The amp breaker you need for a 240V 40 gallon electric water heater is 30 amp breaker and 10-2 copper wire.
Most 240 volt electric water heaters are a 30 amp breaker.
An electric water heater is most often a 30 amp breaker although it can vary depending on the water heater you have and how many watts is uses as well.
However the majority of 240 volt electric water heaters require 10-2 copper wire and a 30 amp breaker.
110 Volt Water heaters are usually 13.75 amps to 15 amps and you would need at least a 20 amp breaker and 12-2 copper wire.
Some 220 volt or 240 volt water heaters use only 22 amps but you need at least a 30 amp breaker and the 10-2 wire to be sufficient enough to power the water heater without tripping the breaker.
240 volt electric water heaters require a 240-volt dedicated circuit, which serves only the electric water heater and no other appliances or devices.
The circuit wiring for a 240 electric water heater typically includes a 30-amp double-pole circuit breaker and 10-2 non-metallic (NM) or MC cable.