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Transformer Buyer’s Guide

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Purchasing a Dry-Type Transformer

Dry-type transformers are designed for minimal maintenance, offering years or decades of reliable and trouble-free service. These transformers are completely static, solid-state devices with no moving parts, ensuring a long lifespan under normal operating conditions. They provide a safe and reliable power source without the need for fireproof vaults, catch basins, or venting toxic gases. These safety features allow dry-type transformers to be installed inside buildings close to the load, improving overall system regulation and reducing costly secondary line losses.

Key Information Needed Before Purchasing a Transformer

  1. Primary voltage
  2. Secondary voltage
  3. Frequency (Hz)
  4. Capacity (kVA). This is determined by the largest load expected at any given time.

Determining the Required Transformer Capacity

You can use our “Transformer Calculator” to find the necessary capacity for your transformer.

Single-Phase Transformers

Multiply the load current (in amps) by the load voltage (in volts).

Example: If the load is 100 amps and the secondary voltage is 240 volts:

240V × 100A = 24200VA or 19.2kVA

Three-Phase Transformers

Multiply the load current (in amps) by the line-to-line load voltage (in volts) by 1.73 (√3)

Example: If the load is 80 amperes and the voltage is 240 volts:

240V×80A×1.73 = 33216VA or 33.26kVA

Note: Always select a transformer with a capacity larger than the actual load (about 20%) to ensure safety and allow for future expansion.

General Tips

  • Identify the loads based on your operational requirements.
  • Calculate the kVA each load will draw.
  • Add them up to determine the maximum load.
  • Include factors for unplanned load growth (commonly 20%) 

Additional Considerations When Choosing a Transformer

  • Efficiency standard 
  • Power factor
  • Impedance
  • Temperature rise
  • Enclosed or Open-Type: Consider footprint, height, access, cooling
  • Weather Resistance: Location and space requirements
  • Seismic requirements
  • Accessories (including taps) and instrumentation
  • Basic impulse level: The withstand rating in kV

Transformer Definitions

  • Rated Voltage: The maximum voltage (phase-to-phase), expressed in Volts RMS (root mean square), for which the equipment is intended, also known as maximum system voltage.
  • Rated Insulation Level: The combination of voltage values characterizing the insulation of a transformer regarding its capability to withstand dielectric stresses.
  • Rated Frequency: The nominal frequency of the system, expressed in Hz, for which the transformer is designed.
  • Ambient Temperature: The average 24-hour ambient temperature above the standardized +20 °C, which influences the thermal design of transformers and must be specified.
  • Primary Windings: Designed as a multi-layer coil of double-enameled wire with special paper layer insulation. 
  • Core: Made of carefully selected laminated sheets of metal, stacked together to provide a flat magnetization curve, over-dimensioned to achieve very low flux at operating voltage.