3-Phase Power Explained

Power Analogy

Imagine a dark room with three treadmills and three distance runners. The runners are wearing special shoes that briefly light up with each step.

Single-phase power is when one person is running while the others stand by. When a step is taken, the room lights up for a brief moment. In the moments between steps, the room stays dark.

Three-phase power is when all three people are running simultaneously. They work together to make sure at least one runner's foot is down at all times to keep the room lit.

In this analogy, each runner represents a hot wire of a power distribution system and the light intensity represents the phase voltage.

Power Distribution to Server Racks

Electrical power is delivered from a generator to the electrical grid to industrial buildings in the form of alternating current (AC) three-phase power. Industrial buildings like data centers must decide how to further distribute this power to the IT equipment on server racks. At the most basic level, there are two options:

Option #1: Distribute single-phase power to the racks. This typically involves power distribution through a single LINE (hot) wire and a NEUTRAL wire.

Option #2: Distribute three-phase power to the racks. This typically involves power distribution through three LINE wires, one for each phase, and in some cases a NEUTRAL wire.

In North America, the LINE wires in a "208V" service typically have an oscillating voltage between -170V to +170V peak-to-peak with respect to the NEUTRAL wire. When discussing AC circuits, voltage RMS (the equivalent DC voltage) is used instead of peak voltages. For example, an AC voltage with 170V peak is referred to as 120VAC (Voltage Alternating Current).

In the single-phase plot below, positive voltages are represented by the left shoe, negative voltages by the right shoe. Notice both shoes provide the same amount of light at the peaks. In other words, the voltage polarity does not matter. Both are useful.

Single-phase rack power distribution units (PDUs) are designed to receive and distribute single-phase power to IT equipment. These PDUs are internally wired to deliver 120VAC (LINE-to-NEUTRAL) directly to the outlets. Equipment connected to these outlets will operate with 120VAC.

In comparison, a three-phase system has three LINE wires each measuring 120VAC with respect to the NEUTRAL wire. The plot below shows three-phase power with the LINE voltages overlaid. Notice how each LINE voltage is shifted in time.