Tutorial: How to size a transformer for industrial loads?

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Correctly sizing a transformer is essential for ensuring reliable power delivery, avoiding overloads, and planning for future expansion.

Follow this step-by-step guide to determine the right transformer size for your industrial application.

Step 1: list all electrical loads

Start by identifying all the equipment that will be powered by the transformer. For each item, note:

• Power rating (in kW, kVA, or HP)
• Voltage
• Phase (single or three-phase)
• Operating current (if available)

Step 2: convert all loads to kVA

Transformers are rated in kVA, so convert all loads accordingly:

• For three-phase loads:
  kVA = (√3 × Voltage × Current) / 1000
• For single-phase loads:
  kVA = (Voltage × Current) / 1000

If you only have kW and power factor (PF): kVA = kW / PF

Step 3: apply demand and diversity factors

Not all equipment runs at full load all the time. Apply:

• Demand factor: reflects the actual maximum load expected.
• Diversity factor: accounts for the probability that not all loads operate simultaneously.

Example:
If your total connected load is 500 kVA and the demand factor is 0.8:
Actual load = 500 × 0.8 = 400 kVA

Step 4: add a safety margin

To allow for future expansion and avoid overloading, add a safety margin of 10–25%.

Example:
Required transformer size = 400 × 1.2 = 480 kVA

Step 5: select the nearest standard transformer size

Choose the next standard transformer size above your calculated requirement.

Common sizes include: 25, 50, 75, 100, 150, 200, 300, 500, 750, 1000 kVA, etc.

In the example above, you would choose a 500 kVA transformer.

Step 6: match voltage levels

Ensure the transformer’s:

• Primary voltage matches the utility supply
• Secondary voltage matches your equipment requirements

Step 7: consider additional factors

• Cooling type: dry-type (indoor) or oil-immersed (outdoor)
• Impedance: affects short-circuit current levels
• Efficiency: higher efficiency reduces energy losses
• Environmental conditions: temperature, humidity, altitude, etc.

In conclusion, proper transformer sizing is essential for ensuring safe, efficient, and reliable power distribution in industrial settings. By calculating the total load, applying realistic demand factors, and including a safety margin, you can confidently select a transformer that meets both current and future needs. Always match voltage levels and consider environmental and operational factors to ensure long-term performance.