The wire size is dependent upon the wattage size of your water heater.
Breaker and wire size for 4500 watt water heater.
You have stated that the calculated amperage load for 4500watt at 240volts is 18 75amps however nec article 422 13 states that the minimum circuit rating for a water heater must be 125 of the rated load so this 4500watt load requires a minimum circuit of 23 44 amps at 240volts.
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.
Multiply the current rating of the water heater with 125.
Hj az since the breaker s function is to protect the wire and most water heaters are wired with 10 wire the 30 amp breaker is the standard in most cases.
80 of breaker max single phase breaker size wire size based on copper thhn wire increase one size when using aluminum conductors at 115 volts at 230 volts 15 amp 14 1 380 2 760 20 amp 12 1 840 3 680.
This means that 10 copper wire will also be used.
Now in order to choose an electric breaker multiply the current draw of the water heater with 125 per cent in order to determine the size of the breaker you will need.
4500 240 18 75 amps will work on a 20 amp ocpd but.
4500 watt vs 5500 watt.
60 amp is correct for such use.
Now lets consider this water heater circuit installation.
A standard 3000 watt heater will use a two pole 20 amp breaker connected to.
Multiply the value for current by 125 percent to determine the size of the breaker you need for the heater.
In a us household the current draw for a 1500 watt water heater is 12 5 amps.
The higher the wattage the larger the wire size.
So a 4500 watt heater circuit must be sized up to either a 25 amp or 30 amp circuit.
4500 watt vs 5500 watt.
If it s 4500 then the 20 amp will work but it won t be code compliant.