The resistance is not constant and is in fact a function of temperature [0]. Line sag is a proxy measurement for line temperature, since things expand as they heat up. From [1], most materials have positive alpha coefficients so the resistance increases with temperature.

0. https://en.wikipedia.org/wiki/Electrical_resistivity_and_con...

1. https://www.allaboutcircuits.com/textbook/direct-current/chp...

The statement referred to heating driving sagging, which increased resistance.

The increase of resistance with temperature is well-known -- I was curious if there is some effect that is directly related to the wire sag (changing morphology, somehow-altered surface area/skin-depth, changing kinks at the connections, some subtlety with increased length (cross-sectional area also increases), etc..).

Theoretically I would thing the sag itself should increase resistance since resistance increases with both length and smaller wire diameter. When the line lengthens the diameter should decrease (similar to stretching a piece of gum or putty).

It's not being stretched by an external force, though. It's expanding due to heat. The diameter would also expand some negligible amount I think.

I don't think it should. Heat expansion should slightly increase the diameter.

Not if the length increases to counteract it.

It doesn’t ”counteract” it. All dimensions increase based on the same coefficient. Same percentage increase on all axis.

See https://en.m.wikipedia.org/wiki/Thermal_expansion and check the coefficient of expansion.

My thought is that after each expansion/contraction cycle, contraction is never exactly back to its previous state.

Like bending a piece of metal back and forth, it weakens each time.

Just FYI, transmission lines are aluminum not copper. Also, both copper and aluminum have positive temperature coefficients of resistance.

My thought was that the gauge of the wire goes down as it sags. The same weight of wire across a longer distance.

Maybe I’m wrong and this kind of wire stretching doesn’t impact resistance?

The volume expands as it heats, so it's not a given that the diameter (gauge) goes down. But the length is longer and that does increase the total resistance.

I think sag means voltage sag.

One of the more impressive things I've seen with electricity is watching a transmission line into a local neighborhood turn orange and stretch to drape itself across a pickup truck parked in the wrong spot. Took a minute to realize what I'd just watched.

You glowing orange from heat?

Yep, only thing that made sense. There was a heavy storm going through and I was driving by at night, during the day probably wouldn't have been visible at all.

In this case sag is meant literally.

https://www.atcllc.com/blog/why-do-power-lines-sag-on-hot-da...