What are Primary, Secondary, and Tertiary Distribution Boxes?

What is a primary distribution box? What is a secondary Distribution box? What is a tertiary distribution box? When hearing these terms, most people likely feel they know what they are but cannot explain them specifically. Today, we will clarify their precise meanings and functions.

A primary distribution box refers to the main distribution panel, usually located in the electrical distribution room. It typically features bottom entry and exit wiring, front-opening access, and uses copper busbars for the main connections to ensure good contact. It includes a metering system, is safe and aesthetically designed, and has a rainproof top suitable for outdoor operation.

A secondary distribution box, also known as a sub-distribution panel, generally supplies power to a specific area. It is designed with inner and outer doors, features a sprayed plastic exterior for safety and appearance, and also has a rainproof top suitable for outdoor work.

A tertiary distribution box refers to a switch box, which is responsible for supplying power to only one device.

What do primary, secondary, and tertiary distribution boxes mean? This is a relative concept. Let’s consider an example: A newly built residential community introduces a 10kV power line and establishes a distribution room. Power is supplied from the low-voltage side of the transformer at 0.4kV to the distribution cabinet (primary distribution cabinet), then from there to the distribution boxes of each building (secondary distribution boxes), and further to the distribution cabinets of each unit (tertiary distribution), finally entering households.

Generally, primary distribution should not directly supply power to electrical devices. Secondary distribution often supplies three-phase power to equipment such as motors, while tertiary distribution provides standard mains power (220V). The internal components also differ: primary distribution cabinets usually include isolation switches, circuit breakers, and earth leakage protectors; secondary cabinets typically contain large three-phase circuit breakers; and tertiary cabinets use single-phase circuit breakers. Of course, these are examples, and equipment selection depends on specific electrical load requirements.

Primary Distribution Box: Can be understood as the main distribution panel for a construction site or project (usually only one).

Secondary Distribution Box: Distribution boxes for each floor or building (as needed).

Tertiary Distribution Box: The final connection box for electrical devices, which can be movable or fixed. Remember, the earth leakage protection switch is the final point, and devices are connected from it. (The rule is one machine, one earth leakage protector—no multiple connections.)

Requirements for distribution boxes can follow the project’s electrical plan. If no plan is available, you can refer to exemplary construction sites (e.g., large projects, which are usually well-regulated).