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 because the same current flows through a smaller cross-section. The current density is thus calculated from the ratio of the current intensity in amperes (A) to the conductor cross-section in square millimeters.
This relationship applies to direct current and low- frequency alternating current, where the distribution of 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 a non-uniform distribution.