The motor back emf is one reason why you need a supply voltage to the drivers much higher than the nominal voltage of the motors. It does not affect the gauge of wire you need.
Bear in mind that if you decide to increase the motor current above 1.5A, then the increase in current from the 24V supply will be much more because the 24V current goes as the square of the motor current. So I think your factor of 1.5 is rather low. Also, I suggest you allow a higher factor for the X and Y motors (or all three motors if it is a delta) because of the back emf during travel moves.
Bear in mind that if you decide to increase the motor current above 1.5A, then the increase in current from the 24V supply will be much more because the 24V current goes as the square of the motor current. So I think your factor of 1.5 is rather low. Also, I suggest you allow a higher factor for the X and Y motors (or all three motors if it is a delta) because of the back emf during travel moves.