Current density

is the physical value for current divided by the cross-sectional surface area. The unit of measure is A/mm2. With mains frequency, air-cooled current rails have a maximum of 2 A/mm2, and with medium frequency up to 500 Hz approx. 1 A/mm2. If the electrical conductor is surrounded by flowing water, as in water- cooled cables, a maximum of 16 A/mm2 will be available in this application. Because of the line losses, the length must also be taken into account in addition to the cross-section. double the length with the same cross-section also means double the losses. A 5 m long cable with 600 mm2 copper cross-section has loss- es of 13.7 kW at 9,600 A of transmitted current, so the losses in a 10 m cable will be 27.4 kW. If the furnace has 2 cables, the losses will therefore be 27.4 kW at a length of 5 m and 54.8 kW with a 10 m cable length. This application example applies to a furnace with an output of 2,150 kW at 1,000 V. The reduction of the current density, in this case from 16 A/mm2 to 10.67 A/mm2 thus produces loss- es of 18.2 kW and 36.4 kW.