Cable sizing for fast transient loads

The most recognized standards for sizing US manufactured cables are ampacity tables published in the National Electrical Code. These tables are only for steady-state loads. This paper addresses the transient temperature response of cables undergoing fast heavy loading. Exponential functions are analytically derived that express responses to step changes in cable currents. Formulas for coefficients and time constants are derived. Cable manufacturers´ data and other sources for physical properties are used to calculate the coefficients and time constants for three copper conductor sizes. Steady-state end temperatures are based on National Electrical Code ampacity tables. The derived formulas are limited to cables in open tray or free air, and in conduit in air. The methods to develop data are detailed for other conductor sizes and aluminum conductors. Also presented are the means to size power cables based on calculated transient temperatures. These calculated temperatures are compared with cable temperature ratings to select a cable size. An example is included that illustrates sizing feeders for five 2000 hp crusher-conveyors.