UL 1562-2013 pdf download – Transformers, Distribution, Dry-Type – Over 600 Volts.
1 0.3 The neutral bus bar and a terminal pad provided for securing the neutral pressure terminal connector on a transformer marked with a K-factor rating in accordance with 1 7.3 shall be sized based on the transformer full load current and the following maximum current densities: a) Five-hundred amperes per square inch (77.5 A/cm 2 ) of cross section for solid copper; b) Three-hundred and seventy-five ampere per square inch (58 A/cm 2 ) of cross section for solid aluminum having a conductivity of at least 55 percent that of the International Annealed Copper Standard; c) One-hundred amperes per square inch (1 5.5 A/cm 2 ) of contact area at bolted contacts between copper bus bars and connecting straps or connectors. In determining the contact area of a bolted or riveted connection, no additions or subtractions shall be made for the area of screws, bolts, or rivets; and d) Seventy-five amperes per square inch (1 1 .6 A/cm 2 ) of contact area at bolted contacts between aluminum bus bars and connecting straps or connectors. In determining the contact area of a bolted or riveted connection, no additions or subtractions shall be made for the area of screws, bolts, or rivets. Exception No. 1 : The current densities of the neutral bus bar and terminal pad may be greater that the specified values if the transformer is found to comply with the temperature test requirements specified in the IEEE Standard Test Code for Dry-Type Distribution and Power Transformers, C57.1 2.91 , while the neutral bus bar and terminal pad are carrying 200 percent of the transformer full load current. Exception No. 2: The current densities of the neutral bus bar and terminal pad may be greater than the specified values if the cross sectional area of the neutral bus bar and terminal pad is no less than 200 percent of the cross sectional area of the phase bus bar and terminal pad, respectively.