240v or 240 volt circuits need 2 breakers which is a double pole breaker.
Each side of the breaker has 120 volts which equals the 240 volts needed.
A 240 volt circuit should always have a double pole breaker and not 2 single 120 volt breakers as using 2 single breakers is a hazard as both sides won't trip when needed.
Having a double pole breaker on a 240 volt circuit ensures that both sides trip and shut off power when needed in the event of an overload or short circuit.
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.