Granny Chargers: Charging an EV From a 3-Pin Plug Safely
Updated: Oct 9
Yes, you can charge an electric car from an ordinary 3-pin socket, using a granny charger: the portable lead, often supplied with the car, with a household plug at one end and the car connector at the other. It is slow, though, at around 2.4 kW, and it asks a household socket to carry a heavy load for many hours at a stretch. On a sound socket and circuit it is fine for occasional use. As the main way to charge every night, it is the option we'd least like to see, and a dedicated EV charger installation on its own circuit is the safer long-term answer.
How slow is charging from a 3-pin plug?
Very slow for a full battery. Which? puts a 3-pin socket at about 2.4 kW and says a full charge can take at least 30 hours depending on the car. In its own example, a car that takes 12 hours on a 7 kW home charger takes 38.5 hours on a 3-pin plug.
The Energy Saving Trust's rule of thumb is that charging power in kW roughly equals the miles of range added per 20 minutes. At about 2.4 kW, that is roughly 2 to 3 miles of range for every 20 minutes plugged in, or somewhere around 7 miles an hour. If you drive 20 miles a day and plug in overnight, that can keep up. If you drive 60 miles a day, it won't.
Why can a normal socket overheat on EV charging?
Because a socket, plug and circuit that are perfectly happy running a kettle for three minutes are being asked to carry close to their full rating for 10, 20 or 30 hours. Electrical Safety First puts it plainly: the extended load and duration of charging an EV are likely to exceed what a standard socket-outlet and standard wiring is designed for.
The weak points are the ones that were never a problem before:
Worn socket contacts. Older sockets lose their grip on the plug pins. A loose contact has higher resistance, and resistance under a sustained load means heat.
Loose terminal screws. A connection that has worked loose inside the socket or anywhere along the circuit heats up the same way.
Old or undersized wiring. A radial or ring circuit that has been extended over the years, or wiring that is simply old, may have less margin than it once did.
Other loads on the same circuit. Your granny charger shares the circuit with whatever else is plugged in. A tumble dryer and a heater on the same ring as the car adds up.
The failure is rarely dramatic at first. It shows up as a plug that is warm to the touch, a socket face that has discoloured or gone brown around the pins, a faint smell of hot plastic, or a breaker that trips partway through the night.
Can you use an extension lead for a granny charger?
We'd say no. Extension leads are rated at 13 A (some older ones at 10 A), so an EV charging lead can use up most or all of that rating for hours. Electrical Safety First warns that running a cable reel while it is still wound can make it overheat and catch fire, and that you should never plug one extension lead into another. Running a lead out through a window or door also damages the cable and leaves connections exposed to rain.
If the car can't reach a socket, the answer is an outdoor socket or a dedicated charger fitted where you park, not a longer lead.
Is it safe to charge from an outdoor socket?
It can be, provided the socket is weatherproof, in good condition and properly protected. BS 7671 requires 30 mA RCD protection for socket-outlets up to 32 A and for mobile equipment up to 32 A used outdoors, and an outdoor socket feeding a car is exactly that case. Older outdoor sockets sometimes have no RCD protection at all, particularly where they were added to an old installation. Which? recommends asking a qualified electrician to check the cable and socket are suitable before you rely on 3-pin charging, and we agree.
How to use a granny charger more safely
If a 3-pin lead is what you've got for now, these habits cut the risk:
Use one socket for the car and nothing else. Electrical Safety First's advice is that a 13 A socket used for EV charging should be the more robust type marked BS1363/EV, on its own dedicated circuit. If yours is an ordinary socket, have an electrician check it before you rely on it.
Plug straight into the wall socket. No extension leads, no adaptors.
If the lead lets you choose a lower current, use the lower setting when you don't need speed.
Check the plug and socket the first few times you charge. If either is more than slightly warm, stop.
Inspect the lead regularly and stop using it if it's damaged, as Electrical Safety First advises.
Don't leave the control box lying in a puddle or hanging by its cable.
When is a dedicated charger the better answer?
When charging from a 3-pin plug has become your routine rather than your backup. A dedicated charger has its own circuit sized for the load, its own RCD protection suitable for DC faults, and a proper outdoor connector, so nothing else in the house is carrying the car. At about 7 kW it also charges roughly three times faster.
We fit chargers on their own dedicated circuit, and we check the supply, earthing and consumer unit before we do. If the board can't take another circuit, that is the point to look at a consumer unit upgrade. Most homes stop at 7 kW because they have a single-phase supply, which we explain in our comparison of 7 kW and 22 kW chargers.
With more homes running cars, heat pumps and induction hobs off the same supply, the load on household wiring is changing fast, as we cover in our look at UK home electrical trends. If you're charging from a 3-pin plug in Wolverhampton, Telford or Shropshire and want the socket checked or a charger fitted, contact us and we'll take a look.
Granny charger questions, answered
Will charging from a 3-pin plug trip my electrics?
It might, if the circuit is already carrying other heavy loads or if there is a fault the extra current brings out. Repeated tripping is a sign to stop and have the circuit checked, not to reset and carry on.
Is a granny charger the same as a Mode 2 cable?
Yes. The in-line control box on the lead is what makes it a Mode 2 charging cable. It manages the charge and provides some protection, but it can't make an unsuitable socket or circuit safe.
Should an electrician check the socket first?
Yes, if you plan to rely on it. A quick inspection of the socket, its connections and the circuit's protection tells you whether it can take hours of load, and costs far less than dealing with a burnt socket later.
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