Know the Difference Between Three-Phase and Single-Phase Power

Why three-phase power not only delivers real cost savings but creates a more efficient data center.

Jawahar Swaminathan

September 22, 2017

4 Min Read
Raritan PDU

Throughout North America, homes are powered by 120-volt single-phase electricity. A typical residential circuit breaker box reveals four wires coming into our homes: two “hot” wires, a neutral wire and ground. The two "hot" wires carry 240 VAC, which is used for heavy appliances like electric ranges and dryers. However, the voltage between both hot wire and the neutral wire is 120 VAC, which powers everything else in our homes.

However, electricity generating manufacturing plants in North America transmit three-phase power at super high voltages ranging between 230 kV and 500 kV. A close look at high tension power lines reveals three separate conductors, each of which carries current, plus a neutral conductor. Three-phase power is less costly to distribute because transmission lines for three-phase power don't need the same heavy gauge copper wires as a single-phase transmission line would need. Further, three-phase offers flexibility at the service connection and can give customers not only the customary 120 VAC service but also 208 VAC. Virtually every industrial building, including yours, receives three-phase power, as it offers many advantages over single-phase.

Designing or retrofitting a data center to use three-phase power pays off, but some centers do not understand the benefits three-phase power brings. Let's take a look at the differences between single and three-phase power to understand why three-phase power not only delivers real cost savings but creates a more efficient data center.

The Problem with Single-Phase

Conventional 120 VAC single-phase service running at 60 Hz cannot deliver continuous power. At that frequency, the alternating current sine wave crosses the zero point 120 times each second. It’s best to understand that power is measured in Watts, and Watts is the product of Voltage applied times Amperes of current flowing in a circuit (W=V x A).

When either voltage or current crosses the zero point, the electrical power being delivered falls to zero. In practice, those instantaneous drops to zero don't visibly affect equipment in the circuit. If the equipment is a motor, for instance, the mechanical inertia of its spinning armature “rides through” the zero points. (However, those zero point crossings do add up. Motors running on single-phase power have shorter life expectancies than those designed for three-phase power). Similarly, if the equipment under load is solid state electronics, smoothing capacitors in the power supply filter “buffers” those zero points.

Three-phase power, on the other hand, consists of three sine waves separated by 120 degrees. This form of power is created by an AC generator with three independent windings, each exactly 120 degrees apart. Each current (phase) is carried on a separate conductor. Due to the phase relationship, neither voltage nor current flow applied to an IT load ever drops to zero. This means three-phase power at a given voltage can deliver more power. In fact, about 1.7 times the power of a single-phase supply.

In recent years, the processing power that can be configured in a single rack has multiplied. Not too long ago a rack might have held as many as ten servers that consumed 5 kW. Now, due to never-ending miniaturization and the unstoppable march of technology, that same rack might hold four or five dozen servers—and consume more than 15 kW.

Powering a 15 kW rack with single-phase power at 120 VAC draws 125 Amps. The copper needed to safely carry that current, AWG 4, is nearly a quarter-inch in diameter. [1] It's hard to work with and expensive. Clearly, single-phase isn't practical for such loads. However, in a three-phase system, each conductor, AWG 11 at just 0.09-inch diameter, would only carry about 42 Amps. If interested in drilling down more into the arithmetic behind this read our blog “3-phase, 208V Power Strips (Rack PDUs) Demystified, Part II: Understanding Capacity”.

How Three-Phase Can Help

Your choice of a power system can bring you efficiency and economy, or inflexibility and excessive costs. Single-phase power is ideal for residential users whose biggest load comes from a dryer or electric range. Data centers, though, need to look at the benefits three-phase power brings. These include:

  • Can run both 120 VAC and 208 VAC devices from the same power source, mixing and matching PDUs as required.

  • Three-phase allows you to run all your devices on 120 VAC today, but upgrade to 208 VAC simply by swapping out your PDUs—which you can do quickly and without significant downtime.

  • The cost of cabling falls dramatically when you deliver three-phase power directly to your server cabinets.

  • The work required of electricians installing AC cabling, and total installation time, are both reduced.

If you're looking for ways to future-proof your data center using three-phase power, learn how PDUs fit into the mix of solutions you will need.

This blog post was sponsored by Raritan.

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