Electricity is everywhere, and could theoretically power electric cars anywhere we wish to go.
If we only charge at home, we're limited to driving within a circle around our home. Public charging stations extends the area we can drive with, but that's still limited to areas with charging stations. Why should we allow either limit where we can drive? Electricity is everywhere, why not access that electricity to drive anywhere? At this time that means developing knowledge of some simple rules for safely using extension cords and adapter cables matching any power outlet, and carrying with you a high power charging station.
Currently (2017) 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. Accessing a high power electric outlet with a portable high power charging station means we don't have to be limited to the public charging station network, and can charge at a reasonable rate nearly anywhere.
The car manufacturers provide a 120 volts 12 amps (1.2 kilowatts) line cord charger with each car. In a pinch that station can be used anywhere to get some electricity into the car, but at 4-5 miles range per hour of charging you'll be parked for a long time. They're great at home or anywhere else where a long recharge time is acceptable.
The outlet shown to the right, a NEMA 14-50 outlet, or other 240 volt 50 amp outlets, are the primary objective for full-speed 6 kiloWatt electric vehicle charging away from both home and the public charging network. This, and other 240 volt 40-50 amp outlets, will let you charge at 6 kiloWatts continuous power. That's enough for 25 miles of range per hour of charging. (see Understanding charging rates and effective trip speed) Many RV parks have NEMA 14-50 outlets, making them potential informal charging locations.
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.
The J1772 charging protocol ensures safety between the car and the portable charging station. The risk comes from the power outlet you're using, and the quality of any extension cord you're using.
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
Is it safe to use extension cords and adapters to charge an electric car? Yes, with care. Can you charge an electric car from a dryer outlet? Yes, with care. The short answer is to use a good quality extension cord with thick wires and beefy plugs. In the lists below we show several choices for 120 volt 10 gauge extension cords that are safe for use with a line cord charger, and several 240 volt 40 amp extension cords safe for use with higher power charging stations.
WARNING: Vehicle manufacturers universally warn against using an extension cord to extend the reach of the supplied line-cord charger. Please, exercise caution in reading what is written here.
The general principles are:
What is a substandard dangerous situation? A skinny cord, frayed wiring, bad power socket, acts like a "resistor" or "heating element". Sending too much current through a bad connection acts like a heater. Say your car is pulling 6 kilowatts, a 10% loss from a bad connection means 600 watts being dissipated as heat, or about the same as a typical hair dryer. That's a common cause of electrical fires.
Avoid violating Electrical codes Smart electricians and electrical engineers have spent decades cooperatively developing the electrical code. Their recommendations were won through hard work and the occasional electrical fire.
Your fate is in your hands - this is for educational purposes. I am offering this for education, and you are responsible for your actions. Applied incorrectly this can cause electrocution or fire. Take care, be smart and educated, and you'll be okay.
The need for safety: Done badly, it's possible to cause damage. The warning against extension cords isn't a whimsy, they're rightfully concerned about safety. Carefully done, you can safely ignore that warning, but you must know what you're doing. There have been multiple fires from electric cars being charged through substandard wiring. How? Bad wiring can easily cause enough heat to raise surrounding materials to the ignition point. This isn't a new issue since electrical fires have happened for as long as we've used electricity. Careful use of good quality extension cords and power outlets is the way to avoid an electrical fire.
The most correct electric car extension cord fits between the J1772 plug from the charging station to the J1772 socket on your car.
Think about the concerns just raised: skinny frayed extension cords plugging into dodgy power outlets that might cause an electrical fire. An extension cord that you know is built for the purpose, from components rated to the load that will be carried, is the safest choice. Unfortunately there are cheap sub-standard J1772 extension cords, just as there are cheap substandard extension cords of other types.
Ratings: As with every other component in the charging system, the J1772 extension cord must be rated to handle the current shown in the chart above. For a 6+ kiloWatt charging session, the cord needs to be rated for at least 40 amps, for example.
Wire size: To be useful, a J1772 extension cord has to be 25 feet or longer, which can mean a significant voltage drop with thin wires. For a 6+ kiloWatt charging session, the wires should be 8 gauge or thicker, going by the ampacity charts referenced earlier.
Flexibility: Your J1772 extension cord will be used in a variety of settings, so a flexible cord is a must.
The following are some useful extension cords for use with an electric car charging station. Since they're designed for regular power outlets, they fit between the charging station and a power outlet.
NOTE CAREFULLY: When in the store looking at extension cords, read the label carefully and look at the thickness of the wire.
Some of these have a molded plug on one end, and bare wire at the other. That makes them excellent starting points for building adapter cords. Simply attach the desired plug or socket to the end with bare wires.
These will be primarily used with higher power 6+ kiloWatt charging stations. Every one of these extension cords have the beefy wires and connectors required to handle a 40 amps continuous charging rate. That's enough to handle any electric car charging situation you'll face.
These are the same as the NEMA 14-50 cords, but with 6-50 connectors instead. They also support a 40 amp charge rate, and will handle any charging situation you'll face.
Coleman Cable 1917 8/3 STW 6-50 Welder Extension Cord, Blue, 25-Foot. 8-Gauge plug. Stw jacket for heavy-duty performance. Lighted receptacle end indicates when power is on. Molded ends offer safety and convenience on the jobsite. Bring portability to welding machines and create convenience. Provide a convenient, safe method to extend the range of welding equipment. Intended for new commercial construction, mro industrial and general welding.
The outlets rated for 30 amps are useful with the 16 amp or 24 amp charging stations. Using these outlets may require buying or building an adapter cord to match the EVSE's plug to the power outlet.
NEMA L6-30 extension power cord allows you to extend an L6-30P power cable up to 20 feet. Rated for 30 Amps, 250 Volts. L6-30P plug to L6-30R connector.
A 15 foot L6-30P to L6-30R locking extension power cable, rated for 30 amps and 250 volts. Used typically in rackmount PDU's, servers, UPS, and generators.
Ditto, and the 20 amp outlets are limited to use with 16 amp charging stations.
The 120 volt power outlets are limited to use with the line cord EVSE sold with the car. When looking at 120 volt extension cords you must be very careful with the wire gauge. The typical skinny 14 or 16 gauge extension cord simply won't cut it for EV charging. You must use a heavy-duty cord with 10 gauge wires and good quality connectors.
You won't always find the nirvana of matching the charging station to the power outlet. An adapter, whether commercially made or home-made, is often necessary. Be careful with adapters to avoid the position of drawing 32 amps from an outlet rated for 16 amps. If you're lucky the circuit breaker will prevent a serious problem.
The TT-30 outlet is 120 volts up to 30 amps (24 amps continuous).
Charging an electric vehicle is almost certainly going to happen outside. It might be raining. Water and electricity is of course a bad idea. GFCI plugs are designed to make it safe.
These products are useful for building custom adapter cords.