I have completely wired my workshop with 20 amp breakers using 12 ga wire. I now want to add 30 amp breakers for 240 vac. I will use 10 ga wire but I am not sure on what socket to use. I am planning to use an air compressor which requires 240VAC and pulls 15 amps. However, I also would like to setup a 240 VAC for future use, say for a welder. For the air compressor and welder 30 amps sockets, but I cannot find any. Also If the compressor uses 15 amps, can I use a 20 amp socket? I just do not know what amps the motor. Also is not 10 ga good for 30 amps? Any recommendations on the type of socket to use?

Receptacle configurations are determined by what the load needs for amperage and voltage, and if a neutral is needed. See the attached chart.

For a welder, 240 volts 30 amp outlet without a neutral, a NEMA 10-30 would be needed. This is the typical outlet used for older dryers. There will always be one blade for an equipment ground.

If the air compressors uses 15 amps, the circuit will need to be a 20 amp circuit.

#10 wire is used for a 30 amp circuit.

Many welders require more than 30 amps. And you may want to purchase the welder before installing a circuit for one. A standard 225 amp stick welder may require 40 amps or more. It all depends on what type welder you purchase.