OIC. The part I was missing is that the electrician goes direct to the breaker box as the electric dryer outlet could have thinner wires. However, I just checked my circuit breaker box and the 240 VAC is on two 30 amp breakers, which should mean the in-house wiring should handle 30 amps or more.The electrician will determine how thick the actual copper (inside the cable) needs to be based on how long the run from the circuit breaker box to the outlet is. Thicker if the run is much longer.
If you add a long extension cord, you're messing with that calculation -- you could find that the copper needs to be thicker than it actually is. It can cause overheating in the circuit. In practice, you usually get "voltage drop" instead.
So basically you have to make sure that all the cables and wall wiring involved have thick enough copper to handle the current for the TOTAL length from the circuit breaker to the car. The copper in the wall might not.
FWIW, the 240 VAC outlet is just a few feet from the outside circuit breakers, so I would not expect much voltage drop.
And perhaps I can measure the voltage at the vehicle end, under max load.
Or by any chance is such info as the charging voltage and /or wattage shown on the Model 3 screen? That could be handy.
My electric motorcycles show the output charging wattage & voltage via Bluetooth to a Smartphone with a Zero app.
-Don- Reno, NV