First, conduct a comprehensive investigation into the supply voltage parameters, the user’s actual load characteristics, and local environmental conditions. Next, reference the technical specifications marked on the transformer nameplate—typically including key parameters such as rated capacity, voltage, current ratings, and environmental adaptability. The capacity selection should be primarily determined by the user’s electrical equipment capacity, load nature (e.g., continuous or intermittent), and load duration requirements, ensuring the chosen transformer capacity aligns with these factors.

Under normal operation, a transformer should ideally operate within ​75% to 90% of its rated capacity. If measurements during operation reveal that the actual sustained load is consistently ​below 50% of the rated capacity, a smaller-capacity transformer should be considered. Conversely, if the transformer’s rated capacity significantly exceeds the actual load demand, immediate replacement with a properly sized unit is necessary to avoid inefficiency or overloading risks.

Additionally, when selecting the transformer, the primary side voltage should be determined based on the grid’s supply voltage, while the secondary side voltage must match the requirements of the connected electrical equipment. For most applications, ​low-voltage three-phase four-wire power supply​ is the optimal choice, as this configuration effectively supports both power loads and lighting systems.

Regarding current rating selection, special attention must be paid to ensuring the transformer can handle motor starting inrush currents while meeting the motor’s steady-state operational requirements. Note that motor starting currents are typically ​4 to 7 times higher than steady-state operating currents, which must be factored into the transformer’s current capacity design.