An analytical quantification of errors due to the use and misuse of cable positive-sequence impedances provided by the NEC

Table 9 of the National Electrical Code^® (NEC) is regularly used by electrical engineers to identify the positive-sequence impedance of low-voltage, three-phase, single-core cables at 60Hz and 75°C, installed in various conduit types. This table comes handy when cable manufacturers do not provide resistance and reactance of cable of various trade sizes necessary to conduct load flow/voltage drop, short circuit, and arc flash calculations. However, a footnote under the same table clearly states that these impedances can only be used for cradled cables in conduit with RHH insulation type and with an operating temperature of 75°C. It is also assumed that Table 9 is in compliance with Annex C of the NEC, i.e. maximum 40% fill requirement of conduits. This paper quantifies the errors that users may incur into by applying these impedances to cables with different insulation types and different operating temperatures than 75°C (misuse). The authors will also analytically calculate the additional errors that can occur if the table is used in correspondence with cables having a different layout than cradled in conduits of size not in compliance with the maximum 40% fill requirement of the conduit (misuse). Last but not least, an analysis will be conducted to determine the error in the impedance calculation, as well as in studies, even if the table is used in the proper context, as it is based on outdated cable physical parameters (use).