How the current affects the field produced by an overhead line

The actual field produced by an overhead line depends on several factors. This page illustrates this for one standard line, a 400 kV L12 transmission line with typical loads. Our detailed calculations of fields all specify the conditions they were calculated for.

The magnetic field depends directly on the current. The largest transmission lines in use have a rating of over 4000 A per circuit, but the average current in a typical circuit is more like 700 A. Distribution lines typically have currents of hundreds of A or less.

graph of how current affects field


As well as depending on the total current, the magnetic field can depend on the balance of the current in the two circuits.  The graph on this page is for balanced currents.

See also: