precautions for a 25kVA transformer (480V to 120V)

The installation requirements and precautions for a 25kVA transformer (480V to 120V) are explained in combination with the characteristics after the capacity upgrade:

# I. Basic Installation Requirements

1. **Selection of Installation Location**

   - **Environmental Conditions**: For indoor installation, good ventilation is required, and environments with high temperature, humidity, or dust should be avoided. For outdoor installation, an enclosure with a protection class of IP54/IP65 should be selected, and sunshade and rainproof facilities should be provided.

   - **Space Requirements**: A maintenance channel of ≥ 1 meter should be reserved around the transformer, and a heat dissipation space of ≥ 0.5 meters should be reserved at the top.

   - **Load-bearing Foundation**: The 25kVA transformer weighs approximately 100 - 150kg and needs to be fixed on a concrete base or channel steel bracket to ensure horizontal stability.

2. **Electrical Connection Specifications**

   - **Cable Specifications**:

     - Primary Side (480V): It is recommended to use copper-core cables, and the current-carrying capacity should meet the calculated current (for example, for three-phase: 25kVA/(√3×480V)≈30A, a 6mm² cable can be selected).

     - Secondary Side (120V): When the load is single-phase, the current is approximately 25kVA/120V≈208A, and a cable of more than 50mm² is required.

   - **Connection Terminals**: Use tinned copper lugs for crimping to ensure good contact and avoid loosening and heating.

# II. Safety and Protection Measures

1. **Configuration of Protection Devices**

   - **Circuit Breaker Specifications**:

     - Primary Side: Configure an 80A circuit breaker for three-phase (overload protection needs to be considered), and a 63A circuit breaker for single-phase.

     - Secondary Side: Configure a 225A circuit breaker for single-phase load (it is recommended to use branch control).

   - **Fuses**: Install a high-voltage fuse (such as 50A) on the primary side and a low-voltage fuse on the secondary side.

   - **Over-temperature Protection**: Configure a temperature sensor to automatically cut off the power or start the cooling fan when the temperature exceeds the limit.

2. **Grounding System**

   - **Neutral Point Grounding**: The neutral point of the three-phase transformer needs to be reliably grounded, and the grounding resistance ≤ 4Ω (in accordance with the GB 50169 standard).

   - **Enclosure Grounding**: Connect the metal enclosure to the grounding main line through a dedicated grounding terminal to ensure the conductivity of the PE line.

# III. Special Precautions (Compared with 5kVA)

1. **Heat Dissipation Management**

   - The full-load loss of a 25kVA transformer is approximately 300 - 500W, and heat dissipation needs to be enhanced:

     - For indoor installation, it is recommended to configure a temperature-controlled cooling fan (which starts when the temperature ≥ 60℃).

     - For outdoor installation, select a protective enclosure with heat dissipation holes to avoid direct sunlight.

2. **Vibration and Noise Control**

   - Large transformers may generate vibrations, and anti-vibration rubber pads need to be installed at the base.

   - The noise value is usually ≤ 65dB. If it is in an environmentally sensitive area, sound insulation treatment is required.

3. **Phase Sequence and Phase Balance**

   - The three-phase transformer must be wired strictly according to the phase sequence (L1/L2/L3) to avoid the reverse rotation of the motor.

   - The single-phase load needs to be evenly distributed among the three phases to ensure the balance of the three-phase current (the deviation ≤ 10%).

# IV. Compliance and Certification

1. **Standard Compliance**

   - Installation in China needs to comply with the GB 50150 "Code for Acceptance of Electrical Installation Works - Handover Test Standards for Electrical Equipment".

   - Select products that have passed the CCC certification and request the factory test report (no-load/load loss data).

2. **Professional Installation Suggestions**

   - A 25kVA transformer belongs to a medium-sized transformer, and it is recommended to be constructed by a licensed electrician or an electrical engineering company.

   - After the installation is completed, a withstand voltage test (2500V for 1 minute on the primary side), a transformation ratio test, and a phase check are required.

# V. Adaptation to Application Scenarios

*Industrial Equipment**: For example, when supplying power to multiple CNC machine tools or production lines, the total load power factor needs to be confirmed (it is recommended to compensate it to above 0.9).

*Imported Equipment**: Check the parameters on the equipment nameplate to ensure that the voltage and frequency (50Hz/60Hz) match.

*Emergency Power Supply**: If it is used in conjunction with a generator, an anti-islanding protection device needs to be installed.

*Reminder**: Before purchasing, it is necessary to clarify the type of the transformer (dry type/oil-immersed type, single-phase/three-phase), whether it has an isolation function, and whether special designs such as moisture-proof and anti-corrosion are required. After installation, regular inspections (once a quarter) are required, clean the internal dust, and check the tightness of the connection terminals.