Calculation of Current Ratings for Durations. Greater Than 2 Hours a max h a Winter and summer ratings for durations greater than 2 hours can be determined as follows: I =I (θ max −θ /θ )1/ n Where, I = Adjusted rated current (amperes) I a = Current rating to be calculated for a specific duration (amperes) θ a = Ambient temperature (°C) θ max h = Allowable maximum hottest spot temperature9 (°C) θ r = Allowable temperature rise at rated current (°C)
