30 amps can be either 120 volts or 220 volts depending on what it's supplying power too.
If the breaker is a double pole breaker then it's 220 volts or 240 volts and if the 30 amp breaker is a single pole breaker then it's 110 volts to 120 volts.
30 amp breakers are often double pole breakers and are 220 volts to 240 volts, although 30 amps can also be a single pole 120 volt breaker.
If the 30 amp breaker has 2 sides and are connected then it's a double pole 240 volt breaker and if it's a single breaker it's a 120 volt breaker.
A 30 amp single pole breaker is a 120 volt circuit and are similar to 20 amp breakers.
Double pole 30 amp breakers are most often 240 volts with 120 volts on each side that make up the 240 volts.
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.