yes, you can attach. but if the current through the line i.e. through the breaker exceeds 20 amp the breaker will trip. As long as the line carries 20 amp or less there is no problem.
You can't.
No, you should not install a 30 amp circuit breaker to a machine which normally requires a 20 amp supply. The circuit breaker is protecting the wire to the machine. That wire is likely only rated for 20 amps, (# 12 awg). Therefore, a 30 amp breaker could allow too much current to go through the wiring to the machine and cause the wire to burn down. Replace the 3 phase 20 amp breaker with the same amperage breaker.
No tandem breakers are of the same value.
A 32 amp fixture can not be fed from a 20 amp breaker as the breaker will trip every time.
Some 200 amp breaker panels are made with only 20 slots.
Maybe you have other electrical drains on that breaker line that cause it to exceed the 15 or 20 amp limit. Fix it.
AWG 12/2 requires the use of a 20 amp single pole breaker.
30 amp breaker with #10 gauge copper wire
If you are referring to a cloths dryer, the answer depends on the requirements of the dryer. Most dryers require AWG#10 wire with a 30 amp fuse. If the wiring is AWG#12 then use a 20 amp breaker but never use it on AWG#10 which requires a 30 amp breaker. If you are referring to a hair dryer then yes a 20 amp breaker is fine.
No more than 13 maximum on a 20 amp circuit.
I would suggest a 20 amp breaker. * Added - I would suggest a 25 amp breaker. A slight surge, depending upon what equipment is the source of the 17.3 amp load, should not result in circuit breaker opening. <<>> In North America, the electrical code only allows circuit loading up to 80% on a continuous load. A 20 amp breaker can be legally loaded to 16 amps. A 25 amp breaker can be legally loaded to 20 amps.
If the 12,000 BTU A/C only requires 20 amps to run then yes you can use the same 12 gauge wire but you cannot change it to a 15 amp breaker. You will need to install a 20 amp double pole breaker. If it requires more than 20 amps you will have to replace the wiring and breaker.