Safely Charging your Electric Vehicle Using a Granny Charger

Is it best to use a charging station?

When purchasing an EV, we would always recommend that you get a professionally installed charging unit outside of your home. This ensures that the supply used has been tested to be safe and will achieve a faster charging time. When not at home, it is always best to use a public charging point.

The charger with a UK plug supplied with most EVs, is known as a 'granny charger', referring to use when visiting family. EV manufacturers generally recommend that a granny charger is not used as the sole means of charging. The fuse clips within the plug becoming loose is the most common cause of an issue. As with any high current appliance, as chargers draw a constant 10As, we recommend checking the plug fuse clips remain tight annually, or if the plug ever feels warm, using these instructions

Can I use an extension lead with my 13A 'Granny' charger?

Electric vehicles are supplied with a charger fitted with a standard 13A plug, often known as a 'Granny' charger. They are designed to charge the vehicle when access to a charging point is not possible. 

The issue often encountered is that the lead is not long enough to reach the nearest socket, leading to owners using an extension lead. The question of whether an extension lead should be used, and if so which type is best, is probably one of the most hotly debated issues on on-line forums! In addition, some manufacturers recommend that extension leads are not used to charge electric vehicles. This is likely to be because most extension leads available online and in DIY stores are:

  • Manufactured from cable which is not sufficiently large in diameter (normally 1mm2 or 1.25mm2) to handle the relatively high sustained 10-13A current required.
  • Most are rather long. Think of a cable like a water pipe. If you want high pressure at the end of your 10m garden, you’re not going to achieve it if you use a hosepipe the size of a drinking straw! The longer the distance, the larger diameter you need to sustain the same flow.
  • Almost always made from PVC or blue ‘arctic’ flex. Despite frequent claims that artic cable is ‘heavy duty’, it is far from it. It is prone to mechanical and UV damage and is not recommended for use outdoors or below 5°C.

There may be occasions when it is not possible to use a charging station:

  • Whilst waiting for a charging station to be installed.
  • Staying away from home.
  • On holiday in the UK or abroad.

In such situations, it is down to the EV user to consider the manufacturers guidance, the safety of the supply and any other risks, before deciding whether to use an extension lead to extend the charger. Given varying advice from manufacturers and varying risks in a whole myriad of charging scenarios, we cannot make a blanket recommendation that an extension lead should be used. Tough Leads simply supply high quality extension leads for higher current applications; it is down to the user to determine what applications they are used for. Throughout the website, the term 'compatible' refers to the socket used being compatible with the larger 13A plugs used on EV chargers. It is does not infer compatibility with the EV charger itself. If in any doubt, consult an electrician. Tough Leads therefore cannot accept responsibility for any damage caused as a result of using an extension lead.  

Why are Tough Leads different to DIY store extension leads?

All of our extension leads are carefully constructed in our workshop from the highest quality and most robust cable and plug/sockets available. The H07RN-F rubber cable is specifically manufactured for harsh environments and its safety and performance under extreme conditions simply cannot be beaten. When combined with heavy duty plugs and sockets, our leads will withstand tough daily use for considerably longer than any other type. H07RN-F rubber cable is perfect for EV charging applications:

  • We only use 1.5mmsq cable, which is suitable for sustained high current applications. As the cable itself is rated at 15 amps, it can easily handle a full 13 amp load, even over longer lengths.
  • Very good mechanical resistance to impact and abrasion, making it ideal for harsh environments.
  • Unlike normal PVC cables, it doesn’t tangle itself into a giant birds nest! Pop it on the floor, give it a wiggle, and it’ll generally straighten out. No matter how long it is stored coiled up, it instantly straightens.
  • Resistant to accidental exposure to a wide range of chemicals, including most oils and greases.
  • Extremely flexible, bending to 6 times its overall diameter.
  • Whist we would never recommend that you submerse an extension lead, it’s good to know that just in case it ends up running through a puddle, the cable is actually submersible to a depth of 100m and can therefore be used outside.
  • Retains its flexibility and performance over a wide temperate range from -25 to 55°C (far more artic than ‘artic’ cable can handle!).
  • Always ensure that leads are fully uncoiled before use, as coiled leads can become warm when used at high currents.


