Three-phase service is typically provided to larger commercial and industrial customers. Any customer, whether it be residential, commercial, or industrial, with a motor larger than 10 horsepower is usually required to have three-phase service. The reason for this is that starting a single-phase motor larger than 10 horsepower creates a voltage dip on the distribution system, which would affect neighboring customers. Three-phase service may be provided using either the delta or wye configuration.
Delta service
Delta connected power is now provided using four wires, although three-wire delta has sometimes been provided in the past. For delta connected power, three independent transformer windings are connected head to toe. There is no single point common to all phases like there is in the wye configuration, but a grounded neutral is connected to one of the transformer windings. Most delta services use the center tapped configuration shown here, meaning the neutral is connected in the middle of one of the transformer windings. The alternative is to connect it to the end of one of the windings in which case the resulting single-phase voltage will be different from those stated here.
Four-wire delta has two different voltages available: a line-to-line voltage (typically 240V or 480V in the U.S.) used for three-phase loads, and a line-to-neutral voltage (typically 120V or 240V in the U.S.) used for small single-phase loads within the building.
Delta services are more common for customers with mostly three-phase equipment and very little single-phase equipment like manufacturers and processors since the small amount of single-phase load does not cause a significant imbalance and the three-phase loads can be balanced across the three “legs.” Delta has historically been preferred for loads with lots of motors since 240V motors are more commonly available than 208V motors, and internal transformers to convert 277V power to 120V power for lighting are more expensive than the internal transformers required for delta service.
Wye service
Wye connected power is developed from three independent transformer windings that are connected at a common point called a neutral or star point. In the case of a wye distribution line, each wire is connected to a transformer in the substation as shown in the diagram. The distribution voltage is dependent upon the connection made between the wires. Here you can see that there are two possibilities for distribution voltages from this particular line: 7.2 kV phase-to-ground and 12.5 kV phase-to-phase.
Service voltages are obtained by either connecting between two phase lines or by connecting between a phase line and a neutral. The connection between one of the phases and the neutral line provides single-phase power while connecting all three phase lines provides three-phase power.
Wye services are best for customers who need three-phase for some equipment but also have large amounts of single-phase load. This is because the single-phase loads can be spread across multiple “legs” of the transformer configuration allowing the load on each transformer to remain balanced.