Distribution lines can be configured in different ways depending on how they are wired into the transformer at the substation. Lines may be single-phase or three-phase. And three-phase lines may be either delta or wye configurations.
In a delta configuration, three wires are connected to a transformer in the substation as shown in the diagram below. The result is three phase lines which, when connected, provide a phase-to-phase voltage. In the example below, which is typical for a delta distribution feeder, the phase difference is 12 kilovolts (kV).
Service voltages delivered to consumers from a delta system depend on how the service transformer is connected to the distribution line and how the customer panel is wired to the service lines provided from the service transformer. Here are examples of service connections from a delta system:
- Single-phase three-wire – This service includes two “hot” wires from a single transformer plus a neutral and provides the customer the option for 120 or 240 volt (V) circuits (commonly written as 120/240V). This service configuration is used for residential and small commercial customers located on a delta circuit.
- Three-phase three-wire – This service includes three “hot” phase wires but no neutral and provides the customer just a single voltage such as 240V, 400V, 480V, 600V, or higher. This service configuration is common for industrial customers.
- Three-phase four-wire – This service includes three “hot” phase wires plus a neutral and provides the customer the option of different voltages for different circuits. Common service voltages include 120/208V and 277/480V. This service configuration is common in larger commercial buildings.
In a delta distribution line, there are three wires (one for each phase). For this reason, delta distribution lines are less expensive to construct than wye distribution lines, which require four wires. However, it is common to add a fourth static line above the three phase lines simply for the purpose of carrying lightning to the ground. If this line is used, it is simply for protection and is not connected to the transformer (unlike the neutral line in a wye configuration). Note the photo above shows only the three phase wires without a static wire.
Delta distribution lines are used primarily in commercial and industrial areas with large amounts of three-phase load and very little residential or single-phase load. They are also common in areas with substantial pumping loads. Delta lines are not used as often in areas with large amounts of single-phase loads due to difficulties in balancing loads across the three phases. Because all single-phase load is connected phase to phase, it is harder to keep the system balanced when there are many single-phase customers on a line. An imbalance can result in three-phase motors running hot, causing them to burn out prematurely. So, in areas where delta lines exist and substantial single-phase load is added, the lines may need to be upgraded to wye distribution lines.