Normal Rating. The Normal Rating of an independent current transformer is its rated continuous current capability, which is determined by its continuous thermal current rating factor (RF; this is standard nameplate information) and the average cooling air temperature. ANSI/IEEE Standard C57.13-2008establishes rated continuous current for current transformers according to thermal current rating factor at a design temperature rise of 55 °C above an ambient temperature of 30 °C. Figure 1 is reproduced from the ANSI/IEEE Standard and provides the percent of rated primary current that can be carried continuously by devices of that design without causing established temperature limits to be exceeded. For example, Normal summer and winter ratings for independent current transformers with an RF of 1.5 are found to be 160% and 180% of nameplate, respectively, using the ambient temperatures specified in Appendix A of Planning Procedure No. 7. Figure 1 55°C Rise Current Transformer Basic Loading Characteristics (In Air)1
