While many of the 5 horsepower and below air compressor models are wired directly into the Pressure Switch, the larger reciprocating air compressors (7.5 hp and above single phase and all three phase units) are equipped with a Magnetic Starter for easier starts.
The power line coming from your breaker box to the air compressor should be routed to the Contactor located inside the Magnetic Starter box.
The power leads should be connected to the terminals on the Contactor labeled L1 & L2. (See connection points marked with green circles on the image below.)
The ground wire should be connected to the dedicated lug near the fuses.
Note – Air compressors that operate on 3 phase electrical service will have 3 terminal connections on the Contactor labeled L1, L2, and L3.
Also on 3 phase units, you will need to check the motor rotation by quickly turning the unit on and off to verify the rotation is correct. If the motor is turning in the wrong direction, then switch the power leads on terminals L1 and L2 to correct the rotation.