12-2 NM/Romex is fine for 240V (120V+120V), and is commonly used for things like large ACs, heaters, etc. Mark the white wire with red electrical tape at both e

12-2 NM/Romex is fine for 240V (120V+120V), and is commonly used for things like large ACs, heaters, etc. Mark the white wire with red electrical tape at both ends (or really any color besides white/grey/green, but avoid black because while technically correct it just blends in as regular electrical tape).