Fast Transient Loads of Low Voltage Power Cables

Fast transient loads are kiloampere current spikes that last several minutes. The most recognized standards for sizing US manufactured cables are ampacity tables published in the National Electrical Code [3]. These tables are only for steady-state loads. For occasional kiloampere spikes of several minutes' duration, the temperature responses of medium-voltage cables have been published in [I]. A method to quickly size medium-voltage conductors undergoing fast transient loads is presented in [2]. Herein, fast transient work extends these methods to cover low-voltage cables of US manufacture. Exponential functions are analytically derived in [I] that express temperature responses to step changes in cable currents. Formulas for coefficients and time constants are derived. In [I] and [2] cable manufacturers' data and other sources for physical properties are used to calculate the responses of four sizes of SkV power cables to these current spikes. The tables listing the results are repeated here with their sizing procedure. Herein are developed temperature responses to current spikes carried by 600 volt cables, and 600V power cable sizing methods. Motors demanding fast transient loads, and variable-speed drives supplying them are also discussed.