Just finished wiring up my Shop and Dewalt 5HP 60 gallon compressor. Some good info on this thread but I did see a couple conflicting points that probably should be clarified.
If using EMT for the run, you will need two line conductors (wires) between the compressor motor and the two pole breaker since the compressor is a 240v single phase load. You need to run wire that is rated for running in conduit. This would typically be THHN wire you can purchase from HD or Lowe’s. I would use one Red and one Black. By the way, dont use wire cut out of NM cable you may have lying around. It’s not rated to go into conduit. As far as a neutral, you don’t need one for the compressor if is the only load you are feeding through the EMT run. I also agree with others running a separate green or bare copper ground in the EMT even though code does not require it. Just ensures you have a low impedance path back to the breaker for ground fault protection.
To size the conductors/wires/ breaker properly meaning code compliant, can you provide the FLA, Service Factor, HP, Temp Rise from the motor nameplate? Better yet a picture of the nameplate. Another key piece of information is if your motor on the compressor has built in overload protection? Would typically look like a reset button on the motor.
Protecting motor loads like compressors correctly need to consider two different fault conditions: 1) Overload and 2) Short Circuit/Ground Fault. There is a entire complicated article 430 in the National Electric Code that covers it. It can be found here for reference: Motor Calculations Part 1: Motors and Branch-Circuit Conductors | EC&M
Assuming your compressor motor has built in overload protection, I suspect your 5HP compressor will work out with a 10awg conductor size and a breaker size of 40 to 50 amps (150 to 300% of the Full Load Current of the motor). The two pole circuit breaker in this case is sized that way to avoid nuisance tripping due to the motor inrush current when starting but still protect for short circuit/ground faults. The breaker is not protecting from overloading the wires because that Fault condition is taken care of by the built in motor overload protection assuming it has it.
Lastly, I would suggest thinking about using a magnetic starter if you plan to use compressor a lot. The contactors built into the pressure switch on the compressor are typically not that heavy duty and tend to fail over many compressor starts. A magnetic starter has much heavier duty contactors that will last much longer. A good mag starter is around $100 to $150 dollars. Model number I used, got it from Amazon:
EATON B27CGF30B040 MAGNETIC MOTOR STARTER 5HP 1 PHASE 208-230 VOLT 30 AMP
!