What is the hot spot allowance of a transformer?

Frequently Asked Questions General 

The coils of a transformer core do not evenly heated during energization. Parts of one coil will be hotter than the surrounding areas because they are farther from any ventilated openings or closer to the core which also produces heat. The hot spot allowance is a set number as defined by industry standards and is associated with the insulation class. Typically the smaller the transformer, the lower the insulation class and the more uniform the heating.

  • 105C Insulation System: 10C Hot Spot Allowance
  • 150C Insulation System: 30C Hot Spot Allowance
  • 180C Insulation System: 25C Hot Spot Allowance
  • 220C Insulation System: 30C Hot Spot Allowance

The Hot Spot Allowance is added the expected ambient temperature and full load temperature rise to get the total expected temperature rise of a transformer.