What size breaker needs to be installed for 4500 watt 240 volt hotwater heater?

Answer for USA, Canada and countries running a 60 Hz supply service.

You need a 30 amp 2-pole breaker and #10 AWG wire.

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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.