There are a variety of configurations and layout options for secondary services, most of which are standardized throughout the industry. The configurations for any specific customer’s electric service depends primarily on the equipment located within the customer premises and the service voltage needed to operate that equipment. However, additional factors such as the geographic size of the property to be served, vehicle and human traffic and congestion in close proximity, and customer preferences also affect the service configuration.
Common variables include whether the service is two-wire, three-wire, or four-wire, and whether the service is single-phase or three-phase. A three-phase service may be in either a wye or delta configuration. The number of wires refers to how many conductors are connected to the service. Electricians who wire buildings provide voltages to different types of electrical equipment by wiring the equipment to the conductors in different configurations. For instance, a 120/240 service would provide 120 V single-phase power for use by appliances and light bulbs while also providing 240 V service for larger appliances such as freezers, air conditioners, and EV chargers.
While there are always exceptions, most smaller customers receive single-phase service because this is suitable to the end-use equipment most often found in homes and offices. And because larger electrical users (typically large commercial and industrial) run more complex motors and machinery they usually receive three-phase service, which is more suited to this purpose.
The decision to take single-phase or three-phase power is mostly dependent upon the size of the motors run by the end user. Historically, engineering design standards and economics have dictated that customers with electric motors smaller than 10 horsepower in size can be adequately served with single-phase power. However, when single-phase electric motors larger than 10 horsepower are started on a single-phase distribution system, their initial requirement for power results in a momentary voltage drop on the system that exceeds allowable industry standards. This can cause power quality problems for other connected electrical equipment. You may have noticed that when large motors like air conditioners are started at a residence there may be a momentary but observable dimming of the lights in the house. Such problems worsen with larger motors, and thus three-phase power is generally chosen to run these devices.
The use of a three-phase motor typically requires that three-phase electrical service be run to the facility (the alternative is to convert single-phase power to three-phase power within the facility). Starting a three-phase electric motor does not result in a problematic voltage drop, so three-phase motors in much larger horsepower sizes can be started without causing severe voltage fluctuation on the system.
For three-phase customers, the service configuration may be delta or wye. This refers to the way that the service is wired to the transformer. 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. Delta services are more common for customers with mostly three-phase equipment and very little single-phase equipment like manufacturers and processors.