Power Distribution in Your Building: How To Differentiate Distribution Configurations
By Brian McDivitt, PE
Building power distributions are based on what electrical service the local utility company provides. In the United States, building power distributions are categorized in three main configurations. The first distinction is between single-phase and three-phase, with three-phase further distinguished as wye (Y) or delta (Δ).
Editor’s Note: This is the first in a three-part series on power distribution systems. Read part one and part three.
As described under Part 1, voltage is measured as line-to-line (VLL) or line-to-neutral (VLN). These designations will be used here.
Most single-family residences and some small commercial buildings have single-phase 120/240V service. This service provides two hot lines (L1, and L2) that are 180 degrees apart, one neutral (N), and one ground, and is referred to as a 3-wire system. The two voltages available with this configuration are VLN = 120V, requiring only a 1-pole breaker, and VLL = 240V, which requires a 2-pole breaker. Most outlets in a house are powered by 120V circuits. A few appliances, such as an oven or clothes dryer, require a 240V circuit. The diagram illustrates this single-phase configuration.
Three-Phase, Wye (Y)
There are two types of three-phase configurations: wye (Y) and delta (Δ). The Y-configuration provides three hot lines and one neutral, which is typically tied to the ground and is referred to as a 4-wire system. The three lines (L1, L2, L3) are evenly spaced at 120 degrees apart. The diagram illustrates the VLL and VLN for a Y-configuration.
The typical three-phase Y-configurations that we see are 480Y/277V and 208Y/120. In each of these configuration names, the larger voltage designates the VLL and the smaller voltage is the VLN. For example, a 480Y/277V configuration has VLL = 480V, and VLN = 277V.
Three-Phase, Delta (Δ)
For a general approach to understanding this type of configuration, a three-wire system, consider a generic 208V Δ-configuration as illustrated below. First, notice that no neutral is present. In this configuration, the VLL = 208V, but a VLN is nonexistent. Another important aspect to note is that a delta configuration is ungrounded. Oftentimes, one leg of the delta is tied to the ground. The grounded leg provides ground protection to the system, and VLL remains 208V.
Split-Phase, Delta (Δ)
Some commercial buildings and factories employ a delta configuration where VLL = 240V. While this configuration provides three-phase and single-phase 240V for equipment, these buildings still need standard 120V outlets. To obtain VLN = 120V, one phase of the delta is center-tapped with a grounded neutral.
Which Configuration is Best?
Common voltage configurations have been explained here, but is one configuration more advantageous than the others? In Part 3, I’ll explain the important aspects of these configurations further to help you understand the pros and cons.