It's still single phase, one AC sine wave. The transformer supplies power using three current carrying conductors, two hots, and a neutral. The voltage, or potential difference, between the two hots is 240V. The neutral is a center tap on the transformer, and the voltage between each of the hots and the neutral is 120V. This allows circuits to use either or both 120/240V.
You are correct, however, in your math for total capacity.
It's still single phase, one AC sine wave. The transformer supplies power using three current carrying conductors, two hots, and a neutral. The voltage, or potential difference, between the two hots is 240V. The neutral is a center tap on the transformer, and the voltage between each of the hots and the neutral is 120V. This allows circuits to use either or both 120/240V.
You are correct, however, in your math for total capacity.