The current density (J) is a measure of the number of charge carriers in a conductor. By definition, it is the current in amperes
(A) that passes through a given conductor cross-section (A). If the conductor cross-section becomes larger, the current density decreases; if the conductor cross-section becomes smaller, the current density increases, since the same current flows through a smaller cross-section
. Thecurrent density is thus calculated from the ratio of the current intensity in amperes (A) to the conductor cross-section in square millimetres.
This relationship applies to direct current and low-frequency alternating current, in which the distribution of the charge carriers is uniform. As soon as alternating current has a low frequency, such as the mains frequency, the skin effect occurs, which presses the current overproportionally to the conductor surface and thus ensures an uneven distribution.