MG4 - charging from a 13A socket

[Gordon]

In 18 months ownership I have only ever used the granny charger. Plugged into a hard-wired extension from the garden shed which itself is a plug-in extension from the house. All in 13A rated cable. never knowingly had an issue. It takes 2.2 kW according to my mains monitor. The granny charger plug gets slightly warmer than ambient but not as bad a an electric kettle can.

 

[Fogey]

It's not the cabling or the granny charger - it's the plugs and sockets (excluding the granny charger's own 13A plug, which has thermal protection). They're not protected against thermal runaway, which can occur at any time when there are contacting metal surfaces. As I've said.

You have also to be confident about your house wiring too.

 

[tsedge]

Wallboxes offer a number of advantages:

• Engineered to last and take high loads.
• PEN fault (and other) protections, that reduce the risk of electrocution.
• Installed by an electrician, typically on a dedicated circuit which is provided and checked to make sure it is rated correctly.
• Load monitoring for the house so can reduce the draw if you have a high domestic load for your supply.
• Wifi connections and apps for remote control and scheduling and integration with other equipment such as solar / battery and special tariffs.

"Granny" chargers are more suitable for occasional / temporary / emergency use, although many people seem to rely on them.

Main risk with these are:

• Poor quality mains wiring (old / unchecked).
• Shared circuits with other high-load appliances causing occasional overload.
• Use of extension leads not rated for the continuous load, not uncoiled and/or not waterproofed.

The risks are generally cumulative:

• Poor connections cause high resistance.
• High resistance causes temperature spikes.
• Temperature spikes cause expansion / contraction.
• Expansion / contraction worsens connections, increasing resistance.
• The loop continues and gets worse over time.

It is easy to try a granny charger and because it worked fine, assume it will always be fine and forget about it - but regular monitoring is needed to check the temperatures and look for any evidence of charring / scorching / burning on connections.

Unfortunately, this is exactly the opposite behaviour to human psychology: once we've done something a few times we tend to switch to automatic pilot and stop checking.

All of that said, fires and electrocution do seem to be rare.

 

[Everest]

re comments from @tsedge and @Fogey.

The main concern with using a regular (rather than EV-rated) 13A plug+socket is that they are not designed for or tested against continual high loads or disconnection of inductive circuits under load.

The weak points are:-

a) the contact of the live and neutral pins which, being square, rely on a small surface area of contact. The resistance of which will increase due to tarnishing on the brass pins and weakness over time of the spring of the socket's connection and

b) the limited contact area and quality of the connection of where the fuse is held within the plug.

The latter was particularly evident from the heat generated from the fuse connection in the plug as @Yorkshire Hillbilly experienced, shown in these postings...

Noting that all had been working fine for 2 years, before this happened...

Thread 'Don't charge from a 3 pin plug when it's 27 degrees outside kids'

Jun 18, 2025

Good job tapo KP115s havs thermal protection, lucky escape there. We set to 8 amps too. Never had a problem for 2 years even at max speed during the winter.

• Yorkshire Hillbilly
• charging electricity pin plug safety temperature
• Replies: 28
• Forum: General Chat Forum

• 

and

Post in thread 'Don't charge from a 3 pin plug when it's 27 degrees outside kids'

Jun 19, 2025

Have you got a picture of the 'granny's' plug?

It wasn't granny's plug thank god. It was an industrial extension cable approx. 25m long to a weather proof socket.

• Yorkshire Hillbilly

• 

 

[GaryMG4]

Oddly enough for the 22 months I used the 3 pin I would check the socket, Tapo (OMG he used a Tapo!) and plug each time I plugged in. Most of the time I just used the (then) 4 hour Go slot without any issues with just the occasional longer slot to use solar excess and one 28 hour charge! Now I only use it when one car is on the Zappi and it is an Octopus free slot, but all I do is make sure the plug is slightly warm.

 

[Everest]

Some fag-packet maths says that limiting it to 6A and only charging at times when we're at home and awake, will add more miles than we use (over 250 miles/week added vs under 200 miles/week used)... so if we do use granny charging as a stop-gap until the proper charger fitted (I'm expecting a quote for that today) we should be able to do so with safe-ish limits.

My gut feel is that 8A would be a good compromise for granny charging from a non 'EV' rated socket. If the plug or socket get significantly hot with an 8A load then they should be replaced - there is a clear fault with them.

Charging at 6A is, obviously, safer still. But bear in mind that there is likely to be 200W or more of quiescent vehicle electronics running when charging, so around 1A of your charge rate will be "wasted" powering the vehicle. This is in addition to the 10 to 15% inverter conversion loss from AC to DC. Hence at 6A, you will be only be putting in around 5A into the inverter (hence a loss of 16% on top of the inverter's losses).

FWIW... The power dissipated at any point of resistance (assuming a largely resistive load) is proportion to the square of the current drawn. From Ohm and Watts laws (V=IR and P=VI) you get P=I * I * R.

Hence, if you dodgy plug and socket has a resistance of (say) 150 mOhms, then the power dissipated by that connection would be:-

At 6A; power = 5.4W
At 8A; power = 9.6W
At 10A; power = 15W
At 13A; power = 25.3W

OMG he used a Tapo

Yes. Not sure how it was used, but it is never a good idea to force disconnect an inductive load through equipment that is not designed for it (such as a regular BS1363 plug/socket).

If one is using a granny charger, I'd always ensure that the charging has been terminated from the vehicle before switching off or unplugging a 13A plug - they are not designed for interrupting inductive loads.

 

[EvTek23]

If one is using a granny charger, I'd always ensure that the charging has been terminated from the vehicle before switching off or unplugging a 13A plug - they are not designed for interrupting inductive loads.

That's why one of the reasons the type 2 connector is locked into the vehicle while charging, to prevent arcing damage by disconnecting while current is flowing, and secondly security / theft.
When you terminate the charge from the car, by say unlocking the car, then the car stops drawing current and signals to the charger and unlocks the charging port so that the type 2 can safely be removed. Of course if you just unplug the granny charger 13A plug this process would not happen so the full current would be flowing at the moment of disconnection.

 

[GaryMG4]

Yes. Not sure how it was used, but it is never a good idea to force disconnect an inductive load through equipment that is not designed for it (such as a regular BS1363 plug/socket).

I set the Tapo to switch on 5 minutes before the car started the charge, and off 5 minutes after the car had terminated the charge. I found it useful to work out exactly how much energy was used and monitor the balancing process.