Where overhead lines have two circuits, there is a choice about how they are wired relative to each other, called the "phasing". One option, usually a phasing known as "transposed", results in more cancellation between the two circuits and lower fields. More on "phasing" including graphs showing this effect.
"We recommend that electricity companies be encouraged to choose the optimal phasing (usually transposed phasing) for all new lines, and also be encouraged to convert existing lines where possible and justifiable."
"We do not expect our recommendation to result in any change for the National Grid"
This is because the National Grid (400 kV and 275 kV lines) was built from the start with a policy of using transposed phasing where possible. About 90% is already transposed. The remainder is mainly made up of single-circuit lines, where phasing is not relevant, or situations where three lines meet at a "T" point, where it is infeasible to achieve transposed phasing for all three lines.
For 132 kV lines, SAGE collected rough estimates from electricity companies. Between 70% and 90% (depending on the company) of lines are double-circuit as opposed to single-circuit. Of these, between 70% and 90% were estimated as already transposed. SAGE estimated that 12% of 132 kV lines, 2000 km, are not currently transposed but reasonably could be.
Resulting from SAGE's recommendation, there is now a formal policy on phasing in the UK, and this has led to the electricity industry collecting and reporting more accurate statistics. We now know that 50% of double-circuit 132 kV lines have optimum phasing.
SAGE estimates the cost of converting a line to transposed phasing as £60k for 275 kV and 400 kV lines or £10k - £30k for 132 kV lines, where it can be done simply by changing the terminations at the end of the line. If it cannot be done that way and requires a new pylon or other new work the cost rises.
"We do not advocate a rigorous definition of what should be done, nor do we believe it should be enshrined in regulation or should have a specific timescale attached. Instead, we consider this will be most effective if expressed as a general encouragement to electricity companies to take the desired action where possible."
In October 2009, the Government formally responded to SAGE's recommendations. See full details of the response. On this particular issue of phasing they said:
"The Government agrees with the SAGE recommendation and urges industry to optimal phase overhead lines wherever possible and reasonable. We will proactively work with industry to consider how best to take this forward. This might include developing a voluntary code of practice on phasing for voltages of 132kV and above." (para 50)
That Code of Practice was duly adopted in February 2011.