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How do you properly size a distribution transformer?

Distribution transformers need to take several items into consideration when sizing including:

  • Maximum Load
  • Potential future load growth (typical is 25%)
  • Load Inrush and voltage regulation
  • Harmonics and Power Factor
  • Ambient Temperature
  • Additional Service Factor

For reference, NEC Article 210, Branch Circuits, and NEC Article 230, Services is used to select panelboards and the size of branch circuits. Typically a transformer must be sized to support the load requirements of the switchgear, panelboards and branch circuits. For drive isolation transformers, it is suggested to take sizing charts provided by manufacturers into consideration due to derating for harmonics. In addition to sizing a transformer, the general types including general purpose, K-Rated, Harmonic Mitigating and Drive Isolation also need to be chosen.

Distribution transformers are often sized from loads based on NEC Article 220. NEC Article 220.87 does allow transformers to be sized based on peak-load data over a 1-year period. NEC also allows loads to be sized using metered data over 30 days if the additional maximum anticipated heating and/or cooling load is also factored in. This often allows a transformer to be sized lower than the base calculations from NEC Article 220. Peak efficiency for 600V class distribution transformers is typically at 35%. Peak efficiency for medium voltage transformers is typically at 50% load.

Additional capacity for future loads can be obtained by A) specifying a lower temperature rise (15%-30% for dry type) or B) utilizing fans (25%-50% for dry type).

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