Power lines: minor_line threshold ?

In general my view is that the choice of minor_line vs. line should not be directly termined by the voltage of the line. Instead the choice should be based on the general visibility of the line in the general landscape.

Lines supported on pylons or very tall towers will be visible over long distances and can be useful navigation markers. Lines supported on poles close to the ground are only useful for local orientation. In general this difference will follow voltage, but there are exceptions. For instance in Britain there are still a number of 33kV and 66kV lines supported on pylons which are considerably smaller than those of the main distribution network. Sometimes low voltage lines are placed on pylons to cross rivers and gorges.

The main issue is a practical one: what can a relatively naive mapper conveniently use as a tag. Discerning the voltage requires somewhat arcane knowledge, and this information is conveniently supported by its own tag. So for me the deciding factor is the overall visibility of the line in the landscape (say immediately obvious from over 200m away).

The absolutely clinching factor for me, was a mapping need which arose earlier this year after the Nepal earthquake. After a US helicopter crashed with the loss of several rescuers' lives, there was a fear that unmapped power lines represented a significant danger to rescue workers. Helicopter pilots have no interest in the voltage of a power line: merely whether it is a significant aerial hazard, and that entirely depends on the height of the supports. Thus an entirely different use-case (hazard maps) would use similar criteria to my own (navigation in the landscape), which broadly corresponds to the height above ground of the highest wires on the power line, and nothing directly corresponding to voltage.

Lastly, in general it is not a good practice to use one tag as a proxy for another (see place & population): it is effectively tagging the same thing twice.

Summary: Use height of line off the ground to assign lines to power=line or power=minor_line. If possible add voltage and/or the tags which enable a 'power mapper' to assign voltage. This was we get both information relevant to the grid network and information relevant for other use cases.

From my point of view, 20 kV and below should be tagged power=minor_line.

These tag is very useful for the authors of map rendering stylesheets because they can easily differ between important (high voltage, high towers) and less important (lower towers/poles, lower voltage) power lines and render important power lines on lower zoom levels because these are good visible landmarks.