What size breaker do you need for a tankless water heater?
Keeping this in view, how many amps do you need for a tankless water heater?
Similarly, does a tankless water heater need a dedicated circuit? A tankless water heater will require a dedicated circuit as specified by the installation manual of the unit that has been selected. If you need additional space in the panel for the circuit breaker then a quad circuit breaker may be allowed as specified by the make and model of the panel.
Beside above, what size breaker do I need for a 220 volt water heater?
A water heater of this type is required to be considered a continuous load meaning that the branch circuit needs to be calculated at 125% of the heater's rated ampacity. 4500/240 = 18.75 amps, will work on a 20 amp OCPD but, 18.75*125% = 23.4 amps, requires a 25 or 30 amp circuit.
What size breaker do I need for a 240 volt water heater?
Since this is a purely resistive load 5500 watts at 240 volts is 23 amps, breaker is sized at 30 amps and 10 awg wiring is used. In your case if the elements are strapped for 4500 watts draw is 18.75 amps, permanent draw loads should be sized at 80% of breaker capacity so a 20 amp breaker is limited to 16 amps.