t covered all of the content from the guide in this article, but hopefully this will give a flavour of the reason for its inception. The guide is due to be published in the first quarter of 2027. I don’t want to give away the ending, but it will be worth waiting for.
A draft of the IET Electrician’s Guide to Domestic Electric Vehicle Charging Equipment Installation is now available for public consultation here, to give you the chance to comment on the content and ensure that the publication truly reflects the needs of the industry.