 If the wire is carrying high frequency currents (above 10 kHz) the skin effect may affect the distribution of the current across the section by concentrating the current on the surface of the conductor.If active cooling is provided by blowing air or circulating water much higher current densities can be achieved - proportionally to the effectiveness of cooling.An aluminium wire must have 1.6 times the cross sectional area as a copper wire to achieve comparable DC resistance.