Three-phase AC power is everywhere. Every major power generation and distribution system in the world uses some variation of it. The reason is simple: Three-phase power systems allow a utility to ship more power over smaller (and cheaper) wires than would be possible in a single-phase system. IT organizations should turn to it in their data centers' racks.
As server gear has undergone relentless waves of miniaturization, with the contemporary equivalent of the behemoth rack servers of years ago now boiled down to a sub-rack-unit blade, the amount of compute capacity that can be delivered in a single cabinet has risen dramatically. However, so too has the amount of power that a single rack of modern servers can consume. Years ago, you might fit eight or nine of the most power-hungry servers into a rack and consume around 5kW in the process. Today, you can easily fit 50 or 60 in the same space -- some blade platforms allow twice that -- and consume more than 30kW in total.
Why your data center's single-phase power can't do the job any longer
The typical single-phase power distribution systems are ill-suited to these kinds of loads. For example, as you start to move beyond a fairly typical 30-amp high-voltage circuit, the conductors, plugs, and sockets required to supply ever-increasing amperages become heavier, more difficult to work with, and progressively more expensive.
Plus, these large single-phase loads ultimately have to be pulled from the building's three-phase power system -- presenting a challenge for facilities electricians to keep those phases in balance. By delivering three-phase power directly to the server cabinet, you can get away with cheaper cabling, simplify your electrician's job, and deliver substantially more power all at the same time. The catch is that effectively using three-phase power requires knowledge many IT shops don't have.
The typical North American single-phase power circuit in a typical data center has three wires: the hot wire, the neutral wire, and the ground wire. The circuit that delivers the power comes from the hot and neutral wires; the ground is there only for fault protection and does not carry power. Because the ground is assumed to always be there, it typically isn't counted in the number of wires. Despite the fact that there are three actual wires, this is called a two-wire system.
When you pull power from two phases, you don't end up with twice the voltage of each individual phase. Instead, you'll end up with a line voltage that is 1.73 (or √3) times that of the individual phase voltage because each phase is 120 degrees out of phase with its neighbors.
In a typical North American commercial three-phase system, you get a 208V line voltage (120V * 1.73) across any two of the phases and a 120V by combining any phase with the neutral. This gives you flexibility in the voltages provided for different applications. In larger commercial settings, the utility provides a 480V three-phase service (which uses a 277V phase voltage). You must step this down to 120V using an on-premises transformer before it can service IT gear.
Elsewhere in the world, standards are wildly different -- both in terms of the voltages used and how it's typically deployed. In the United Kingdom, for example, phase voltage is typically 240V and line voltage is 415V (240V * 1.73). Europe has standardized on the 230V line and 400V phase voltages. That's why why you see IT equipment designed with autoranging AC/DC power supplies that can accept voltages from 120V to 240V; this is far easier for manufacturers to do than to maintain different power supply stocks for various parts of the world.
No matter where it's used, three-phase power brings two big benefits to the table:
- By drawing power from three separate conductors, much smaller and easier-to-manage conductors can be used to move the same amount of power.
- For very high-density power environments, using three-phase power all the way to the equipment can make it much easier to balance upstream loading of building power phases. Most building electricians would be much happier to give you a three-phase circuit for a heavy load than to use a single, very large single-phase circuit.