Comparing Single-phase and Three-phase

Single-phase power is a two-wire alternating current (AC) electrical power distribution. A third wire is used as a grounding (earthing) conductor as required by electrical codes. Single-phase electrical power is the easiest to calculate. The single-phase power is volts (V) x amps (A). The result is VA (volt amps). To convert to watts (W), multiply the VA x power factor (pf). For most modern pieces of IT equipment with a power consumption > 75 watts, the power factor is close to unity due to active power factor correction circuits in the IT equipment power supply units (PSUs). In the absence of pf data from the manufacturer, use a pf of 0.95.

Three-phase electrical power is either a three-wire (delta-wired) or four-wire (WYE-wired) AC electrical power distribution. Both delta-wired and WYE-wired three-phase distributions include a grounding (earthing) conductor as required by electrical codes. The delta-wired and WYE-wired circuits with their equations for balanced loading conditions are shown in Figure 1.

A line conductor (L = A, B, C) is a voltage and current carrying conductor. The neutral conductor (N) is a current carrying conductor only. A phase (P) connects two individual line conductors or a line and neutral conductor through a load. While the input side of the rack PDU that is downstream of the circuit breaker in the facility electrical panel can be three-phase, the output side of the rack PDU is always single-phase. This greatly simplifies the process of connecting IT equipment loads to the rack PDU and will be covered in a later section of the appendix.