Most of the calculations on this website of fields from overhead lines in general, from specific overhead lines, or demonstrating the effect of particular factors, are for a single overhead line.  What happens if there are two lines close to each other? 

General principles

The general principles of what happens are shown in this graph:

The blue curve shows the magnetic field from a single line.  It's calculated for the same standard conditions as many of the other graphs - an L12 line, with 500 A in each circuit, transposed phasing, and 12 m ground clearance.

The red curve shows the field when a second identical line is placed 50 m away from the first line.

Each line produces its own elevated field underneath it, falling off to the sides.  The maximum field under one line is affected by the other, but only by a few percent, as can be seen from the difference between the red and blue curves at the "0 m" point.  Close to a line, it is that line which dominates the field, and the other line has only a small effect.  In this particular case, the presence of the second line actually reduces the field under the first line slightly.  That is a fluke of the specific conditions, in particular the relative loads and the relative phasing, which determine the direction of the two fields and whether they add or subtract.  The field could go up or down depending on the specific conditions, but it will only be a slight effect either way.  And in a real situation, the loads in the two lines would be constantly varying anyway.  See more on adding fields together.

To the sides of the lines, where the field is lower, again, it is the closer line which dominates the field, with the farther line making a difference, but not a big one.  Only if you are half way between the lines is the combined field markedly greater than you would get from the closer line only.

Particular application: 132 kV + 400 kV lines

One situation is where there is an existing 132 kV line, and there is a proposal to build a 400 kV line alongside.  This graph shows what happens:

The blue lines show the magnetic fields from each line separately - the left hand one the 132 kV line at 0 m, the right hand one the 400 kV line at 50 m.  The red line shows the combined field.  Again, the details of where the two fields reinforce each other and where they cancel each other depend on the relative phasing and loads in the line.

(In this example, the 400 kV lines has 500 A currents, 12 m ground clearance, and transposed phasing.  The 132 kV line has 130 A currents, 10 m grouind clearance, and untransposed phasing.)

We stress that these graphs illustrate the general principles only.  The details of what happens to the field are highly dependent on the specifics of each situation.