Currently (2018) the public charging station network isn't enough to let us freely drive our electric vehicles anywhere we want. Most cities have public level 2 and even fast charging, but the inter-city charging network is weak. Electricity is everywhere, but has to be adapted into a charging plug for use with the car. Charging stations adapt electricity to fit safety constraints designed by the automotive industry for safe electric car charging. We're not allowed to simply plug-in to a regular power outlet, because of the safety interlocks designed into the system.
A few scenarios for informal electric car charging at power outlets
It's always preferable to charge at an approved electric car charging station. The UL approval is meant to ensure safety, that the charging station has met all safety requirements, and functions according to the charging specification. Therefore informal charging at a regular power outlet should be seen as a move of last resort.
Many folks are using a regular power outlet to charge their electric car at home. It's great - the typical 10-12 hours the car is parked overnight means perhaps 50 miles of added range. This is low cost - the power outlet may already be there - and puts the 120 volts 12 amps (1.2 kilowatts) line cord charger provided by the manufacturer to good use.
That 120 volts 12 amps line cord charger which came with the car can be used anywhere to get some electricity into the car. Some electricity is better than no electricity. Your travel needs may bring you to places a simple 120 volt outlet makes the difference between a successful trip or a failure.
For example, during a trip to Florida in October 2012 I managed to rent a Nissan Leaf for driving between Orlando and Daytona Beach. That's more distance than the Leaf had, which meant finding places to charge. Neither the hotel nor the Daytona International Raceway had regular charging stations, and 120 volt outlets did the job. The hotel allowed me to use an outdoor 120 volt outlet seemingly set aside for RV's. An overnight charge gave enough range to reach the racetrack. At the I was allowed to use 120 volt outlets in the paddock, and the all-day stay at the track gave enough range to reach the hotel. That worked for a couple days of driving back-and-forth between hotel and race track, and then back to Orlando for our flight home.
The pieces of this puzzle are:-
Which end to extend? We can use an extension cord between the power outlet and the EVSE, or between the EVSE and the car. The safest is to use a J1772 extension cord between the EVSE and the car.
Safety Every step must be done safely. We bought our electric vehicles to, for example, help the climate. Getting on the evening news by causing an electrical fire negates the gain you might have achieved.
What's the charging rate? How many amps will be used by your car through a specific charging unit? Your car will support a maximum charging rate, often 6 kiloWatts. The owners manual will list the maximum charging rate, and some electric cars support changing the rate. The car and EVSE negotiate the charging rate depending on the capacity of the EVSE and car:
|EVSE type||Range per hour of charging||Power required||Circuit|
|Typical line charger||4 miles||120 volts 12 amps||120 volts 20 amps|
|3 kW EVSE||10-12 miles||240 volts 16 amps||240 volts 20 amps|
|4-5 kW EVSE||18 miles||240 volts 24 amps||240 volts 30 amps|
|6 kW EVSE||20-25 miles||240 volts 32 amps||240 volts 40 amps|
What's the required power outlet and cord ratings? The electrical code's 80% rule says a "continuous load" must be limited to 80% of the rating of the circuit breaker and power outlets. The last column in this table tells you the required rating to support a given charging rate.
Ampacity is the measure of the maximum safe current (amps) we can use in a wire of a given thickness. Generally, the thicker the wire the more current it can carry. Wire thickness is measured in "gauge", and you may have seen this number used. A typical home extension cord is 14 gauge, for example. That wire gauge is completely insufficient and unsafe for the line-cord charger that comes with electric cars.
The 120 volt line-cord charger usually runs at 12 amps, and requires a 10 gauge extension cord. The 6 kiloWatt charging stations run at 32 amps continuous, and requires a 6 gauge extension cord.
More details see: Electric car charging within electrical code and power outlet limits