A water heater is 240 volts although they are sometimes referred to as 220 volts.
A 220 volt water heater will work on 240 volts.
The standard power incoming on each leg of the service entrance to your home is 120 volts each which equals 240 volts.
Some places you may only get 110 volts on each leg depending on the power coming to your house.
But a 220 volt water heater and a 240 volt water heater are wired with 240 volt breakers.
However over the years the 110 Volts electric supply was increased to 117 volts and now 120 Volts, due to increased demand, and reduce some voltage losses in wiring.
Most residential 240 volt water heaters use a 30 amp double pole breaker and 10 gauge wire.
40 amp breakers would use 8 gauge wire and is too large for most water heaters.
The size of wire that you need for a 30 amp breaker is 10 gauge wire also known as 10 AWG wire.
The size wire that you need for a 40 amp breaker is a size 8 AWG which is an 8 gauge wire.
8 AWG wire is the standard wire size for electrical circuits that are on a 40 amp circuit breaker.
The gauge number of a wire indicates its thickness, with lower numbers representing thicker wires that can carry more current.
10 AWG wires and 12 AWG wires are not suitable for 40-amp circuit breakers.
10 AWG and 12 AWG copper wires have 35 Amp and 25 Amp wire ampacity.
Using the 10 AWG or 12 AWG for a 40 Amp circuit is inherently wrong and might result in an electric accident.
A 20-amp 240-volt circuit calls for 12-gauge wire.
A 30-amp circuit calls for 10-gauge wire.
A 40-amp circuit calls for 8-gauge wire.
And a 50-amp circuit calls for 6-gauge wire.