Three-Phase Power Supply: Why do we use it? | Three-phase power benefits

Most electrical appliances used in homes and businesses operate on alternating current (AC), which means that the voltage supplied is pulsating, as opposed to the constant output from a battery (direct current, DC). In the US, the voltage supplied by utilities has a frequency of 60 Hertz, which means it alternates between positive and negative polarity 60 times per second.

Most AC power supplies can be classified as single-phase or three-phase depending on the characteristics of the voltage supplied. As the name suggests, a three-phase system has three separate AC voltages, each with a frequency of 60 Hz. However, these voltages alternate between positive and negative in sequence, not simultaneously, providing a constant supply that is not possible with a single-phase system.

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How three-phase energy makes electrical installations cheaper

The capacity of AC power systems is measured in volt-amperes (VA) and is calculated by multiplying voltage and current.

• For example, a 120V circuit with 20A wiring can carry 2,400 VA.
• A three-phase circuit using 20A wiring can carry 7,200 VA.

Consider that you need a neutral conductor and a grounding conductor in both cases, plus a live conductor for each voltage output. This means you need three wires for the single-phase system and five wires for the three-phase system. In other words, the three-phase system has 300% of the capacity of the single-phase system, using only two additional wires (only 67% more copper). When you consider the reduction in wiring resulting from using three-phase power in a large commercial or industrial facility, the savings are significant.

Single-phase power is typically used in residential applications where loads are too small to justify the complexity of a three-phase system. However, single-phase supply to individual homes is typically derived from a larger three-phase system.

• Single-family homes and other small buildings obtain single-phase power from the utility company's proprietary three-phase distribution system.
• Larger multi-family buildings typically have their own three-phase service entrance.

Performance advantages of three-phase equipment

In addition to saving on wiring, three-phase systems have notable performance advantages over their single-phase counterparts. This is especially true for electric motors:

• For a given power, three-phase motors have higher efficiency than single-phase motors. Considering the high kilowatt-hour prices in New York, this is a significant advantage.
• Three-phase motors also have a higher power factor, meaning they draw fewer volt-amperes for a given load and efficiency. Some electricity rates include charges for poor power factor, and three-phase motors can help reduce these.
• Because single-phase systems provide pulsating power, motors tend to experience more vibration, while the constant supply of three-phase systems results in more stable operation.
• Single-phase motors cannot start alone, requiring external devices. On the other hand, three-phase motors can start with just the power supply, and can even reverse direction if you swap two conductors together.

A three-phase system is also more versatile than a single-phase installation. If you need to operate a single-phase device with a three-phase power supply, you can use only one of the three conductors. However, the opposite does not apply: three-phase appliances cannot operate on single-phase power. Motors are an exception: you can run a three-phase motor with a single-phase power supply, but the motor's mechanical power is drastically reduced and its lifespan is drastically reduced.

Wiring color requirements

The National Electrical Code establishes wiring color requirements for electrical systems. This makes it easier to identify drivers, reducing the chance of human error and improving safety. The requirements are summarized in the following table.

When a three-phase system feeds both three-phase and single-phase loads, the recommended practice is to balance the single-phase loads between the three phases. An unbalanced voltage source can be harmful to some types of equipment. The neutral conductor also carries a higher current when the system is poorly balanced, and this causes energy loss in the form of heat dissipation.

Note that wiring is not the only circuit element that changes between single-phase and three-phase installations. Components such as protective devices, distribution boards and transformers are also constructed differently. In the case of transformers, you can use three single-phase units to step up or step down the three-phase voltage, but a three-phase transformer is cheaper and more compact in most cases.