If you know the geometry of the line and the currents (or voltages for electric fields) it is possible to calculate fields quite accurately ( download a tutorial on how to calculate the fields from a three-phase circuit).

Often, it is acceptably accurate to approximate the conductors as infinitely long straight lines and to approximate the currents as exactly balanced (the three currents in the three phases exactly equal).

If necessary, however, you can model the actual sag of the conductors and the actual currents in each conductor (more on unbalanced currents). The results from this exercise are shown here. The red dots are the measured magnetic field, the blue line the calculated magnetic field for the same instant of time. This comparison has been published in a scientific journal - abstract at bottom of this page.

Power-line calculations and measurements are usually performed for 1 m above ground.  This is for a sound scientific reason explained here.

Straight lines and curves

Overhead lines and underground cables are generally pretty straight, and we usually model them as straight lines.  But of course, overhead lines sag between pylons, and sometimes, the conductors of underground cables diverge from each other.  This doesn't usually make a great deal of difference to the fields but we give more detail on the effect of sag and diverging conductors.

If the conductors are straight but only finite in length, they produce less field than the infinite conductors we usually calculate for - but we can calculate how much less quite easily.

Calculating fields for yourself

If you want to calculate fields yourself we provide:

dimensions for some common UK power line designs.

an online calculator for magnetic fields from some common UK power line designs