National standards, like BS7452 above, give values for the permitted increase in temperature that a transformer can achieve when working at full power.

  • Class A = 75 degree rise
  • Class B = 95 degree rise
  • Class E = 90 degree rise
  • Class F = 115 degree rise
  • Class H = 140 degree rise

These figures are based on an ambient not normally exceeding 25 degrees but occasionally reaching 35 degrees.