You need to run a 10-2 romex[30 amps].You will also need a 2-pole breaker[30 amp]need to know the make of the panel.The white wire of the 10-2 is going to be a hot wire so put a piece a black tape on it, at the water heater wire nut the white wire to the red of the hot water heater and the black to black then at the panel put the white on the breaker and the black on the the breaker there are two screws and ground with grounds in panel.