[frosted-flakes]

240V circuits are simply both legs of the 120V split phase connected together, so the amperage is measured at 120V. The circuit breakers for 120V and 240V circuits are the same, except 240V circuits use two breakers with the handles tied together[1]. Electric resistive heaters like ovens and clothes dryers are always perfectly efficient, so I'm not sure how you even thought it would be possible to have an inefficient heater.

[1]: The 120V rails in the middle of the breaker panel alternate between each split phase, so if you connect a circuit to any two adjacent breakers you will get 240V. A 120V circuit is connected to neutral and a single breaker.

[Symbiote]

As well as using a heat pump, a more efficient dryer can be better-insulated, or make better use of the heat produced in drying clothes, however that might be done.

[mcbishop]

> I'm not sure how you even thought it would be possible to have an inefficient heater

They're inefficient relative to a 250-500% efficient heat pump.