Alternating Current

• Single-phase AC current rises and falls in a wave. 3-phase current has three wires that rise and fall, each one 120 degrees out of phase with the previous wire. The lines on 3-phase wires are called L1, L2 and L3.
  
  

Transformers

• 3-phase transformers can be thought of as three single-phase transformers grouped together, each one operating partly behind its nearest neighbor. Each single transformer has two coils, a primary and secondary. Used together, the three primary or three secondary coils can be wired in either Delta or Wye configurations.
  
  

Winding

• A coil of wire or winding has a wire leading in and a wire leading out. Current enters through the inlet wire, passes through the coil and exits through the outlet wire. In Delta a configuration, the outlet of each winding is connected to the inlet of the next winding. The three incoming or outgoing wires are also connected to the inlet of each of coil or winding.
  
  

Voltages

• Measuring the voltage between any two lines from the secondary coils of a transformer will show a consistent voltage between them. All three measurements will be the same, which is what makes the Delta configuration good for neighborhoods; homes are supplied with two equal lines of 120 volts each.