Obviously electric car manufacturers are afraid owners of cars they've manufactured will cause an electrical fire. On multiple occasions sloppy charging situations have caused electrical fires while an electric car was charging.
Electrical fires happen, typically, because an electric circuit is overloaded. If the power cord is getting warm, that's a sign the cord is overloaded or else shoddily built. If the power outlet is getting warm, that's a sign it is overloaded or else shoddily built. If the circuit breaker pops, that's a sign something on the circuit is either drawing too much electricity, or else something is shoddily built.
Every wire or electrical connection is rated for the maximum current (Amps) it can carry. An overloaded circuit is carrying more current than its rating. This creates heat in the wires, and with enough heat things can catch fire.
A shoddily built wiring system or power outlet can also create heat. That dodgy outlet in the garage probably gets hot when in use -- the dodgy connection acts as a resistor, emitting heat as electricity flows. And with enough heat things can catch fire.
That's the technical issue which would cause an auto manufacturer to warn against using an extension cord while charging an electric car. The manufacturer cannot trust that all their customers are savvy enough about electricity to recognize a dodgy situation that must be avoided.
Important electrical facts about extension cords and electric cars
What I'm about to say is from my knowledge of USA electrical code learned while studying the design of solar arrays and ancillary equipment. Solar array design boils down to safe electrical system design.
Every component for electrical systems are designed for a maximum current, measured in Amps.
That rating is a maximum allowable current through the device. The maximum current for a "continuous load" is 80% of the rating printed on the label. This means a circuit rated for 20 amps can at most be used for 16 amps continuous load.
Charging an electric car is a continuous load. Hence every circuit you look to charge your car with must be rated for 125% of the current required to charge your car. (1/80% is 125%)
|Charge rate||Circuit rating required|
|12 amps||16 amps|
|16 amps||20 amps|
|24 amps||32 amps|
|32 amps||40 amps|
|40 amps||50 amps|
For example the typical 12 amp line cord charger which comes with the electric cars requires a circuit rated for 16 amps. A 15 amp circuit is not sufficient, and since circuits are never rated for 16 amps the next higher size is 20 amps. The same holds true for the 24 amp device, which would then be required to use a 40 amp circuit.
Notice the capacity is rated in amps, not volts, and not watts. That's because current carrying capacity is based on how thick the wires are. These devices are also rated for maximum voltage. In most cases the maximum voltage rating is way above what we'll be using for electric car charging.
To read more about the electric code, see: Electric car charging within electrical code and power outlet limits
Advice we can derive from these electrical facts
Thicker wires carry more current. If we want to charge at 240 volts 32 amps, that means an entire electric circuit where every component is rated for each 40 amps.
Therefore successfully using an extension cord to charge an electric car means using good quality cord with good connectors and THICK wire.
Safely using an extension cord to charge an electric car
Given the above summary it seems the manufacturers are guilty of an overabundance of caution. Someone who IS savvy about electricity could safely use an extension cord with an electric car charging station.