Why do posts on many on-line forums say that only 2.5mmsq flex should be used?

The 1.5mmsq vs 2.5mmsq flex debate seems to be the most common one with regards to EV leads on on-line forums, and sadly the most uninformed. For lengths of up to 20m, our advice is that the 1.5mmsq flex used in all of our leads is the best option for the following reasons (apologies for getting rather technical!):

  • BS 1363-1:1995 A4:2012 the British Standard for 13A plugs, specifically states that flex must not exceed 1.5mmsq with the line ‘having nominal conductor cross-sectional areas not exceeding 1.5 mmsq’. Therefore, other than manufacturer fitted molded plugs, 2.5mmsq flex should not be terminated in a 13A plug.
  • The IET Code of Practice for portable appliance testing allows a safe max length for 1.5mmsq flex without an RCD of 15m, and 30m with an RCD. When PAT testing an extension lead you check for the resistance of the earth- a 15m 1.5mmsq lead would be within the 0.2 ohm limit. Voltage drop on the same lead would be calculated according to the wiring regs as 5.17v, again well within the limit.
  • The data sheets for 1.5mmsq H07RN-F cable generally state a max current of 15A. If the cable is a sensible length, the manufacturers recommended max current is nowhere near exceeded.
  • 1.5mmsq H07RN-F rubber cables do not degrade over time if used up to the full 13A, as this is well within their safe load limit.
  •  In contrast, 2.5mmsq is more expensive, heavier, and more challenging to manage.

How do I know the electricity supply is safe?

Irrespective of whether an extension lead is used, it is important that you consider how safe the supply may be before plugging in your charger.

  • Domestic electrical installations can deteriorate with use and age. It is therefore important that a periodic inspection is carried out every 10 years for an owner-occupied home and every five years for rented properties or HMOs.
  • As sockets age the contacts can loosen, meaning that the contact between the socket and plug pins is not as firm as it should be, which may increase the risk of over heating. We would always recommend charging your EV from a relatively new branded socket, such as one manufactured by MK, Crabtree, Schneider, Hamilton etc. 
  • Carefully check the socket for signs of damage, such as brown marks around the receptacles or cracks.
  • Only use either a single socket, or a double socket which has nothing else connected to it.
  • We would always recommend that the supply is checked using a plug in socket tester. Whilst the unit does not provide the same level of testing or assurance provided by a periodic inspection, it does at least provide a simple check of the supply in order to identify basic problems. The unit simply plugs in, and a row of lights illuminate to confirm that the supply is safe to use e.g. there is an earth connection and the live/neutral are the correct way around.  

Is it worth using an RCD?

Whenever you mix electricity, the great British weather and a metal car, it is absolutely vital to ensure that the entire lead is protected by an RCD. A residual current device (RCD) is a life-saving device which is designed to prevent you from getting a fatal electric shock if you come into contact with a live wire or current. RCDs offer a level of personal protection that ordinary fuses and circuit-breakers cannot provide.

Due to their size, RCD plug units often prevent a lead from being used in an outside socket, or where there is anything above the socket. All of our leads therefore come with the option of an RCD plug or an RCD unit which is fitted in-line with the cable, to prevent such issues.

How about travelling abroad?

Standard travel adaptors must never be used with an EV granny charger. Most are only designed to power low current appliances and some often dangerously do not connect the earth pin.

Our guide to Using UK EV granny chargers in Europe and Scandinavia will provide the support you need to select the correct adaptor from our range of heavy duty EV compatible adaoptors.




Large plugs

  • The plug is rather large around the cable tail, shaped quite unlike anything ever seen within the UK.
  • The cable is 2.5mmsq. The largest flex normally connected to a 13A plug in the UK is 1.5mmsq, as anything larger is specifically prohibited by BS 1363-1:1995 and A4:2012. However, in this case, as the plug is fitted by the manufacturer, it is perfectly safe.

The plug didn’t fit into the majority of sockets, as the body of the plug protruded too far around the flex tail. For the small number of sockets that the plug actually did fit, with one exception, the lid then could not be closed to maintain the waterproof rating, due to the larger overall diameter of the 2.5mmsq flex.