The IEC 61439-1 sets the thermal limit in busbars working at the maximum working load. Here, 140°C (which is 105K over the ambient temperature of 35°C) is the upper safe temperature limit. 23-1987 "American National Standard Guide for Metal-Enclosed Bus and Calculating Losses in Isolated-Phase Bus" 1. Jointing of Copper Busbars Not open for. The current rating is calculated from the conductor cross-sectional area, material (copper or aluminium), and maximum temperature rise per IEC 61439-1 (typically 70K above 35 degrees C ambient for bare copper). For terminals connecting external conductors, the allowable thermal rise is tighter — 55 K — to protect cable insulation at connection points. This assumption is widespread in workshops, on job sites, and even during procurement reviews. However, real-world testing and.