Circuits and conductors
Current density
- where
- current density (A/mm²)
- current (A)
- conductor area (mm²)
Explain this equation
Works out current per square millimetre of conductor. A quick comparison of how hard a conductor is being worked.
J is current density in A/mm². I is the current and A is the conductor’s cross-sectional area in mm².
Spreading the same current over a bigger area means less current through each square millimetre, so it is a straight division.
Sense check: 10 A in 2.5 mm² is 4 A/mm². 32 A in 6 mm² is about 5.3 A/mm². Double the area and J halves.
Watch out: this is not a cable rating. Installation method, grouping, insulation and ambient temperature decide what a cable can carry.
Current density relates current to conductor cross-section. It helps compare loading but is not a complete current-carrying-capacity calculation.
Thermal conditions, insulation, grouping and installation method still matter for cable selection.
Enter your values to see the result.
Common mistake
Using a fixed A/mm² rule as a substitute for cable design.
Limits
Average uniform current density; area units define output units.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.