Unplug the refrigerator from the wall. Commercial refrigeration systems may require turning the electricity off at the circuit breaker box. The correct circuit breaker should read either kitchen or refrigeration.
Double-check the voltage at the refrigerator's terminal block with a multi-meter turned to AC Volts. Place a lead on each insulated terminal. The meter must read zero before continuing.
Identify the terminals on the compressor. The identification tag, located on either the side of the compressor or stamped into the terminal cover, has three letters representing the terminal position. The terminal marked "C" stands for common, the terminal marked "R" stands for run and the terminal marked "S" stands for start.
Remove the terminal cover. Use a flat screwdriver to pry the cover out of the locking tabs.
Connect the wire coming from the capacitor or the start relay, if equipped, to the "S" terminal. A capacitor looks like a galvanized cylinder with wires coming from the top. One of these wires leads to the "common" side of the contactor. One wire leads to the compressor and if the capacitor has a third wire, it would lead to a fan. A start relay, shaped like a black box, connects to a capacitor. One wire of the relay connects to the compressor and one would either go to a run capacitor or the common side of the contactor.
Connect the wire coming from the common side of the contactor to the "C" terminal on the compressor. The contactor acts as a high voltage switch. The high voltage coming from the circuit breaker connects to one side of the contactor. When a low voltage signal, sent by a thermostat, tells the contactor to turn on, high voltage exits the contactor to feed electricity to the fans and the compressor.
Connect the wire coming from the hot side of the contactor to the "R" terminal. The contactor has two legs, one hot and one common.