Primary distribution boxes typically operate at medium voltages ranging from 2 kV to 35 kV.

Primary distribution boxes receive power from a substation where transmission voltages are stepped down to medium voltage levels, usually between 2 kV and 35 kV, depending on the network design and regional standards . These boxes distribute power to feeders that supply distribution transformers, which then step the voltage down to low-voltage levels suitable for residential, commercial, or industrial use, such as 120/240 V in the US .

Typical Voltage Ranges

• Industrial plants: Primary distribution systems often use 2.4 kV to 15 kV to accommodate higher loads and reduce voltage drop over long distances .
• Urban and rural networks: Medium-voltage feeders in distribution networks generally operate between 5 kV and 35 kV, with the exact voltage chosen based on load density, distance, and reliability requirements .

Design Considerations

When selecting the voltage for a primary distribution box, several factors must be considered:

• Load requirements: Higher voltage allows for lower current for the same power, reducing conductor size and losses.
• Distance to transformers: Longer feeder distances favor higher voltages to minimize voltage drop.
• Safety and insulation: Equipment must be rated for the chosen medium voltage, including breakers, busbars, and protective devices .
• Regulatory standards: Local electrical codes and utility requirements dictate acceptable voltage ranges and safety margins.

Summary

A primary distribution box is designed to handle medium-voltage power, typically 2 kV to 35 kV, which is then stepped down by distribution transformers to low-voltage circuits for end users. The exact voltage depends on the application, load, and network configuration, with industrial plants often using 2.4–15 kV and utility feeders ranging up to 35 kV . Proper design ensures safe, reliable, and efficient power delivery.