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Throughout North America, homes are powered by 120 volt single-phase electricity. A typical residential circuit breaker box shows four wires entering our home: two "hot" wires, a neutral wire, and a ground wire. Two "hot" wires carry 240 VAC and are used for heavy equipment such as electric furnaces and dryers. However, the voltage between the live and neutral wires is 120 VAC, which powers everything else in our house.

However, power plants in North America transmit three-phase power at ultra-high voltages between 230 kV and 500 kV. A closer look at the high-voltage power line will reveal three separate conductors, each of which carries current, plus a neutral conductor. The distribution cost of three-phase power is lower because the three-phase power transmission line does not require the same thick copper wire as the single-phase transmission line. In addition, the three-phase provides the flexibility of service connection, not only can provide customers with the usual 120 VAC service, but also 208 VAC service. Almost all industrial buildings, including yours, use three-phase power because it has many advantages over single-phase power.

Designing or modifying a data center to use three-phase power is rewarding, but some centers do not understand the benefits of three-phase power. Let's take a look at the difference between single-phase power and three-phase power to understand why three-phase power can not only save costs, but also create a more efficient data center.​​​

The traditional 120 VAC single-phase service running at 60 Hz cannot provide continuous power. At this frequency, the AC sine wave passes through the zero point 120 times per second. It is best to understand that power is measured in watts, which is the product of the applied voltage multiplied by the amperage of the current in the circuit (W=V x A).

When the voltage or current exceeds the zero point, the transmitted electric power drops to zero. In fact, these momentary drops to zero will not significantly affect the equipment in the circuit. For example, if the device is a motor, the mechanical inertia of its rotating armature "crosses" the zero point. (However, these zero-crossing points do add up. The life expectancy of a motor running on a single-phase power supply is shorter than that of a motor designed for a three-phase power supply). Similarly, if the load device is a solid-state electronic device, the smoothing capacitor in the power filter will "buffer" these zeros.

On the other hand, a three-phase power supply consists of three sine waves separated by 120 degrees. This form of electricity is generated by an alternator, which has three independent windings, each of which is exactly 120 degrees apart. Each current (phase) is carried on a separate conductor. Due to the phase relationship, the voltage and current applied to the IT load will not drop to zero. This means that a three-phase power supply at a given voltage can provide more power. In fact, it is about 1.7 times the power of a single-phase power supply.

In recent years, the processing power that can be configured in a single rack has doubled. Not long ago, a rack could hold up to 10 servers consuming 5 kW. Now, due to never-ending miniaturization and unstoppable technological advancements, the same rack may hold four to five dozen servers and consume more than 15 kW.

It takes 125 amps to power a 15 kW rack with a 120 VAC single-phase power supply. The diameter of the copper wire AWG 4 required to safely carry this current is close to a quarter of an inch. [1] Difficult to use and expensive. Obviously, it is not practical to compare this type of load alone. However, in a three-phase system, each wire (AWG 11 with a diameter of only 0.09 inches) can only carry about 42 amps. If you are interested in learning more about the algorithm behind this, please read our blog "The secret of 3-phase 208V power supply board (rack PDU), Part 2: Understanding capacity".

The power system you choose will bring you efficiency and economy, or inflexibility and excessive costs. Single-phase power supply is very suitable for residential users whose maximum load comes from clothes dryers or electric stoves. However, data centers need to consider the benefits of three-phase power. These include:

If you are looking for a way to use three-phase power to make your data center future-proof, learn how PDUs can be adapted to the combination of solutions you will need.

This blog post is sponsored by Raritan.

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