Ampacity of low-voltage power cables under nonsinusoidal currents

This paper investigates the ac resistance in the presence of harmonies and proposes ampacity derating factors for cables made according to CENELEC Standard HD603. These cables are widely used in low-voltage industrial and building installations. Four-conductor cables of small, medium, and large conductor cross sections are considered. The fourth conductor is used as the neutral conductor. The cables are modeled using finite-element analysis software. The ac/dc resistance ratio is shown to increase with the frequency of the current and the cross section of the conductor, the increase being much larger when zero-sequence harmonics are present. A derating factor is defined and calculated for five typical nonsinusoidal current loads, for ex-ample, computer equipment. The derating of the cable's ampacity is shown to be very large when zero-sequence harmonics are present. The cross section of the neutral conductor is shown to be significant only when zero-sequence harmonics are present. The validity of the method is verified by comparison with data given in IEEE Standard 519-1992 and with measurements conducted on a cable feeding a large nonlinear load.