The second step in the selection process is to determine what input voltage the Tripp Lite PDU will be connected to. Some Tripp Lite PDUs accept single-phase input voltage, and other models accept 3-phase input voltage. What does phase refer to? Power moves along a transmission line in the form of a wave. Each of these waveforms is called a phase. For efficiency's sake, utility companies deliver three waveforms (or three phases) on the same transmission line from their generator to a facility. Also for efficiency's sake, a facility's electrical panel will deliver 3-phase power along some circuits to high wattage loads. For lower wattage loads, the panel will split the power into single phases delivered on separate circuits to separate sets of devices. The phase and voltage of the PDU must match the phase and voltage of the outlet it is plugged into.