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Home / Technical Articles / We can load distribution transformer heavily when it is cold, but when it’s hot…

Transformer loading and sizing

Distribution transformers (commonly mineral-oil-immersed type) are output rated and their loading and sizing guidelines of many utilities are based on ANSI/IEEE C57.91.

We can load distribution transformer heavily when it is cold, but when it's hot...
We can load distribution transformer heavily when it is cold, but when it's hot...

Distribution transformers can deliver their rated kVA without exceeding temperature rise limits when the following conditions apply:

  1. The secondary voltage and the voltage/frequency do not exceed 105% of rating.
    So, a transformer is a constant kVA device for a voltage from 100 to 105% (the standards are unclear below that, so treat them as constant current devices).
  2. The load power factor ≥ 80%
  3. Frequency ≥ 95% of rating

Modern distribution transformers are 65°C rise units, meaning they have normal life expectancy when operated with an average winding temperature rise above ambient of not more than 65°C and a hottest spot winding temperature rise of not more than 80°C.

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

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