By Brian McDivitt, PE, and Ryan Maroney, PE

Many devices and equipment we use in our daily tasks require a power distribution system that makes 120V available for users. This 120V power can be achieved by a single-phase or a three-phase (wye or split-phase delta) configuration, but is one configuration better than another for your building? Ultimately, the configuration used for your building depends on your equipment needs, overall building electrical load, and what is most economical.

Before stepping through the different configurations, let’s look at some essential components to ensure effective and safe power distribution.

  • Grounding. A properly grounded system is important to ensure protection for both system users and personnel maintaining the system. While current typically travels through an intended equipment load, sometimes a loose or disconnected wire causes current to take an unintended path, such as metal, on a piece of equipment. A grounded system provides a path to the earth for the current to dissipate energy safely.
  • Neutral. As Part 2 of this series illustrates, having a neutral wire is another important consideration in a power distribution configuration. Most electrical system designs include access to a neutral anywhere equipment may need it. While single-phase 120V circuits include a neutral by default to form a safe and complete circuit, multi-pole circuits do not always have a neutral connection. However, certain multi-pole equipment does have 120V components (typically controls) that require the presence of a neutral.

Consider a typical residential clothes dryer, which may or may not require a neutral. The dryer plug configuration makes this distinction and determines what receptacle is needed to power the dryer. Many older dryers require a 3-prong NEMA 10-30R receptacle (no neutral), while most new dryers require a 4-prong NEMA 14-30R receptacle (neutral included). The latter option provides a safer system. Because a dryer is sometimes located in a wet location, the insulated neutral provides the grounded current path instead of the equipment ground itself. In the late 1990s, the National Electric Code was updated to require 4-prong dryer receptacles in all new construction installations.